MOISTURE

MOISTURE, CLOUDS and PRECIPITATION

Meteorology - study of the atmosphere

Weather - study of short-term changes in the atmosphere.
Climate - the combination of weather elements that characterize an area over a long term. Takes into account seasonal changes as well as short term extreme fluctuations.

Composition of the Atmosphere
Gas	Volume %
Nitrogen	78
Oxygen	21
Argon	1
All others	<1
Locally	up to 3% water vapor

Structure of the Atmosphere (Figure)

Troposphere - lowest layer of the atmosphere where weather occurs. Temperature falls 3.5°F per 1000 feet. Top lies at about 6-7 miles. Temperature varies from an average of 60° at the surface to -60° at the top of the troposphere. Tropopause - top of the troposhere.

Stratosphere - top of the tropopause to about 30 miles. Temperature is constant to about 20 miles then increases to the stratopause. At the stratopause the temperature is about 40°F. The temperature rise is due to the presence of an ozone layer. The ozone absorbs incoming UV radiation increasing the heat content of the gas. At about 18 miles the ozone reaches its maximum concentration, about 5ppm. Without the ozone layer the Earth's surface would be warmer and the UV level much higher.

Mesosphere - extends from 30 miles to about 50 miles. Temperature falls to -120°F at the mesopause.

Thermosphere - extends above the mesosphere more than 100 miles. Zone of increasing temperature, but this is meaningless since there is in effect no atmosphere present.

Heat

Heat is a form of energy. Temperature is a measure of the quantity of heat energy. Actually each is a function of molecular motion, the faster a molecule is vibrating/moving the greater the heat energy and the higher the temperature.

Temperature is measured with a thermometer. The most common, the alcohol thermometer, measures the differential expansion of red alcohol and the glass that encloses it.

All weather is a function of heat transfer. There are three forms of heat transfer.

Conduction - heat travels through material by molecular collisions. The analogy is a hot spoon.
Convection - heat transfer by actual motion. For example (Figure) heated water is less dense so it rises and the colder water at the surface sinks to take its place

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Radiation - transfer of energy by electromagnetic waves. The radiant heat of the sun passes through the vacuum of space and is absorbed by the Earth.

Heat Balance

Sun emits energy in the wavelength from 0.5 m m to 10 m m. This includes some ultraviolet and infrared energy as well as visible light.

Ozone and oxygen absorb most of the ultraviolet. Water vapor and carbon dioxide absorb infrared.

70% of the Sun's radiant energy penetrates the Earth's atmosphere (Figure). 30% is reflected by the atmosphere- albedo.

Greenhouse Cycle

Greenhouse Effect - (Figure) Earth like the Sun radiates energy, but since it is a smaller body it radiates in the infrared. Carbon dioxide in the atmosphere absorbs significant quantities (Figure) of this energy and reradiates it to Earth. It is estimated that the temperature on Earth is 63°F warmer than it would be if there were no greenhouse cycle.

Sun's Illumination

Effected by the inclination of the Earth's axis, which in turn causes the seasons. Earth's geographic poles are tilted 23.5° from the vertical. This dramatically influences the amount of radiant energy striking various points on the Earth's surface.

The days when the axis is tilted exactly 23.5° toward the Sun are termed the solstices:

June 21 - summer solstice
December 22 - winter solstice

Equinox - days when the Earth's axis is at right angle to a line between the center of the Earth and Sun.

March 21 - vernal equinox
September 23 - autumn equinox (Figure)

Tropic of Cancer and Capricorn - point on the Earth's surface where rays from the Sun strike the Earth at a perpendicular angle at noon on the solstice. Located 23.5° north or south of the equator.

Arctic Circle - boundary marking the part of the Earth which remains in daylight or darkness for 24 hours during the solstice. Located 66.5° north or south of the equator.

Obviously, the tropics receive more thermal energy than the poles. Weather and seasons result from the tendency of the Earth to disperse this uneven heat distribution.

Moisture in the Atmosphere

Humidity - measure of the amount of water vapor in the atmosphere:

Absolute humidity - the mass of water vapor in a given volume of air (g/m³).
Relative humidity - the amount of water vapor in air relative to the maximum it can hold at a given temperature. Expressed as a percentage. Relative humidity is temperature dependent, the higher the temperature the air, the more moisture it can hold.

Dew Point - point at which the relative humidity = 100%. At the dew point theoretically condensation will occur. In reality, unless dust or other particulate matter is present the air must be supercooled to produce condensation.

Dew - condensation in the form of water vapor.

Frost - condensation as a solid, below the freezing point.

Clouds form when warm moist air rises (Figure). The process of adiabatic cooling accompanies this. As the air cools, it looses its capacity to hold moisture and condensation in the form of clouds appears.

Dry adiabatic lapse rate = 10°C per 1000 meters

Wet adiabatic lapse rate = 5°C per 1000 meters

Normal lapse rate = 6°C per 1000 meters

Clouds (See slides in class)

Clouds are classified by 1) shape and 2) elevation.

Elevation

Low - ground level - 6500 feet
Medium - 6500-23,000 feet
High - 23,000 feet +
Clouds of vertical development (span two or more elevation zones)

Shape

Stratus - layered
Cumulus - look like cotton balls, generally have flat bottoms and rounded tops
Cirrus - thin, wispy

Low Clouds

Fog - cloud layer very close to the ground
Nimbostratus - low layered cloud
Stratocumulus - low level cumulus cloud often seen during the clearing stages of a storm

Medium Clouds

Altostratus - mid level layered clouds
Altocumulus - mid level cumulus clouds

High Clouds

Cirrostratus - high, thin layers, often invisible to the eye from the ground
Cirrocumulus - high cotton ball-like clouds
Cirrus - typical high level thin, wispy clouds

Vertical Clouds

Cumulonimbus - thunderheads with tremendous vertical extent (25,000 meters)

Precipitation

Not nearly as straightforward as it may seem. The average cloud droplet is 0.01 - 0.02 millimeters in diameter. At that size, it would take 48 hours to reach the surface of the Earth if it began to fall. It would evaporate before it reached the surface. How then does precipitation occur? This remains largely a matter of speculation. Clearly, the droplets must grow in size, but how? Two theories:

Coalescence - some droplets are able to fall to earth under the influence of gravity. These collide with others and grow. This is probably the least important process.
Supercooling - at the top of a cloud ice forms because the temperature is below freezing. If supercooling occurs the process literally sucks all the available moisture from around the ice crystal forming a much larger ice crystal, which then melts as it falls to earth.

Types of Precipitation

Rain - droplets >.5 mm in diameter

Drizzle - droplets .1 - .5 mm in diameter

Snow - aggregates of ice crystals several mm in diameter

Sleet - rain or drizzle which freezes before reaching the ground

Freezing rain - rain which freezes upon reaching the ground