1. By: Lori Sedlak

2. Humidity Measure of water vapor in atmosphere Water vapor is gaseous form of water - Also called atmospheric moisture Increased air temperature increases water vapor Warm air has higher moisture level than cold During winter, low atmospheric pressure

3. Humidity (continued) Colder months of winter considered dry Warmer air has higher moisture content - Considered humid Visibility is reduced - When high water vapor present Water vapor scatter light - Difficult to see over long distances

4. Sources of Atmospheric Moisture Main source is evaporation - Liquid changes to a gas Evapotransporation - Water Moves from soil into plant roots - Moves through plant and evaporates off leaves Sublimation - Ice changes directly to water vapor

5. Relative Humidity Measure of water vapor in atmosphere - Ratio between: Amount of water in air Compared to saturation point -At a specific temperature and pressure -Saturation Total amount of water air can hold Expressed in a percentage

6. Dew Point Temperature Temperature air must be cooled for saturation If dew point is 65°F: - At air temperature of 65°F, air becomes saturated Psycrometer measures dew point and temperature - Consists of two thermometers

7. Dew Point Temperature (continued) One thermometer measures dry bulb temperature -Equal to current air temperature Other measures wet bulb temperature - Records cooling temperature of evaporation water - Drier the air, the lower the temperature Because more water is evaporating

8. Cloud Formation Mass condensing water droplets and ice crystals - Form when air is cooled to its dew point Once air is saturated, condensation occurs - Water vapor changes to a liquid Vapor condenses forming droplets of water or ice Attach to small solid particles in air

9. Cloud Formation (continued) Condensation nuclei: - Sea salt, ash, dust, and other substances Water droplets/ ice crystals forming condensation nuclei - Together make a cloud Form when air forced to rise and cool - Rising air is called an uplift

10. Cloud Formation (continued) Lifting mechanisms: - Heat radiates by Earth’s surface -Topography – orographic lifting Tops of mountains hidden in clouds - Convergence of surface winds Where winds converge or come together - Cold air mass underneath warm air mass

11. Types of Clouds Low clouds: -Form at low levels to a height of 6,500 feet Include cumulus clouds-large cotton balls Stratus-one gray, uniform layer covering the sky Nimbostratus-thicker than stratus and darker Cumulonimbus-large with vertical development -Up to 23,000 feet

12. Types of Clouds (continued) Middle clouds: - Elevations between 6,600 and 23,000 feet - Temperatures between 32°F and -13°F - Common forms: Altostratus-one uniform white or gray layer Altocumulus-thick, white, puffy in long bands

13. Types of Clouds (continued) High clouds: cirrus clouds-thin and wispy -Heights above 23,000 feet - Composed of ice crystals - Cirrostratus or cirrocumulus Clouds close to Earth-fog - Morning or night When air temperature is at the dew point

14. Formation of Precipitation Water vapor condenses and falls to Earth Action of updraft starts process - Upward flow of wind by convection - Cumulus clouds associated with updrafts Cumulus clouds build Water droplets and ice crystals form Get knocked around by updrafts

15. Formation of Precipitation (continued) Collide with each other and stick together Forms larger water droplets and ice crystals Continue to collide and grow in size Eventually become too heavy - Updraft cannot keep them suspended Fall to Earth as precipitation

16. Types of Precipitation Drizzle-water droplets 0.5 mm in diameter - Develops in stratus clouds Rain-liquid precipitation 1 to 6 mm in diameter - Cumulonimbus and nimbostratus clouds Freezing rain-super cooled droplets of water - Freeze on contact

17. Types of Precipitation (continued) Snow-water condenses as ice crystals - Stick together and grow larger Ice pellets-rain freezes before it hits the ground (sleet) - Appear white and bound off the ground Hail-ice pellets melt and freeze - Clouds with strong updrafts- cumulonimbus

18. Orographic Precipitation Forms from lifting air mass over mountains - Moist air forced to rise over the mountains - Rapid formation of clouds and precipitation - Once over mountain, decreases in elevation - Descending air is compressed and warms - Precipitation stops and clouds dissipate