By: Lori Sedlak
Humidity, Clouds, and Precipitation
Chapter 17
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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