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