Moisture in the Air

Key Terms

• Evaporation – the process by which water molecules escape into the air.

• Humidity – water vapor or moisture, in the air.

• Relative humidity – the percent of moisture the air holds relative to the amount it could hold at a certain temperature.

Measuring Relative Humidity

• Psychrometer – device used to measure relative humidity.– Consists of 2 thermometers.– One is covered with a moist cloth.– When air passes over wet cloth, water

evaporates and temperature cools.

Measuring Relative Humidity

• Differences in temperature between 2 bulbs is used to determine relative humidity

• Compare temp difference (degrees difference) to dry thermometer temp– Look up relative humidity in chart

Clouds

• Form when moisture in the air condenses on small particles of dust or other solids in the air.

• Mixture in which particles of water are suspended in air.

Cloud Formation

• Warm air can hold more moisture than cold air.• As air rises it cools.• Air becomes saturated with water.

– Holds maximum amount of water.• Water vapor in air condenses clouds form.

– Dew point – the temp. at which water vapor condenses.

Cumulus clouds

• “Fluffy” clouds• Flat bottom• Usually indicate fair

weather– Can produce

thunderstorms

Stratus Clouds

• Smooth, gray• Cover the entire sky• Associated with light

rain and drizzle• When close to ground

fog

Cirrus Clouds•Thin and feathery, made of ice crystals

•Can be seen in fair weather

•May indicate that rain or snow will fall in several hours.

•A.k.a. – mares’ tails

Special Types of Clouds

• Cirrostratus -high-level clouds composed of ice crystals– can cover the entire sky

– relatively transparent, as the sun or the moon can easily be seen through them

• Cumulonimbus – thunderclouds• Nimbostratus - dark, low-level clouds

accompanied by light to moderately falling precipitation

Precipitation

• Water vapor that condenses and falls to Earth– Ex. – rain, sleet, snow

• Formation:– Cloud droplets increase in size by colliding

and combining with other droplets– Droplets become to heavy, and are pulled down

by gravity

Snow

• Forms when water vapor turns directly into a solid

• 6-sided flakes (ice crystals)

Hail

• Forms in cumulonimbus clouds

• Small chunks of ice• Formed when water

droplets hit ice pellets in clouds and freeze

• Can be held in clouds by winds and grow in size before falling

Measuring Rainfall

• Rain gauge– Straight sided

container with flat bottom

– Collects rain over a given period of time

– Usually measure in mm or cm