Water in the Atmosphere Chapter 20 1 1

Forms of Water › Water exists in the atmosphere in 3 forms or phases: Gas called water vapor Solid known as ice Liquid › Water changes from one phase to the next when heat energy is absorbed or released

Some Vocabulary to Know Latent heat – heat energy that is absorbed or released by a substance during a phase change Evaporation – Most evaporation takes place in the oceans; water vapor can also enter the atmosphere by evaporation from lakes, streams, ponds, and soil. Sublimation – process in which solid changes directly into a gas

Humidity Water vapor in the atmosphere is known as humidity Controlled by rates of condensation & evaporation Rate of evaporation is determined by temperature of air Higher temperature, higher evaporation rate Rate of condensation is determined by vapor pressure Vapor pressure is part of the total atmospheric pressure caused by water vapor

Absolute humidity = mass of water vapor (grams) Humidity Continued Dew point – Temperature at which the condensation rate equals the evaporation rate. Temperatures below dew point, water droplets form Absolute humidity – mass of water vapor contained in a given volume of air Absolute humidity = mass of water vapor (grams) Volume of air (cubic meters) Relative humidity – ratio of the actual water vapor content of the air to the amount of water vapor needed to reach saturation. Measure of how close the air is to reaching dew point.

Measuring Humidity Relative humidity can be measured by: thin polymer film, a psychrometer, a dew cell, and a hair hygrometer.

Clouds & Fog Clouds are a collection of small water droplets or ice crystals that fall slowly through the air.

Cloud Formation

Cooling Processes That Help Form Clouds Four major processes that cause cooling for clouds to form: 1. Adiabatic Cooling – temperature of an air mass decreases as the air mass rises & expands 2. Mixing – when 1 body of moist air mixes with another body of moist air of a different temp 3. Lifting – forced upward movement of air results in cooling of air& cloud formation 4. Advective Cooling – temperature of an air mass decreases as the air mass moves over a cold surface

Classification of Clouds CUMULUS CLOUDS CHARACTERISTICS --- Puffy Vertical Growing Clouds Thick Looks like popcorn Form thunderstorms Cumulus – piled, heaped

STRATUS CLOUDS CHARACTERISTICS -- Stratus - Sheet like Low clouds Layered Covers large areas of the sky Blocks the sun “Covers” the earth

CIRRUS CLOUDS CHARACTERISTICS -- Highest Clouds Wispy Feathery Cirrus – curly Made of ice crystals

Produce Hail, Lightning, and Thunder CUMULONIMBUS CLOUDS CHARACTERISTICS -- Thunderstorm Clouds Produce Hail, Lightning, and Thunder Anvil shaped Can reach into the stratosphere Nimbo - Rain

Fog Result of condensation of water vapor in the air. Near the surface of Earth 2 types: Radiation fog – results from the loss of heat by radiation; forms from the nightly cooling of Earth; layer of air in contact with the ground becomes chilled below dew point; Advection fog – forms when warm, moist air moves across a cold surface; forms mostly along coasts.

Precipitation 4 forms of precipitation: Rain – liquid precipitation Snow – common solid precipitation Sleet – forms when rain falls through a layer of freezing air near the ground Hail – solid precipitation in the form of lumps of ice

Causes of Precipitation Coalescence – formation of a large droplet by the combination of smaller droplets Supercooling – a condition in which a substance is cooled below its freezing point, condensation point, or sublimation point without going through a change in state

Measuring Precipitation Meteorologists use a variety of instruments Rain gauge Doppler radar