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Chapter 18
Water in the Atmosphere
18.1 Humidity and Condensation
Water Characteristics:
 Found in all 3 states of matter
o Below 0o – Ice, snow, hail
o Between 0o-100o – rain, cloud droplets
o Above 100o – water evaporates into water vapor
 When water changes state, energy is gained or lost
o Vapor to liquid – condensation
 dew, fog, clouds
 releases heat
o Liquid to vapor – evaporation
 Absorbs heat
 Cooling process- leaves cool surfaces behind
o Vapor to Solid – deposition
 Frost
 Reverse is sublimation (solid to vapor)
Humidity
 Specific humidity- that actual amount of water vapor in the air
o Represented by grams of water vapor per kilogram of air
 Capacity – The total amount of water vapor air at a given temperature can hold.
o The warmer the air is, the larger its capacity
 Saturation – When the air is holding the maximum amount of water vapor in
relation to its temperature
o Ex. If the air’s capacity for water vapor is 22g/kg, and it is holding 22g,
then it is said to be saturated.
 Relative humidity – The percentage of saturation
o How close to capacity is the air?
o Measured by a %. Air that has no water vapor is at 0%. Air that is at
capacity is 100%
o Specific Humidity / Capacity
x 100 = % Relative humidity
o Ex.
 Air’s capacity is 22g/kg
 It’s holding 11g
11 g/kg
22 g/kg
= 0.5
0.5 x 100% = 50%
The Relative humidity is 50%
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Condensation
o As temperature drops, so does its capacity to hold water vapor
o Eventually, air cools to a point where its capacity has dropped to the level
of its specific humidity
o The temperature at which that happens is called the dew point.
o In order for dew to form, the temperature must reach the dew point, and
the vapor must condense on condensation nuclei, which are tiny particles
of dust, soot etc.
o If there are no nuclei present for water to condense on, the air is said to be
supersaturated.
Dew – water vapor condensing on a cool surface
Frost – water vapor deposing on a surface that’s below freezing
Fog – a cloud at ground level
o Advection fog- warm, moist air flows over a cool surface, cools to its dew
point and condenses
o Radiation fog- ground loses heat rapidly through radiation, mixes with
cool air, whole layer cools to its dew point and condenses.
18.2 Clouds
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Types of Clouds
o Classified by altitude and shape
o Stratiform clouds form horizontally Stratus/strato
 These are layered, low clouds
o Cumuloform clouds form vertically Cumulus/cumulo
 These are fluffy with flat bases
o Clouds above 7000 meters are classified as Cirro (Cirrus) clouds.
 These are high altitude and feathery
o Clouds from 2000-7000 meters are classified as a type of Alto cloud
o Any cloud that has nimbo/nimbus in its name produces precipitation.
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Some Examples:
o Altostratus
o Nimbostratus
o Stratocumulus
o Cirrocumulus
o Cumulonimbus
o Altocumulus
o Stratus
o Cirrus
o Cumulus
Cloud Formation
o The shape of a cloud will show the type of air movement that created it
o Heated air will rise and cool to its dew point.
o The condensation level is the altitude at which the condensation occurs
o Adiabatic Lapse Rates – the rate at which air cools as it rises
 Dry – dry air cools at a rate of 10 degrees Celsius for every
kilometer
 Moist – moist air cools at a rate of 5-9 degrees Celsius for every
kilometer
 Why is this important? Moist rising air that condenses releases
heat. This heat can fuel very large and dangerous clouds.
Cumulonimbus Clouds
o Moist air rises, cools and condenses at the condensation level.
o The base of the cloud is low, but the top of the cloud can grow to
enormous heights.
o Unstable air is air that rises rapidly, this can create these large clouds
o Stable air is air that rises slowly, this produces smaller cumulus clouds.
Layered clouds
o Stratiform clouds form in stable air.
o Air has trouble rising in stable conditions so it will spread out
18.3
Precipitation – any form of water that falls from a cloud to earth’s surface
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Rain – Falling water. Raindrop size varies.
Ice crystals – form by deposition (see 18.1 notes)
Sleet- rain that falls through cold air and freeze
Freezing Rain – rain that falls and freezes when it hits the surface
Hail – layered ice. Frozen raindrop is kept aloft by winds, collides with more
raindrops, falls through cloud when heavy enough to overcome winds. Can be
different sizes.
Measuring Precipitation
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Rain Gauge
On average, 1 foot of snow = 1 inch of rain
Where does precipitation Occur?
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Rising and cooling air causes precipitation
The higher air rises, the more moisture it can release
So…. Areas where warm, moist air rises are the places where it is most likely
to rain more often.
o Near equator, heat from the sun makes air rise
o Storms cause air to rise and cool
o Air rises up the side of a mountain, cools and forms precipitation
clouds. This is the windward side of the mountain, the leeward side
gets no precipitation, but plenty of wind. These winds are dry.
 Chinook, Santa Ana Winds
Weather Modification
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Sometimes, it does not rain where it is needed
o Cloud seeding – putting particles into clouds to produce rain