Water in the Atmosphere Chapter 23
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
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.
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
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