By: Lori Sedlak
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 updraftscumulonimbus
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