Unit 14

Clouds can form in one of three ways:

1. Warm, moist air rises, expands and cools in a convection current. As the air reaches its dew point, the water vapor in the air condenses, forming a cloud. AIR RISES, COOLS, AND CONDENSES. 2. Orographic (Oro means mountain), lifting occurs when wind encounters a mountain and the air has no place else to go but up. It cools and hits its dew point, condensing into a cloud. 3. As warm air moves into an area, it is forced above a more-dense, cooler air mass. The warm air cools as it contacts the colder air, causing the water vapor to condense.

The gases making up the atmosphere include:

78% nitrogen 21% oxygen 0.93% argon

Dry Adiabatic Lapse Rate

A dry (unsaturated) air mass will cool at a rate of about 10°C per kilometer. If the air continues to rise, it will eventually cool to its condensation temperature UNTIL IT REACHES THE DEW POINT.

Ozone

A form of oxygen containing 3 oxygen atoms. It is usually found in the stratosphere, and is responsible for filtering out much of the sun's ultraviolet radiation. Without the protection ozone provides, our fragile skin could not tolerate exposure to the Sun for long. The atmosphere is made up of several different layers, each with its own composition and temperature.

Temperature

A measurement of how fast or slow molecules move around. If there are more molecules or faster-moving molecules in a given space, a higher temperature will be generated. The more movement, the higher the temperature

Moist Adiabatic Lapse Rate

A moist (saturated) air mass will cool at a rate of about 5°C per kilometer. Just like all matter, air has mass and therefore it exerts pressure on our bodies. The Earth's gravity attracts this mass, so that the particles of gas are pulled towards the Earth's center. Therefore, as we saw earlier, as height increases, air molecules become less dense, resulting in less pressure. Occasionally, temperature and pressure with a particular layer of the atmosphere do not behave as expected. This is referred to as an inversion.

Heat

A transfer of energy that occurs because of a difference in temperature between substances. Heat will always flow from an object of higher temperature to an object of lower temperature.

Wind

Air moving (sometimes with considerable force) from an area of high pressure to an area of low pressure.

Coalescence

All forms of water, both liquid and solid that falls from clouds.

Moisture in the atmosphere

Almost all air contains some amount of water vapor. Under the right conditions, this can lead to cloud formation. Clouds, therefore, are one of the most important indicators of weather.

Adiabatic

An air mass that does not exchange heat with its surroundings

Inversion

An unusual increase in temperature with height in the atmosphere. Inversions can occur on cold, clear winter nights, as air near the Earth's surface loses heat and cools. This cool air sometimes traps polluted air below a layer of heated air above it. In this case, temperature will increase with height. Because cooler air is denser, it tends to sink below warmer, less dense air. In the lower atmosphere, air generally moves from areas of high density to areas of low density. These movements are in response to imbalances in created by the unequal heating and cooling of Earth's surface. These areas of high and low density translate into areas of high and low pressure and result in wind.

Stratus

Describes featureless sheets of clouds.

Nimbus

Describes low, gray rain clouds. They're dark because they block the sunlight.

Cumulus

Describes puffy, lumpy-looking clouds. (Think of cotton balls.)

Cirrus

Describes wispy, stringy clouds. (Mostly ice crystals.)

Radiation

Emission or transfer of energy in the form of electromagnetic waves or particles. Means to give off. While Earth absorbs solar radiation, it is also continuously sending energy back into space. As much as 35% of the Sun's radiation is reflected back into space by the Earth's surface, the atmosphere or clouds. The rate that the remaining energy is absorbed by the Earth depends on the characteristics of that area. For example, water heats up and cools down slower than land. Also darker objects absorb energy faster than lighter ones.

Convection

Heat transfer by currents that flow from a warm area to a colder one. Pockets of air near the Earth's surface are heated, become less dense than the surrounding air, and rises. As the warm air rises, it expands and starts to cool. When it cools below the temperature of the surrounding air it increases in density and sinks, where the process begins again. These convection currents are responsible for the different types of weather we see.

Water vapor in the atmosphere

In addition, water vapor can comprise from 0 - 4% of the atmosphere. The amount varies with the season, with the altitude of that particular mass of air, and the surface features beneath that air. For instance, air over deserts is drier than air over oceans. The levels of both carbon dioxide and water vapor are critical because they play an important role in regulating the amount of energy that is absorbed by the atmosphere. Water is unique because it is the only substance in the atmosphere that exists in all three states: solid, liquid and gas. This is important because when water changes from one state to another, heat is either absorbed or released. It is this exchange of heat that affects atmospheric motions, and drives weather. Weather- Earth trying to balance heat.

Troposphere

The lowest layer of the atmosphere containing about 95% of the mass of air in the Earth's atmosphere. The troposphere extends from the Earth's surface up to about 10 to 15 km. All weather processes take place in the troposphere, and temperature decreases with height. The troposphere ends at the tropopause.

Cirro-

Prefix referring to high clouds with bases starting above 6 kilometers. (Ice crystals, not water.)

Strato-

Prefix referring to low clouds below 2 kilometers.

Alto-

Prefix referring to middle clouds with bases from 2-6 kilometers.

The four main types of precipitation are:

RAIN SNOW SLEET - rain that freezes on the way down, only in winter. HAIL - occurs at both summer and winter.

Mesosphere

Region of the atmosphere between approximately 50 and 85 km. There is no concentration of ozone here, so temperature again decreases with height. The mesosphere ends at the mesopause.

What is the difference between temperature and heat?

So—heat is the transfer of energy that fuels atmospheric processes, while temperature is used to measure and interpret that energy.

Relative Humidity

The amount of water vapor in the air, compared to the amount the air could hold if it was totally saturated. (Expressed as a percentage) Relative humidity is always expressed as a percentage, and it varies with temperature. In other words, warm air is capable of holding more moisture than cool air. If a certain volume of air is holding as much water vapor as it possibly can (saturated), then its relative humidity is 100%; if it is holding half as much water vapor as it can, its relative humidity is 50%. Relative humidity has important implications for the development of clouds and precipitation.

Solids in the atmosphere

The atmosphere also contains small amounts of solid in the form of tiny particles of dust (picked up by wind) and salt (from ocean spray). Ice is also found in the atmosphere in the form of hail or snow.

Humidity

The concentration of water vapor in the air. (AMOUNT) Air in the lower part of the atmosphere usually contains at least some water vapor. This is referred to as humidity. The air can usually contain more water than it is actually holding at a given time.

Saturation

The condition of the atmosphere when the amount of water vapor present is the maximum possible at the existing temperature. Remember that the air temperature on a mountaintop is cooler than air at lower elevations because the mountaintop is further away from the heat source (Earth's surface).

Atmosphere

The envelope of air surrounding the Earth (or other celestial body, planet-like) Air is a combination of many gases, each with its own characteristics. Together, these gases form Earth's atmosphere, which extends from Earth's surface to outer space.

Lifted Condensation Level

The height at which condensation occurs. Since clouds form when water vapor condenses into water droplets, the height of the LCL often corresponds to the base of the clouds. Above the LCL, the air becomes saturated and cools more slowly. Sometimes they call it wet.

Thermosphere

The outermost layer of the atmosphere, where gas molecules split apart into ions (ionosphere). Temperature increases with height (to more than 1000°C).

Stratosphere

The portion of the atmosphere ranging from approximately 14 km to 22 km. The Earth's ozone is concentrated here, trapping heat, so temperature increases with height. The stratosphere ends at the stratopause.

Condensation

The process in which water vapor changes into liquid water. Condensation can only occur if the atmosphere is saturated.

Dew Point

The temperature to which air with a given quantity of water vapor must be cooled to cause condensation of the vapor in the air. WATER VAPOR IS INVISIBLE. Temperature that the air has to cool to condense.

Conduction

The transfer of heat energy through a substance or from one substance to another by direct contact of atoms or molecules. Through conduction, energy is transferred from the particles of air near Earth's surface to the particles of air in the lowest layer of the atmosphere. For conduction to occur, substances must be in contact with one another. (So, why does conduction only occur at lower levels of the atmosphere?) More atoms at lower levels because of gravity.

Exosphere

This region is considered the very outer limits of the earth's atmosphere. It is estimated to be some 640 km above the surface, where atmospheric gases can escape into outer space.