Chapter 6: Physical and Earth/Space Sciences // Core Content: Hydrologic and Atmospheric Systems

Wind

Air moves from areas where the air pressure is relatively high toward areas where the air pressure is relatively low. This movement is called wind. Measuring air pressure as well as wind direction and wind speed assists forecasters in predicting weather changes. For example, fronts are the boundaries between two air masses. Winds usually converge, or come together, at the fronts and temperature differences can be quite noticeable from one side of the front to another. The pressure on either side of a front can vary significantly. Fronts are classified according to which type of air mass (cold or warm) is replacing another.

Transpiration

Plants also absorb water and release it into the atmosphere through transpiration. Transpiration is especially important in thickly forested areas, such as tropical forests.

Precipitation

Precipitation—rain and snow—then falls to Earth. Precipitation that falls into oceans has completed one full cycle and is ready to begin another. However, water that falls on land must make its way back to the ocean to complete the full cycle.

Condensation

Winds transport this air containing water vapor until conditions cause the vapor to condense into clouds and fall as rain or snow, far from where it evaporated.

Air pressure

Air pressure is the force exerted by the weight of a column of air above a particular location. Air pressure always decreases with increasing altitude because as you move higher there is less and less air above you. Air pressure is, on average, highest at sea level. Air pressure also changes from place to place across Earth's surface. Part of this change is due to differences in land elevation. Most of the remainder is caused by changes in air temperature. Cold air is relatively dense—it has more air molecules per unit volume and so it exerts relatively high pressure. Warm air is less dense and exerts relatively low pressure. Generally, the weather stays fair or improves if air pressure rises. If the air pressure falls steadily, however, the weather may turn cloudy and rainy or snowy.

Temperature

Air temperature generally varies from day to night and from season to season because of changes in the amount of radiation heating Earth's atmosphere. For example, days usually are warmer than nights because Earth receives the heating rays of the sun only during the day. At night, infrared radiation from the planet streams off into space and the air temperature drops.

Glaciers

Another important hydrologic subsystem is glaciers. A glacier is a thick ice mass that moves slowly over the land surface. Glaciers originate on land in places where more snow falls each winter than melts each summer. There are two kinds of glaciers: alpine glaciers and continental glaciers. Alpine glaciers form above the snowline at the crests or slopes of mountains or in mountain valleys. Continental glaciers are enormous ice masses that flow in all directions and cover everything but the highest land. These very thick sheets of ice covered much of North America during the recent ice age. Two ice sheets remain; combined, they cover almost 10 percent of Earth's land area. One ice sheet covers about 80 percent of Greenland; the other is the huge Antarctic ice sheet. This glacier accounts for 80 percent of the world's ice. Glaciers scour, pluck, and abrade the landscape as they move. Glaciers transport rocks, sand, and gravel as they move across the land, depositing these as various landforms when the glacier melts.

Earth's Atmosphere

Earth is surrounded by a blanket of air, which we call the atmosphere. The composition of air is a complex mix of various chemical components including 78 percent nitrogen and nearly 21 percent oxygen. Trace elements—carbon dioxide, methane, hydrogen, argon, and helium—make up the other 1 percent of the earth's atmosphere. The atmosphere can be divided vertically into four layers based on temperature: the troposphere, the stratosphere, the mesosphere, and the thermosphere. The troposphere is the layer of air closest to the surface of Earth where all weather phenomena occur. It begins at ground level and extends 12km (7.5 miles) up into the sky. The stratosphere begins at the 12km (7.5 mile) point and reaches 50km (21.1 miles) into the sky. This important layer of atmosphere contains ozone, a special form of oxygen. The ozone layer is very important to all life on Earth, as it blocks large amounts of harmful solar ultraviolet radiation from entering the troposphere. The mesosphere begins 50km (21.1 miles) above Earth's surface. Temperatures are warmest at the lowest level of the mesosphere and coldest at its highest level. The thermosphere begins 80km (49.7 miles) above Earth. Temperatures increase in the thermosphere because oxygen and nitrogen absorb short-wave, high-energy solar radiation. The troposphere, stratosphere, mesosphere, and thermosphere act together as a giant safety blanket. They keep the temperature on Earth's surface from dipping to extreme cold that would freeze everything solid and from soaring to blazing heat that would burn up all life. See Figure 6.21 below.

Earth's Oceans

Earth's oceans are, in reality, one huge ocean. This ocean stretches from the North Pole to the South Pole and encircles the globe. However, because the continents loosely divide this ocean, it can be separated into four main ocean basins—the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, and the Arctic Ocean. Beneath the world's oceans lie rugged mountains, active volcanoes, vast plateaus, and very deep trenches. The ocean's waters are constantly being moved about by powerful currents. Ocean currents are masses of ocean water that flow from one place to another. Solar energy plays a major role in ocean circulation and in maintaining Earth's heat balance. Ocean currents and winds transfer heat from the tropics, where there is an excess of heat, toward polar regions, where less heat exists. Ocean currents can be at the surface or deep below. In some areas, density, salinity, and temperature differences create deep-ocean circulation. This circulation is important for ocean mixing and nutrient recycling. 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 the ocean's surface. Some of these currents do not last long, and 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.

The Water Cycle

Earth's water cycle is balanced, meaning that the average annual precipitation over Earth equals the amount of water that evaporates. There are local imbalances, however. For example, precipitation exceeds evaporation over continents. Over oceans, evaporation exceeds precipitation. See Figure 6.20 below.

Weather

Four interrelated factors influence weather and weather patterns: temperature, humidity, air pressure, and wind.

Climate Changes

Natural processes have certainly contributed to climatic changes throughout Earth's 4.6 billion year history. To change climate on a global scale, either the amount of heat that comes into the atmosphere changes, or the amount of heat that leaves the atmosphere changes. The heat that enters into Earth's systems comes from the sun. The warmed Earth then radiates heat away. However, gases in our lower atmosphere, such as water vapor, carbon dioxide, methane, and nitrous oxide (called greenhouse gases), tend to absorb this radiation, warming the atmosphere. This is known as the greenhouse effect. If it were not for naturally occurring greenhouse gases, the average temperature of Earth would be much colder. Without the greenhouse effect, Earth would be much too cold to support most types of life that presently inhabit it. But, an increase in the greenhouse effect could also make dramatic climate changes. The National Academy of Sciences has determined that Earth's surface temperature has risen almost 1°C in the past century, with accelerated warming during the past two decades. Scientists have evidence that human activities have altered the chemical composition of the atmosphere through the buildup of greenhouse gases—primarily carbon dioxide, methane, and nitrous oxide. Levels of these greenhouse gases have increased by about 40 percent since large-scale industrialization began around 150 years ago. During the past 20 years, about three-quarters of human-caused (anthropogenic) emissions have come from burning fossil fuels. As a result of increase in carbon dioxide levels, as well as other greenhouse gases, global temperatures have increased. This increase in global temperatures is called global warming. Scientists predict that without better management of fossil fuel burning, global temperatures could increase by more than another 5°C by the year 2100.

Climate

The difference between weather and climate has to do with time. Weather has to do with the conditions of the atmosphere over a short period of time, while climate describes the averages of weather conditions in one location over a longer period of time. Climates differ in various regions of the world because each region receives different amounts of sunlight and has geographical differences, such as proximity to oceans and the presence of mountains. Climatic zones can be classified according to average temperatures and the average amount of precipitation that falls. Examples of climatic zones might include tropical, arid, or polar climatic zones.

Evaporation

The ocean plays a key role in this part of the water cycle. Water evaporates into the atmosphere from the surface of the ocean, mostly in warm, cloud-free subtropical seas, and to a lesser extent from lakes and streams.

Humidity

The percentage of moisture in the air compared to the amount of moisture the air can hold at a particular temperature is known as relative humidity. Relative humidity depends on the temperature of the air, as warm air can hold more moisture than cold air. A relative humidity of 100 percent indicates that the air is holding all the water it can at the current temperature and any additional moisture at that point will result in condensation. After water vapor condenses, forming ice crystals and water droplets, it can take on a variety of forms as it falls to Earth as precipitation. Precipitation in liquid form includes rain. Rain develops when growing cloud droplets become too heavy to remain in the cloud and, as a result, fall toward the surface.

The Water (Hydrologic) Cycle

The water cycle, shown in Figure 6.20 below, is a gigantic worldwide system powered by energy from the sun and by gravity. This cycle is possible because water readily changes from one state of matter—solid, liquid, or gas—to another at temperatures and pressures common on Earth's surface. Evaporation, precipitation, freezing and melting, condensation, and infiltration are all part of the hydrologic cycle—a never-ending global process of water circulation from clouds, to land, to the ocean, and back to the clouds.

Infiltration, runoff, and glaciation

When precipitation falls on land, some of it slowly soaks into the ground through infiltration. Infiltration is the movement of surface water into rock or soil through cracks and pore spaces to form groundwater. When the rate of rainfall exceeds Earth's ability to absorb it, the excess water flows over the surface into lakes and streams in a process called runoff. When precipitation falls in very cold areas at high elevations or high latitudes the water may become part of a glacier. Glaciers store large amounts of water on land. If present-day glaciers were to melt and release all their water, ocean levels would rise by several dozen meters.