Moisture in the Air

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 • 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