CR Earth Science 3/4

The height, length, and period that are eventually achieved by a wave depend on three factors:

(1) wind speed; (2) length of time the wind has blown; and (3) fetch.

Heat is

the energy transferred from one object to another because of a difference in their temperatures. All matter is composed of atoms or molecules that possess kinetic energy, or the energy of motion.

On June 21 or 22 each year, the axis is such that the Northern Hemisphere is "leaning" 23.5 degrees toward the sun. This date is known as

the summer solstice, or the first "official" day of summer.

The influence of the sun on tides is most noticeable near

the times of new and full moons. During these times, the sun and moon are aligned, and their forces are added together.

How does the sun's gravity affect tides?

It changes the size of tidal ranges in a weekly cycle. The gravitational attraction of the moon is the main force that causes tides. In roughly a weekly cycle, the sun makes tidal ranges wider (spring tides) and then narrower (neap tides).

How is conduction different from convection?

Objects must be in contact with one another to transfer heat through conduction. Convection is a movement of heat through liquids created by mass movement of molecules.

What is ozone and how is it formed?

Ozone is a form of oxygen in which every molecule contains three atoms of oxygen. Ozone is formed when a single atom of oxygen (O) and a diatomic molecule of oxygen (O2) collide.

To understand how the atmosphere is heated, it is useful to think about four laws governing radiation:

1. All objects, at any temperature, emit radiant energy. Not only hot objects like the sun, but also Earth—including its polar ice caps—continually emit energy. 2. Hotter objects radiate more total energy per unit area than do colder objects. 3. The hottest radiating bodies produce the shortest wavelengths of maximum radiation. For example, the sun, with a surface temperature of nearly 6,000°C, radiates maximum energy at 0.5 micrometers, which is in the visible range. The maximum radiation for Earth occurs at a wavelength of 10 micrometers, well within the infrared range. 4. Objects that are good absorbers of radiation are good emitters as well. Gases are selective absorbers and radiators. The atmosphere does not absorb certain wavelengths of radiation, but it is a good absorber of other wavelengths.

When radiation strikes an object, there usually are three different results:

1. Some energy is absorbed by the object. When radiant energy is absorbed, it is converted to heat and causes a temperature increase. 2. Substances such as water and air are transparent to certain wavelengths of radiation. These substances transmit the radiant energy. Radiation that is transmitted does not contribute energy to the object. 3. Some radiation may bounce off the object without being absorbed or transmitted. The figure shows what happens to incoming solar radiation, averaged for the entire globe.

How do deep-ocean currents move differently than surface currents?

Deep-ocean currents experience mostly vertical movement, whereas surface currents experience mostly horizontal movement.

The surface waters of the eastern Mediterranean Sea have a salinity of about

38 parts per thousand. In the winter months, this water flows out of the Mediterranean Sea into the Atlantic Ocean. At 38 parts per thousand, this water is denser than the Atlantic Ocean surface water at 35 parts per thousand, so it sinks. This Mediterranean water mass can be tracked as far south as Antarctica. Next, look at the illustration of deep water circulation caused by density currents. Notice that warm water moves away from the equator. North Atlantic Deep water moves solely toward southern latitudes. Also notice that Antarctic Bottom Water flows north to the equator.

Two gases—nitrogen and oxygen—make up

99 percent of the volume of clean, dry air. Although these gases are the most common components of air, they don't affect the weather much. The remaining 1 percent of dry air is mostly the inert gas argon (0.93 percent) plus tiny quantities of a number of other gases. Carbon dioxide is present in only small amounts (approximately 0.039 percent), but it is an important component of air. Carbon dioxide is an active absorber of energy given off by Earth. Therefore, it plays a significant role in heating the atmosphere.

Why are beaches and shorelines constantly changing?

Beaches and shorelines are constantly changing because waves along the shoreline are constantly eroding, transporting, and depositing sediment.

What is the difference between weather and climate?

Climate is a description of aggregate weather conditions for an area, and weather is the state of the atmosphere at a given moment in time.

To illustrate how tides are produced, consider

Earth as a rotating sphere covered to a uniform depth with water. Think about the gravitational forces in the Earth-moon system, ignoring the influence of the sun for now. This gravitational pull is strongest on the side of Earth closest to the moon and weakest on the far side of Earth from the moon. This difference causes Earth to be stretched slightly. The shape of the solid Earth is not affected much by this difference in pull. However, because water is a fluid, it can easily flow from location to location in response to differences in the pull of the moon's gravity.

The length of daylight compared with darkness also is determined by

Earth's position in orbit. All latitudes receive 12 hours of daylight during the vernal and autumnal equinoxes (equal night). The length of daylight on the summer solstice in the Northern Hemisphere is greater than the length of darkness. The farther you are north of the equator on the summer solstice, the longer the period of daylight. When you reach the Arctic Circle, at 66.5 degrees north latitude, the length of daylight is 24 hours.

The ability of substances to conduct heat varies greatly. Metals are good conductors, as those of us who have touched hot metal have quickly learned. Air, however, is a very poor conductor of heat. Because air is a poor conductor, conduction is important only between

Earth's surface and the air directly in contact with the surface. For the atmosphere as a whole, conduction is the least important mechanism of heat transfer.

What determines the direction of gyres in both the Northern Hemisphere and Southern Hemisphere?

Earths rotation

We know that it is colder in the winter than in the summer. But why?

Length of day and a gradual change in the angle of the noon sun above the horizon affect the amount of energy Earth receives. Seasonal changes occur because Earth's position relative to the sun continually changes as it travels along its orbit. Earth's axis is not perpendicular to the plane of its orbit around the sun. Instead, it is tilted 23.5 degrees from the perpendicular. Because the axis remains pointed toward the North Star as Earth moves around the sun, the position of Earth's axis relative to the sun's rays is constantly changing. If the axis were not tilted, we would not have seasonal changes.

Earth has two principal motions: rotation and revolution.

Rotation is the spinning of Earth about its axis. The axis is an imaginary line running through the north and south poles. Our planet rotates once every 24 hours, producing the daily cycle of daylight and darkness. Revolution is the movement of Earth in its orbit around the sun. Earth travels at nearly 113,000 kilometers per hour in an elliptical orbit about the sun.

Ocean waves are energy traveling along the boundary between ocean and atmosphere. Waves often transfer energy from a storm far out at sea over distances of several thousand kilometers. That's why even on calm days the ocean still has waves that travel across its surface.

Seawalls are often built along coastal areas to protect against erosion and flooding.

How is the atmosphere affected by radiation?

Solar radiation can be absorbed, transmitted, or reflected. The radiant energy that is absorbed is converted to heat and causes an increase in temperature.

Barrier islands probably formed in several ways:

Some began as spits that were later cut off from the mainland by wave erosion or by the general rise in sea level following the last glacial period. Others were created when turbulent waters in the line of breakers heaped up sand that had been scoured from the bottom. Finally, some barrier islands may be former sand-dune ridges that began along the shore during the last glacial period, when sea level was lower. As the ice sheets melted, sea level rose and flooded the area behind the beach-dune complex.

What is the difference between spring tides and neap tides?

Spring tides occur when the sun, Earth, and moon are in a straight line. Neap tides occur when the sun and moon are at right angles to one another.

A cork is floating in the open ocean. How does the cork move as a wave passes by?

The cork moves in a circular path. A wave advances across the ocean, but the water itself does not move with the wave. Instead, the water moves in a circular path. It moves up and forward when the wave arrives, then down and back as the wave passes.

What happens as currents move from low-latitude regions to higher latitude regions?

The currents transfer heat.

What causes the friction that fuels surface currents?

The friction between the ocean and the wind that blows across the surface of the ocean causes surface currents to form.

Great Britain and Newfoundland are located directly opposite of each other across the Atlantic Ocean. Why do the two places have different climates?

The gulf stream brings warm water to Great Britain

Why is solar energy not distributed evenly over the Earth?

The movement of the Earth in relation to the sun, and the various topographical features found both on land and in the oceans, cause solar energy to be distributed unevenly across Earth.

How can density currents form in warm waters, such as the Mediterranean Sea?

The salinity of surface water increases from evaporation.

Anatomy of a wave

The tops of the waves are the crests, which are separated by troughs. Halfway between the crests and troughs is the still water level, which is the level that the water would occupy if there were no waves. The vertical distance between trough and crest is called the wave height. The horizontal distance between two successive crests or two successive troughs is the wavelength. The time it takes one full wave—one wavelength—to pass a fixed position is the wave period.

An irregular coastline includes headlands and bays. When refracting waves approach this kind of coastline, how is their erosive power distributed?

Their power is stronger on the sides and ends of headlands.

Where does the atmosphere end and outer space begin?

There is no sharp boundary. The atmosphere thins as you travel away from Earth until there are too few gas molecules to detect.

A beach is the shore of a body of water that is covered in sand, gravel, or other larger sediments. Sediment eroded from the beach is transported along the shore and deposited in areas where wave energy is low. Such processes produce a variety of

Where longshore currents and other surf zone currents are active, several features related to the movement of sediment along the shore may develop. A spit is an elongated ridge of sand that projects from the land into the mouth of an adjacent bay. Often the end in the water hooks landward in response to the dominant direction of the longshore current.

The isotherms generally trend east and west and show

a decrease in temperatures from the tropics toward the poles.

Another important variable component of the atmosphere is ozone. Ozone is

a form of oxygen that combines three oxygen atoms into each molecule (O3). Ozone is not the same as the oxygen we breathe, which has two atoms per molecule (O2).

There is very little ozone in the atmosphere, and it is not distributed evenly. It is concentrated in

a layer located between 10 and 50 kilometers above Earth's surface. In this altitude range, oxygen molecules (O2) are split into single atoms of oxygen (O) when they absorb ultraviolet (UV) radiation emitted by the sun. Ozone is then produced when a single atom of oxygen (O) and a molecule of oxygen (O2) collide. This collision must happen in the presence of a third, neutral molecule that acts as a catalyst. A catalyst allows a reaction to take place without being consumed in the process.

Although waves are refracted, most still reach the shore at a slight angle. As a result, the uprush of water, or swash, from each breaking wave is at an oblique angle to the shoreline. These angled waves produce currents within the surf zone. The currents flow parallel to the shore and move large amounts of sediment along the shore. This type of current is called

a longshore current.

Temperature is

a measure of the average kinetic energy of the individual atoms or molecules in a substance. When energy is transferred to the gas atoms and molecules in air, those particles move faster, and air temperature rises. When air transfers energy to a cooler object, its particles move more slowly, and air temperature drops.

Sometimes the term air is used as if it were a specific gas, which it is not. Air is

a mixture of different gases and particles, each with its own physical properties. The composition of air varies from time to time and from place to place. However, if the water vapor, dust, and other variable components were removed from the atmosphere, its makeup would be very stable worldwide up to an altitude of about 80 kilometers.

A tombolo is

a ridge of sand that connects an island to the mainland or to another island. A tombolo forms in much the same way as a spit.

On the side of Earth closest to the moon, the pull of the moon's gravity on the oceans is greater than it is on the solid Earth. Ocean water flows toward this area, producing

a tidal bulge, or high tide.

About 50 percent of the solar energy that strikes the top of the atmosphere reaches Earth's surface and is

absorbed. Most of this energy is then reradiated skyward.

The most important measurable properties of weather and climate are

air temperature, humidity, type and amount of precipitation, air pressure, and the speed and direction of the wind.

Air pollutants are

airborne particles and gases that occur in concentrations large enough to endanger the health of organisms. Primary pollutants, shown in the figure, are emitted directly from identifiable sources. Emissions from transportation vehicles account for nearly half the primary pollutants by weight.

The energy contributed by the wind to the water is transmitted not only along the surface of the sea but

also downward. However, beneath the surface, the circular motion rapidly diminishes until—at a depth equal to one-half the wavelength measured from still water level—the movement of water particles becomes negligible. The dramatic decrease of wave energy with depth is shown by the rapidly decreasing diameters of water-particle orbits in the figure.

Reflection occurs when

an electromagnetic wave bounces off an object. The reflected radiation has the same intensity as the incident radiation. About 30 percent of the solar energy reaching the outer atmosphere is reflected back to space. This 30 percent also includes the amount of energy sent skyward by scattering. This energy is lost and does not heat Earth's atmosphere.

A temperature control is

any factor that causes temperature to vary from place to place and from time to time. The most important cause for temperature variations is differences in the receipt of solar radiation. Because variations in the angle of the sun's rays and length of daylight depend on latitude, they are responsible for warmer temperatures in the tropics and colder temperatures toward the poles. Seasonal temperature changes happen as the sun's vertical rays move toward and away from a particular latitude during the year. Factors other than latitude that exert a strong influence on temperature include heating of land and water, altitude, geographic position, cloud cover, and ocean currents.

The term baymouth bar is

applied to a sandbar that completely crosses a bay, sealing it off from the open ocean. Find the baymouth bar in the figure. Such a feature tends to form across bays where currents are weak. The weak currents allow a spit to extend to the other side and form a baymouth bar.

The Atlantic and Gulf Coastal Plains are relatively flat and slope gently seaward. The shore zone in these areas is characterized by

barrier islands. Barrier islands are narrow sandbars parallel to, but separated from, the coast at distances from 3 to 30 kilometers offshore. From Cape Cod, Massachusetts, to Padre Island, Texas, nearly 300 barrier islands rim the coast.

The movement of ocean water is a powerful thing. Waves created by storms release energy when they crash along the shoreline. Sometimes the energy of water movement can

be harnessed and used to generate electricity.

At night, clouds have the opposite effect. Clouds act as a

blanket by absorbing outgoing radiation emitted by Earth and reradiating a portion of it back to the surface. Thus, cloudy nighttime air temperatures do not drop as low as they would on a clear night. The effect of cloud cover is to reduce the daily temperature range by lowering the daytime maximum and raising the nighttime minimum.

There are smaller temperature variations in

coastal areas, relative to inland areas, since the ocean water keeps the temperatures of the surrounding areas more stable.

Three mechanisms of energy transfer as heat are

conduction, convection, radiation. These mechanisms operate to transfer energy between Earth's surface (both land and water) and the atmosphere.

Ocean tides result from

differences in the gravitational attraction exerted upon different parts of Earth's surface by the moon and, to a lesser extent, by the sun.

Most water involved in deep-ocean density currents begins in high latitudes at the surface. These deep ocean currents are driven by

differences in water density. Deep ocean currents occur far below the surface of the oceans. Water can become denser with a decrease in temperature or an increase in salinity. At the Earth's poles, air chills the water molecules at the water surface, causing the molecules to slow down and move closer together. This decrease in volume makes the water denser. Seawater at the poles can also get cold enough to freeze. As seawater freezes, the dissolved solids get left behind, making the water below the ice saltier and, therefore, denser. Water can also become saltier in warmer areas of the globe due to evaporation. The denser water sinks to the ocean floor and forms deep ocean currents. This process is called thermohaline (thermo = heat, haline = salt) circulation.

Fetch is the

distance that the wind has traveled across open water. As the quantity of energy transferred from the wind to the water increases, both the height and steepness of the waves also increase. Eventually, a critical point is reached where waves grow so tall that they topple over, forming ocean breakers called whitecaps.

At high latitudes, which process helps form a deep-ocean density current?

sea ice forming

Three main tidal patterns exist worldwide:

diurnal tides, semidiurnal tides, and mixed tides. A diurnal tidal pattern is characterized by a single high tide and a single low tide each tidal day, as shown in the graph. Tides of this type occur along the northern shore of the Gulf of Mexico. A semidiurnal tidal pattern exhibits two high tides and two low tides each tidal day. The two highs are about the same height, and the two lows are about the same height. This type of tidal pattern is common along the Atlantic Coast of the United States. A mixed tidal pattern is similar to a semidiurnal pattern except that it is characterized by a large inequality in high water heights, low water heights, or both. In this case, there are usually two high and two low tides each day. However, the high tides are of different heights, and the low tides are of different heights. Such tides are found along the Pacific Coast of the United States and in many other parts of the world.

Wave deposition can create

dramatic land features. The long, curving arm of Cape Cod, Massachusetts is a prime example. Cape Cod was originally a moraine—a pile of till deposited at the leading edge of a glacier that covered the Northeast during the most recent ice age. At that time, about 20,000 years ago, the sea level was low, and the moraine was deposited on dry land. Since that time, the sea level has risen and the moraine has become subjected to wave erosion. Sediment from the loose pile of till has been carried northward along the shore, continually extending the arm of land in that direction.

The water in the surf zone is turbulent. Turbulence allows longshore currents to

easily move the fine suspended sand and to roll larger sand and gravel particles along the bottom. For a 10-year period at Oxnard, California, more than 1.4 million metric tons of sediment moved along the shore each year.

Secondary pollutants are

emitted directly into air. They form in the atmosphere when reactions take place among primary pollutants and other substances. For example, after the primary pollutant sulfur dioxide enters the atmosphere, it combines with oxygen to produce sulfur trioxide. Then the sulfur trioxide combines with water to create sulfuric acid, an irritating and corrosive substance.

The sun is the ultimate source of

energy that creates our weather. The sun emits light and heat—as well as the ultraviolet rays that cause a suntan. These forms of energy are only part of a large array of energy called the electromagnetic spectrum. This spectrum of electromagnetic energy is shown in the figure. All radiation, whether X-rays, radio waves, or infrared waves, travels through the vacuum of space at 300,000 kilometers per second. Radiation travels only slightly slower through our atmosphere.

Shoreline features that originate primarily from the work of erosion are called

erosional features. Sediment that is transported along the shore and deposited in areas where energy is low produce depositional features.

Many coastal landforms owe their origin to

erosional processes. Such erosional features are common along the rugged and irregular New England coast and along the steep shorelines of the West Coast of the United States. Wave erosion is steadily wearing away the California coast. Where the coast is made up of sedimentary rock, average erosion is 15-30 centimeters per year. But where the coast consists of soil and sand, erosion can be as high as 2-3 meters per year. Coastal erosion is a hazard to structures built on cliffs and bluffs along the shore. The cliffs along California's coast form as tectonic processes slowly uplift coastal land. At the same time, the energy of ocean waves undercuts the cliffs. Over time, this process produces features such as wave-cut cliffs and platforms, sea arches, and sea stacks.

Processes that increase the salinity of water include

evaporation and the formation of sea ice. Processes that decrease the salinity of water include precipitation, runoff from land, icebergs melting, and sea ice melting. Density changes due to salinity variations are important in very high latitudes, where water temperature remains low and relatively constant.

Density currents can also result from increased salinity of ocean water due to

evaporation. In the Mediterranean Sea, conditions exist that lead to the formation of a dense water mass at the surface that sinks and eventually flows into the Atlantic Ocean. Climate conditions in the eastern Mediterranean include a dry northwest wind and sunny days. These conditions lead to an annual excess of evaporation compared with the amount of precipitation. When seawater evaporates, salt is left behind, and the salinity of the remaining water increases.

The planet's next atmosphere is thought to have come from

gases emitted during volcanic eruptions. At the time, the Earth's crust was still forming, so there was a great deal of volcanic activity. Volcanoes emit a mixture of ammonia, methane, carbon dioxide, and steam. Over time, the Earth began to cool and water vapor in the atmosphere condensed to form the hydrosphere.

If you make waves by tossing a pebble into a pond, or by splashing in a pool, or by blowing across the surface of a cup of coffee, you are

giving energy to the water. The waves you see are just the visible evidence of the energy passing through the water. When observing ocean waves, remember that you are watching energy travel through a medium, in this case, water.

Upwelling brings

greater concentrations of dissolved nutrients, such as nitrates and phosphates, to the ocean surface. These nutrient-enriched waters from below promote the growth of microscopic plankton, which in turn support extensive populations of fish and other marine organisms. Upwelling is essential to the fishery industry.

Without these absorbing gases in our atmosphere, Earth would not be a suitable habitat for most types of living things found on Earth today. This phenomenon has been termed the

greenhouse effect because it was once thought that greenhouses were heated in a similar manner. (A more important factor in keeping a greenhouse warm is that the greenhouse itself prevents the mixing of air inside with cooler air outside.)

Huge circular-moving current systems dominate the surfaces of the oceans. These large whirls of water within an ocean basin are called

gyres (gyros = a circle).

Near Antarctica, surface conditions create the

highest density water in the world. This cold, salty water slowly sinks to the sea floor, where it moves throughout the ocean basins in slow currents. After sinking from the surface of the ocean, deep waters will not reappear at the surface for an average of 500 to 2000 years.

The composition of the atmosphere has changed dramatically over Earth's nearly 4.6 billion year history. It is likely that the Earth's earliest atmosphere was composed mainly of

hydrogen and helium. These were the primary gases found within the dusty disk from which the planets formed. Hydrogen and helium are very light gases, and they eventually escaped Earth's gravity and floated off into space.

Because the position of the moon changes only moderately in a single day, the tidal bulges remain in place while Earth rotates "through" them. For this reason:

if you stand on the seashore for 24 hours, Earth will rotate you through alternating areas of higher and lower water. As you are carried into each tidal bulge, the tide rises. As you are carried into the troughs between the tidal bulges, the tide falls. Most coastal locations experience two high tides and two low tides each day.

Different materials or substances have different properties. These properties can include things you may be familiar with, such as color, texture, hardness, porosity, and permeability. Other properties include

interactions of a material and energy. A particular material might absorb energy, transfer energy, or convert energy. The relationship between a material, such as land or water, and energy, can provide important information about temperature variations in different parts of the world.

Circulation by convection: Radiation from the fire warms the bottom of the pan, which conducts heat to the water near the bottom of the container. As the water is heated-

it expands and becomes less dense than the water above. The warmer water rises because of its buoyancy. At the same time, cooler, denser water near the top of the pan sinks to the bottom, where it becomes heated. As long as the water is heated unequally, it will continue to circulate. In much the same way, most of the heat acquired by radiation and conduction in the lowest layer of the atmosphere is transferred by convective flow.

Water heats more slowly than

land. Water also cools more slowly than land. This means that the temperature of a body of water varies less than an area of land.

In contrast, scattering produces a

larger number of weaker rays that travel in different directions. Scattering disperses waves both forward and backward. However, more energy is dispersed in the forward direction. Small dust particles and gas molecules in the atmosphere scatter some incoming radiation in all directions. This explains how light reaches into the area beneath a shade tree, and how a room is lit in the daytime through windows in the absence of direct sunlight. Scattering also accounts for the brightness and even the blue color of the daytime sky. About half of the solar radiation that is absorbed at Earth's surface arrives as scattered radiation.

Solar energy is not distributed evenly over Earth's surface. The amount of energy received varies with

latitude, time of day, and season. These variations in solar heating are caused by the motions of Earth relative to the sun and by variations in Earth's land and ocean surface.

Isotherms are

lines that connect points that have the same temperature.

Ocean currents also play a major role in

maintaining Earth's heat balance. They do this by transferring heat from the tropics, where there is an excess of heat, to the polar regions, where less heat exists. Ocean water movement accounts for about a quarter of this heat transport. Winds transport the remaining three-quarters.

Ocean currents are

masses of ocean water that flow from one place to another. The amount of water can be large or small. Ocean currents can be at the surface or deep below. The creation of these currents can be simple or complex. In all cases, however, the currents that are generated involve water masses in motion.

In the third layer, the

mesosphere, temperatures again decrease with height until the mesopause. The mesopause is more than 80 kilometers above the surface and the temperatures approach -90°C.

Surface currents are

movements of water that flow horizontally in the upper part of the ocean's surface. Surface currents develop from friction between the ocean and the wind that blows across its surface. Some of these currents do not last long, and they affect only small areas. Such water movements are responses to local or seasonal influences. Other surface currents are more permanent and extend over large portions of the oceans. These major horizontal movements of surface waters are closely related to the general circulation pattern of the atmosphere.

Shorelines are among Earth's most dynamic places. They change rapidly in response to

natural forces. Storms are capable of eroding beaches and cliffs at rates that far exceed the long-term average erosion. Such bursts of accelerated erosion not only affect the natural evolution of a coast but can also have a profound impact on people who reside in the coastal zone. Erosion along the coast causes significant property damage. Huge sums of money are spent annually not only to repair damage but also to prevent or control erosion.

Earth's current atmosphere is made up primarily of

nitrogen and oxygen. Today's atmosphere continues to exchange material with the oceans and life on the Earth's surface.

Waves can travel great distances across

ocean basins. In one study, waves generated near Antarctica were tracked as they traveled through the Pacific Ocean basin. After more than 10,000 kilometers, the waves finally expended their energy a week later along the shoreline of the Aleutian Islands of Alaska.

Movements of the atmosphere allow a large quantity of solid and liquid particles to be suspended within it. Although visible dust sometimes clouds the sky, these relatively large particles are too heavy to stay in the air for very long. Still, many particles are microscopic and remain suspended for longer periods of time. These particles include

sea salts from breaking waves, fine soil blown into the air, smoke and soot from fires, pollen and microorganisms lifted by the wind, and ash and dust from volcanic eruptions.

Reactions triggered by strong sunlight are called

photochemical reactions. For instance, when nitrogen oxides absorb solar radiation, a chain of complex reactions begins. If certain volatile organic compounds are present, secondary products form that are reactive, irritating, and toxic. This noxious mixture of gases and particles is called photochemical smog.

Groins, breakwaters, and seawalls are some structures built to

protect a coast from erosion or to prevent the movement of sand along a beach. Groins are sometimes constructed to maintain or widen beaches that are losing sand. A groin is a barrier built at a right angle to the beach to trap sand that is moving parallel to the shore.

Protective structures can also be built parallel to the shoreline. A breakwater is one such structure. Its purpose is to

protect boats from the force of large breaking waves by creating a quiet water zone near the shore. A seawall is another protective structure built parallel to the shore. A seawall is designed to shield the coast and defend property from the force of breaking waves. Waves expend much of their energy as they move across an open beach. Seawalls reduce this process by reflecting the force of unspent waves seaward.

The third mechanism of heat transfer is

radiation. As shown in the figure, radiation travels out in all directions from its source. Unlike conduction and convection, which need material to travel through, radiant energy can travel through the vacuum of space. Solar energy reaches Earth by radiation.

Headlands that extend into the sea are vigorously attacked by waves because of

refraction. The surf erodes the rock selectively and wears away the softer or more highly fractured rock at the fastest rate. At first, sea caves may form. When two caves on opposite sides of a headland unite, a sea arch like the one in the figure results. Eventually, the arch falls in, leaving an isolated remnant, or sea stack, on the wave-cut platform.

Tides are

regular changes in the elevation of the ocean surface. Their rhythmic rise and fall along coastlines has been noted throughout history. Other than waves, they are the easiest ocean movements to observe. But the cause of tides was not well understood until Sir Isaac Newton applied the law of universal gravitation to them. Newton showed that there is a mutually attractive force, gravity, between any two bodies, as between Earth and the moon.

Which change to Earth would eliminate seasons and seasonal weather?

removing the tilt of Earth's axis

As long as a wave is in deep water, it is unaffected by water depth. However, when a wave approaches the shore, the water becomes

shallower and influences wave behavior. The wave begins to "feel bottom" at a water depth equal to half of its wavelength. Such depths interfere with water movement at the base of the wave and slow its advance.

You now know the basic causes and types of tides. However, many factors—including

shape of the coastline, the configuration of ocean basins, and water depth—greatly influence the tides. Consequently, tides at various locations respond differently to the tide-producing forces. That being the case, the nature of the tide at any coastal location can be determined most accurately by actual observation. The predictions in tidal tables and tidal data on nautical charts are based on such observations.

Because of refraction, wave energy is concentrated against the

sides and ends of headlands that project into the water, whereas wave action is weakened in bays. This type of wave action along irregular coastlines is illustrated in the figure. Waves reach the shallow water in front of the headland sooner than they do in adjacent bays. Therefore, wave energy is concentrated in this area, leading to erosion. By contrast, refraction in the bays causes waves to spread out and expend less energy. This refraction leads to deposition of sediments and the formation of sandy beaches.

Observations of a floating object reveal that it moves not only up and down but also

slightly forward and backward with each successive wave. This movement results in a circle that returns the object to essentially the same place in the water. Circular orbital motion allows energy to move forward through the water while the individual water particles that transmit the wave move around in a circle.

Although the sun is farther away from Earth than the moon, the gravitational attraction between the sun and Earth does play a role in producing tides. The sun's influence produces:

smaller tidal bulges on Earth. These tidal bulges are the result of the same forces involved in the bulges created by the moon.

Regardless of the composition, the sediment that makes up the beach does not

stay in one place. The waves that crash along the shoreline are constantly moving it. Beaches can be thought of as material in transit along the shoreline. Earth's surface is constantly being reshaped as material is eroded from one place and deposited in another. Water, wind, ice, gravity, and volcanic lava all take part in changing the Earth's surface. Material along a shoreline is mainly eroded, transported, and deposited through the action of water and wind. Many types of shoreline features can result from this activity. During calm weather, wave action is minimal. During storms, however, waves are capable of causing much erosion.

As a wave advances toward the shore, the slightly faster waves farther out to sea catch up and decrease the wavelength. As the speed and length of the wave decrease, the wave steadily grows higher. Finally, a critical point is reached when the wave is too steep to support itself, and the wave front collapses, or breaks, causing water to advance up the shore. The turbulent water created by breaking waves is called

surf. On the landward margin of the surf zone, the turbulent sheet of water from collapsing breakers, called swash, moves up the slope of the beach. When the energy of the swash has been expended, the water flows back down the beach toward the surf zone as backwash.

Although wind is the force that generates surface currents, other factors also influence the movement of ocean waters. The most significant of these is

the Coriolis effect. The Coriolis effect is the deflection of currents away from their original course as a result of Earth's rotation. Because of Earth's rotation, currents are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. As a consequence, gyres flow in opposite directions in the two different hemispheres.

There are five main ocean gyres:

the North Pacific Gyre, the South Pacific Gyre, the North Atlantic Gyre, the South Atlantic Gyre, and the Indian Ocean Gyre.

Ozone is concentrated 10 to 50 kilometers above Earth because

the UV radiation from the sun is sufficient to produce single atoms of oxygen. In addition, there are enough gas molecules to bring about the required collisions. The ozone layer is crucial to life on Earth. Ozone absorbs potentially harmful UV radiation from the sun. If ozone did not filter most UV radiation and all of the sun's UV rays reached the surface of Earth, our planet would be uninhabitable for many living organisms.

Albedo is

the fraction of total radiation that is reflected by any surface. Many clouds have a high albedo, and therefore reflect a significant portion of the sunlight that strikes them back to space. The extent of cloud cover is a factor that influences temperatures in the lower atmosphere. By reducing the amount of incoming solar radiation, the maximum temperatures on a cloud-covered day will be lower than on a day when the clouds are absent and the sky is clear.

Beaches and shorelines are constantly undergoing changes as the force of waves and currents act on them. A beach is

the accumulation of sediment found along the shore of a lake or ocean. Beaches are composed of whatever sediment is locally available. They may be made of mineral particles from the erosion of beach cliffs or nearby coastal mountains. This sediment may be relatively coarse in texture. Some beaches have a significant biological component. For example, most beaches in southern Florida are composed of shell fragments and the remains of organisms that live in coastal waters.

Beach nourishment is

the addition of large quantities of sand to the beach system. It is an attempt to stabilize shoreline sands without building protective structures. By building the beaches seaward, both beach quality and storm protection are improved. However, the same processes that removed the sand in the first place will eventually wash away the replacement sand as well. Beach nourishment can be very expensive because huge volumes of sand must be transported to the beach from offshore areas, nearby rivers, or other source areas for sand. Beach nourishment can also have detrimental effects on local marine life. For example, beach nourishment at Waikiki Beach, Hawaii, involved replacing the natural coarse beach sand with softer, muddier sand. Destruction of the softer sand by breaking waves increased the water's turbidity, or "cloudiness," and killed offshore coral reefs.

Nearly all of the energy that drives Earth's variable weather and climate comes from the sun. Earth absorbs only a tiny percentage of the energy given off by the sun—less than one two-billionth. This may seem insignificant, but

the amount is several hundred thousand times the electrical-generating capacity of the United States.

The amount of incoming solar radiation, called insolation, that reaches the Earth's surface differs from place to place. Consequently, different locations have different climates. The two main factors that determine how much solar energy, and therefore how much heat, a location receives are

the angle of the sun and the duration of daylight.The angle at which sunlight strikes the Earth's surface at a particular location depends on the location's latitude, the time of day, and the season. Because of the curvature of the Earth, the sun is at a lower angle in the sky and solar rays must travel though a larger area of atmosphere to reach the surface at the Earth's poles. The equator, on the other hand, receives more direct sunlight. The tilt of the Earth on its axis is the key to the seasons and duration of daylight. During the summer, when the Northern Hemisphere is tilted toward the sun, sunlight strikes the surface at a more direct angle and this hemisphere experiences longer days. As a result, the Northern Hemisphere receives more solar heating. At the same time, the Southern Hemisphere is tilted away from the sun, so sunlight strikes it at a more oblique angle and the days are shorter. Thus, when it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere.

Differences in the amount of incoming solar radiation play a major role in weather and climate across the globe. Although incoming solar radiation is absorbed, reflected, or re-radiated differently from place to place, there is a balance of energy transfer into and out of Earth's atmosphere. As a result,

the atmosphere's average temperature tends to remain constant from year to year. But the atmosphere's average temperature can and does change. It changes in response to factors that disturb its energy balance.

The equinoxes occur midway between the solstices. September 22 or 23 is the date of

the autumnal equinox and the start of autumn in the Northern Hemisphere. March 21 or 22 is the date of the spring equinox and the start of spring for the Northern Hemisphere. On these dates, the vertical rays of the sun strike the equator (0 degrees latitude) because Earth is in a position in its orbit such that the axis is tilted neither toward nor away from the sun.

Wave refraction is

the bending of waves, and it plays an important part in shoreline processes. Wave refraction affects the distribution of energy along the shore. It strongly influences where and to what degree erosion, sediment transport, and deposition will take place.

About 3.4 billion years ago, the first photosynthetic bacteria appeared and thrived in

the carbon dioxide-filled atmosphere. Oxygen, a byproduct of photosynthesis, began to accumulate in the atmosphere about 2.5 billion years ago. In the meantime, solar energy broke down ammonia molecules in the atmosphere and released nitrogen and hydrogen. The hydrogen, being the lightest element, floated to the top of the atmosphere and, eventually, out into space.

Some incoming solar radiation is not absorbed and reradiated. Instead, it is absorbed by

the chlorophyll in green plants. Plants use the energy from this radiation in photosynthesis. Thus solar energy is the main energy source for virtually all life on Earth.

Wave-cut cliffs result from

the cutting action of the surf against the base of coastal land. As erosion progresses, rocks that overhang the notch at the base of the cliff crumble into the surf, and the cliff retreats. A relatively flat, benchlike surface called a wave-cut platform, is left behind by the receding cliff. The platform broadens as the wave attack continues. Some debris produced by the breaking waves remains along the water's edge as sediment on the beach. The rest of the sediment is transported farther seaward.

Conversely, at about the time of the first and third quarters of the moon, the gravitational forces of the moon and sun act on Earth at right angles. The sun and moon partially offset the influence of the other. As a result,

the daily tidal range is less. These tides are called neap tides.

The combined gravity of these two tide-producing bodies causes larger tidal bulges (higher high tides) and larger tidal troughs (lower low tides). This combined gravity produces a large tidal range. The tidal range is:

the difference in height between successive high and low tides.

Longshore currents can change direction because

the direction that waves approach the beach changes with the seasons. Nevertheless, longshore currents generally flow southward along both the Atlantic and Pacific shores of the United States.

The strength of the force of gravity between two objects decreases as

the distance between the objects increases. At any given time, different areas of Earth's surface are different distances from the moon. The pull of the moon's gravity is greater at parts of Earth's surface that are closer to the moon and less at more distant locations. The relationship between gravity and distance is inverse, meaning that the greater the distance between two objects the smaller the force of gravity between them.

The impact of large, high-energy waves against the shore can be awesome in its violence. Each breaking wave may hurl thousands of tons of water against the land, sometimes causing

the ground to tremble. It is no wonder that cracks and crevices are quickly opened in cliffs, coastal structures, and anything else that is subjected to these enormous impacts. Water is forced into every opening, causing air in the cracks to become highly compressed by the thrust of crashing waves. When the wave subsides, the air expands rapidly. This expanding air dislodges rock fragments and enlarges and extends preexisting fractures.

The atmosphere efficiently absorbs the longer wavelengths emitted by Earth. Water vapor and carbon dioxide are

the major absorbing gases. When a gas molecule absorbs these waves, this energy is transformed into molecular motion that can be detected as a rise in temperature. Gases in the atmosphere eventually radiate some of this energy away. Some energy travels skyward, where it may be reabsorbed by other gas molecules. The rest travels earthward and is again absorbed. In this way, Earth's surface is continually supplied with heat from the atmosphere as well as from the sun

The primary body that influences the tides is

the moon, which makes one complete revolution around Earth every 29 and a half days. The sun, however, also influences the tides. It is far larger than the moon, but because it is much farther away, its effect is considerably less. In fact, the sun's tide-generating effect is only about 46 percent that of the moon's.

Protective structures often only offer temporary solutions to shoreline problems. The structures themselves interfere with

the natural processes of erosion and deposition. Then more structures often need to be built to counteract the new problems that arise. Many scientists feel that using protective structures to divert the ocean's energy causes more harm than good.

At a windward coast, the prevailing winds blow from

the ocean to the land. These winds help to moderate the climate, meaning they bring cooler weather in summer and warmer weather in winter.

Visible light is

the only portion of the spectrum you can see. White light is really a mixture of colors. Each color corresponds to a specific wavelength. By using a prism, white light can be divided into the colors of the rainbow, from violet with the shortest wavelength—400 nanometers (1 nanometer is 1.0 x 10-7 centimeters) to red with the longest wavelength—700 nanometers.

In addition to producing surface currents, winds can also cause vertical water movements. Upwelling is

the rising of cold water from deeper layers to replace warmer surface water. Upwelling is a common wind-induced vertical movement. One type of upwelling, called coastal upwelling, is most characteristic along the west coasts of continents, most notably along California, western South America, and West Africa. Coastal upwelling occurs in these areas when winds blow toward the equator and parallel to the coast. Coastal winds combined with the Coriolis effect cause surface water to move away from shore. As the surface layer moves away from the coast, it is replaced by water that "upwells" from below the surface. This slow, upward movement of water from depths of 50 to 300 meters brings water that is cooler than the original surface water and results in lower surface water temperatures near the shore.

In addition to the erosion caused by wave impact and pressure, erosion caused by abrasion is also important. In fact, abrasion is probably more intense in the surf zone than in any other environment. Abrasion is

the sawing and grinding action of rock fragments in the water. Smooth, rounded stones and pebbles along the shore are evidence of the continual grinding action of rock against rock in the surf zone. Such fragments are also used as "tools" by the waves as they cut horizontally into the land, like the sandstone shown in the figure. Waves are also very effective at breaking down rock material and supplying sand to beaches.

Sea levels change. On a day-to-day basis, the levels of the oceans rise and fall due to tides. However, they can also change on a much longer time scale. Global sea level change takes place because of two reasons:

the shape of the ocean basins change or the volume of water does. Over time, tectonic movements can change the shape of ocean basins. If tectonic movements bring continents closer together, the ocean basins get smaller and sea levels rise. If tectonic movements push continents away from each other and make the ocean basins larger, sea levels fall. This process takes millions of years because tectonic plates move very slowly. Sea levels can also change on a much shorter time scale if the climate changes. At the beginning of an ice age, for example, sea levels drop as water is taken out of the hydrological cycle and frozen in glaciers and polar ice caps. Conversely, at the end of an ice age, sea levels rise as polar ice caps and glaciers melt and water is returned to the cycle.

Waves seldom approach the shore straight on. Rather, most waves move toward

the shore at a slight angle. However, when they reach the shallow water of a smoothly sloping bottom, the wave crests are refracted, or bent, and tend to line up nearly parallel to the shore. Such bending occurs because the part of the wave nearest the shore touches bottom and slows first, whereas the part of the wave that is still in deep water continues forward at its full speed. The change in speed causes wave crests to become nearly parallel to the shore regardless of their original orientation.

Weather is constantly changing, and it refers to

the state of the atmosphere at any given time and place. Climate, however, is based on observations of weather that have been collected over many years. Climate helps describe a place or region.

Beyond the tropopause is

the stratosphere. In the stratosphere, the temperature remains constant to a height of about 20 kilometers. It then begins a gradual increase in temperature that continues until the stratopause, at a height of nearly 50 kilometers above Earth's surface. Temperatures increase in the stratosphere because the atmosphere's ozone is concentrated here. Ozone absorbs ultraviolet radiation from the sun. As a result, the stratosphere is heated.

Convection is

the transfer of heat by mass movement or circulation within a substance. It takes place in fluids, like the ocean and air, where the atoms and molecules are free to move about. Convection also takes place in solids, such as Earth's mantle, that behave like fluids over long periods of time.

Conduction is

the transfer of heat through matter by molecular activity. The energy of molecules is transferred by collisions from one molecule to another. Heat flows from the higher temperature matter to the lower temperature matter.

The atmosphere can be divided vertically into four layers based on temperature. The figure illustrates these layers. The bottom layer, where temperature decreases with an increase in altitude, is

the troposphere. It is in this layer that essentially all important weather phenomena occur. The thickness of the troposphere is not the same everywhere. It varies with latitude and the season. On average, the temperature drop continues to a height of about 12 kilometers, where the outer boundary of the troposphere, called the tropopause, is located.

Shoreline features vary depending on

the type of rocks exposed along the shore, the intensity of waves, the nature of coastal currents, and whether the coast is stable, sinking, or rising.

The water itself does not travel the entire distance, but

the wave does. As a wave travels, the water particles pass the energy along by moving in a circle.

Atmospheric pressure is caused by

the weight of the air above. At sea level, the average pressure is slightly more than 1,000 millibars (mb), or slightly more than 1 kilogram per square centimeter. One half of the atmosphere by mass lies below an altitude of 5.6 kilometers. Above 100 kilometers, only 0.00003 percent of all the gases making up the atmosphere exist.

Most ocean waves obtain their energy and motion from

the wind. When a breeze is less than 3 kilometers per hour, only small waves appear. At greater wind speeds, more stable waves gradually form and advance with the wind.

Electromagnetic waves are classified by

their wavelength, or the distance from one crest to the next. Radio waves, like the AM waves shown in the figure, have the longest wavelengths, ranging to tens of kilometers. Gamma waves are the shortest—less than a billionth of a centimeter long.

The fourth layer extends outward from the mesopause and has no well-defined upper limit. It is the

thermosphere, a layer that contains only a tiny fraction of the atmosphere's mass. Temperatures increase in the thermosphere because oxygen and nitrogen absorb short-wave, high-energy solar radiation.

Spring tides are

tides that have the greatest tidal range due to the alignment of the Earth-moon-sun system. They are experienced during new and full moons.

After this water sinks, it is removed from the physical processes that increased its density in the first place. Its temperature and salinity remain largely unchanged during the time it is in the deep ocean. Because of this, oceanographers can

track the movements of density currents in the deep ocean. By knowing the temperature, salinity, and density of a water mass, scientists are able to map the slow circulation of the water mass through the ocean.

Ocean currents have an important effect on climates. When currents from low-latitude regions move into higher latitudes, they

transfer heat from warmer to cooler areas on Earth.

Each month there are

two spring tides and two neap tides, each about one week apart

The changing orientation of Earth relative to the sun causes the sun's apparent path to

vary with latitude and season. The angle of the noon sun can vary by up to 47 degrees (-23.5 degrees to +23.5 degrees) for many locations during the year. A mid-latitude city like New York, located about 40 degrees north latitude, has a maximum noon sun angle of 73.5 degrees when the sun's vertical rays reach their farthest northward location in June. Six months later, New York has a minimum noon sun angle of 26.5 degrees.

Density currents are

vertical currents of ocean water that result from density differences among water masses. Denser water sinks and slowly spreads out beneath the surface. An increase in seawater density can be caused by a decrease in temperature or an increase in salinity.

On the side of Earth farthest from the moon, the pull of the moon's gravity on the oceans is weaker than it is on the solid Earth. As a result:

water flows toward that area and a second, equally large tidal bulge forms on the side of Earth directly opposite the moon.

Important materials that vary in the air from time to time and place to place include

water vapor, dust particles, and ozone. These components also can have significant effects on weather and climate. The amount of water vapor varies from almost none to about 4 percent by volume. Water vapor is the source of all clouds and precipitation. Like carbon dioxide, water vapor absorbs heat given off by Earth. It also absorbs some solar energy.

Different land surfaces absorb varying amounts of incoming solar energy. The largest contrast, however, is between land and water. Land heats more rapidly and to higher temperatures than

water. Land also cools more rapidly and to lower temperatures than water. Temperature variations, therefore, are considerably greater over land than over water.

Sea level changes can produce many shoreline features. When sea levels fall, erosional shoreline features, such as

wave-cut cliffs, arches, and stacks, may be found in areas that are no longer anywhere near an ocean. Rias, fjords, and Dalmatian coastlines are features that form when sea levels rise. A ria is basically a flooded river valley. Rias have similar features to ordinary river valleys, but they have higher water levels. A fjord is a feature formed when a glacier retreats and the sea fills the deep, narrow valley carved by the glacier. Dalmatian coastlines are made when parallel valleys fill with seawater but the tops of the valleys remain above water. The result appears to be a series of islands that run parallel to the coast.

The state of the atmosphere at a given time and place is known as

weather. The combination of Earth's motions and energy from the sun produce a variety of weather

It is the unequal heating of Earth that creates

winds and drives the ocean's currents. These movements transport heat from the tropics toward the poles, thus driving the phenomena we call weather.

Six months later, in December, when Earth has moved to the opposite side of its orbit, the Northern Hemisphere leans 23.5 degrees away from the sun. December 21 or 22 is the

winter solstice, or the first day of winter. On days between these extremes, Earth's axis is leaning at amounts less than 23.5 degrees to the rays of the sun.