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Chapter 9: Weather Chapter 9: Weather FactorsFactors
Section 4: Water In the Atmosphere
The Water CycleThe Water CycleWater Cycle – the movement of
water between the atmosphere and Earth’s surface.◦Evaporation◦Condensation◦Precipitation◦Surface runoff
The Water CycleThe Water CycleEvaporation- water molecules in
liquid water escape into the air as water vapor.
HumidityHumidityHumidity - a measure of the
amount of water vapor in the air.Relative Humidity – is the
percentage of water vapor that is actually in the air. ◦Compared to the maximum amount
of water vapor the air can hold at a particular temperature.
Measuring Relative Measuring Relative Humidity Humidity Relative humidity can be measured
with an instrument called a psychrometer.◦ (sy KRAHM uh tur)◦2 thermometers◦Dry bulb◦Wet bulb
How Clouds FormHow Clouds FormClouds form when water vapor in
the air condenses to form liquid water or ice crystals.
Condensation – molecules of water vapor in the air become liquid water.
Dew point – the temperature at which condensation begins.◦Above freezing - water droplets◦Below freezing – ice crystals
How Clouds FormHow Clouds FormThe Role of Particles
◦For water vapor to condense, tiny particles must be present so the water has a surface on which to condense.
◦In cloud formation, most of these particles are salt crystals, dust from soil and smoke.
◦These tiny particles are called nuclei.
Types of CloudsTypes of CloudsCirrus Clouds
Cumulus Clouds
Stratus Clouds
Cirrus CloudsCirrus CloudsWispy, featherOnly at high levels with low
temperatures◦Above 6 kilometers
Made of ice crystals
Cumulus CloudsCumulus CloudsLook like fluffy, rounded piles of
cottonForm less than 2 kilometers
above groundMay extend upward as much as
18 kilometers
Stratus CloudsStratus CloudsForm in flat layersCover all or most of the skyA uniform dull, gray color
Altocumulus and Altocumulus and Altostratus Altostratus Middle level clouds; higher than
regular cumulus and stratus clouds, lower than cirrus clouds.
Between 2 and 6 kilometers above the ground
FogFogFog – clouds that form at or near
the groundAir above the ground is cooled to
the air’s dew point.Near bodies of water, low-lying
marshes, mountain slopes