Water in the AtmosphereWater in the Atmosphere

Chapter 20

Forms of WaterForms of Water

› Water exists in the atmosphere in 3 forms or phases:

1. Gas called water vapor2. Solid known as ice3. Liquid

› Water changes from one phase to the next when heat energy is absorbed or released

Some Vocabulary to Some Vocabulary to KnowKnow

• 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

HumidityHumidity• 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

Humidity ContinuedHumidity 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 HumidityMeasuring Humidity

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

Clouds & FogClouds & Fog• Clouds are a collection of small water droplets

or ice crystals that fall slowly through the air.

Cloud Formation

Cooling Processes That Cooling Processes That Help Form CloudsHelp 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 CloudsClassification of Clouds

CUMULUS CLOUDSCHARACTERISTICS --- • Puffy• Vertical Growing Clouds• Thick• Looks like popcorn• Form thunderstorms• Cumulus – piled, heaped

STRATUS CLOUDSCHARACTERISTICS -- • Stratus - Sheet like • Low clouds• Layered• Covers large areas

of the sky• Blocks the sun• “Covers” the earth

CIRRUS CLOUDSCHARACTERISTICS -- • Highest Clouds• Wispy• Feathery• Cirrus – curly• Made of ice crystals

CUMULONIMBUS CLOUDS

CHARACTERISTICS --• Thunderstorm Clouds• Produce Hail,

Lightning, and Thunder

• Anvil shaped • Can reach into the

stratosphere• Nimbo - Rain

FogFog

• 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.

PrecipitationPrecipitation

• 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 PrecipitationCauses 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 PrecipitationMeasuring Precipitation• Meteorologists use a variety of instruments

– Rain gauge

– Doppler radar