Earth Systems Chapter 18 Notes
18.1 Humidity and Condensation
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Molecules of liquid water are always in motion
Characteristics of water
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Water is the only substance that commonly exists in all three states of matter
o Solid – 0*C or below; ice, snow, hail, and ice crystals
o Liquid – between 0*C and 100*C; rain and cloud droplets
o Vapor (gas) – 100*C clouds and steam are liquid, not gas
Water often changes state in the atmosphere
o Condensation – the change from water vapor to liquid water
 Dew, fog, and clouds
 Releases heat
 Slows down the rate at which air cools
 Dew point – the temperature at which saturation occurs and condensation
begins
 In order for water vapor to condense
 There must be material for water vapor to condense onto
 The air must cool to or below its dew point
 Condensation nuclei – tiny particles that water vapor condenses onto to form
clouds
o Evaporation – the change from liquid water to water vapor
 Absorbs heat
 Is a cooling process
Humidity
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Amount of water vapor present in the air varies
o Specific humidity – actual amount of water vapor in the air at a given time and place
There is a limit to the amount of water vapor that can be present in the air
o Saturated – so much water vapor in the air that the rate of condensation equals the rate
of evaporation
o Amount of water vapor present in saturated air depends on the temperature of the air
o Warmer air can contain more water vapor
Relative Humidity – how near the air is to its maximum capacity for holding water vapor
o Compares the actual amount of water vapor in the air with the maximum amount of
water vapor that can be present in air at a given temperature and pressure
o Stated as a percentage
Psychrometer – an instrument that works on the principle that evaporation causes cooling, used
to measure humidity
18.2 Clouds
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Form when the air cools to its dew point
Form at any altitude in the troposphere
Types of Clouds
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Low clouds, middle clouds, high clouds, and clouds of vertical development
Clouds are classified according to their height or altitude – low, middle, or high – and shape
o Stratiform clouds- air movement is mainly horizontal, layers of clouds
o Cumuliform clouds – air movement is mainly vertical, clouds grow upward in puffs
Altitudes of Clouds
o 2000 – 7000 meters – add “alto”
o Above 7000 meters - add “cirro”
o Below 2000 meters – add “strato”
Names of Clouds
o Stratus and strato – describe clouds that form in layers. Stratus clouds are layered, low
clouds
o Cumulus and cumulo – describe clouds that grow upward. Cumulus clouds are fluffy
clouds with flat bases
o Cirrus and cirro – describe feather clouds. Cirrus clouds are high, feathery ice clouds
o Alto- describes clouds between 2000 and 7000 meters
o Numbus and numbo- refer to dark rain clouds
Cloud Formation
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Cloud shape shows how the air moves through it
o Condensation level – the atmospheric level at which condensation occurs
Clouds need a steady amount of moist, warm air otherwise it will evaporate
o Dry-adiabatic lapse rate – the rate at which unsaturated air cools as it rises
o Moist-adiabatic lapse rate – the rate at which saturated air cools as it rises
18.3 Precipitation
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Precipitation – any form of water that falls from a cloud to Earth’s surface, rain, snow, sleet, hail
Growth of Water Droplets
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Droplets grow by bumping into and combining with other droplets
Large droplets fall faster than smaller ones
Drops that have been in the cloud longer have had more time to grow
Growth of Ice Crystals
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Temperature in the upper layers of clouds are usually below freezing
Supercooled water evaporates, and the resulting water vapor becomes deposited on the ice
crystals
If ice crystals get heavy enough, they start to fall
Kinds of Precipitation
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Many forms – drizzle, rain, snow, sleet, freezing rain, and hail
Sleet – supercooled rain drops that freeze
Freezing rain – causes sheet ice, or glaze on sidewalks, trees, roofs, and power lines
Hail – precipitation in the form or balls of ice or irregular clumps of ice
Measuring Precipitation
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National Weather Service reports rainfall in hundredths of an inch, measured by a rain gauge
Precipitation occurs all over the world
One of the main causes of precipitation is the rising and cooling of moist air