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