1. Moisture in the Air

2. 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.

3. 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.

4. 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

5. 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.

6. 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.

7. Cumulus clouds “Fluffy” clouds Flat bottom
Usually indicate fair weather
Can produce thunderstorms

8. Stratus Clouds Smooth, gray Cover the entire sky
Associated with light rain and drizzle
When close to ground  fog

9. 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

10. 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

11. Precipitation Water vapor that condenses and falls to Earth Formation:
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

12. Snow Forms when water vapor turns directly into a solid
6-sided flakes (ice crystals)

13. 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

14. Measuring Rainfall Rain gauge
Straight sided container with flat bottom
Collects rain over a given period of time
Usually measure in mm or cm