Ch. 24.4 Water in the Atmosphere

Objectives: •What are some ways in which water condenses?•How do clouds form?•What are major cloud types?•What are the major forms of precipitation?

Humidity = Amount of water vapor in air

Warm tropical air contains more water than cold, polar air

•Relative Humidity =Ratio of the amount of water vapor in the air to the maximum amount of water vapor that can exist as that temperature.

•Expressed as a %

•Ex.: Relative Humidity is 60%, Air contains 60% of maximum amount of water vapor that can exist at that temperature

•Relative humidity = 100 %, Air is SATURATED

Relative Humidity & Dew Point

• Warmer air can hold more water

• As temps. decrease (at night), amt. of water air can hold decrease and relative humidity increases. During day, temps. increase and relative hum. Decreases.

(even though the total amount of water in air stays the same).

DEW POINT =

Temp. at which air is SATURATED (RELATIVE HUMIDITY 100%).

If temps. Drop further water vapor condenses (air can’t hold any more water)

Water condenses at Dew Point (temperature)

• Dew – water vapor condenses on earth’s surface.

• Frost – dew point of air is below freezing.– Water doesn’t condense.

G S (ice)

• Fog – water vapor condenses to form a cloud that is near / touching ground

• Clouds

Cloud Formation

Clouds = dense, visible mass of tiny water droplets or ice crystals (very small) that are suspended in the atmosphere.

Cloud formation:1.Warm, moist air rises in atmosphere2.Water vapor condenses when temp. drop below

dew point.3. Condensation Nuclei – also needed

Dust, salt, solid particles needed as platforms for water to condense on.

Cloud Type: Cumulus

• Cumulus Clouds – Puffy, piles/heaps of cotton balls w/ flat bottoms.

• “fair weather clouds”• Form less than 2 kms. From

ground– Cumulonimbus = Dark towering

clouds that are ass. w/ thunder storms. Grow vertically. (Anvil top) “Thunder heads”

Cloud Type: Stratus• Stratus Clouds – Flat

layers, cover much or all of sky– Nimbostratus = low

stratus that produce steady, widespread rain or snow.

– Altostratus = Middle level clouds (2000 – 6000 meters) that produce light rain or snow.

Cloud Type: Cirrus

Cirrus Clouds = Thin wispy feathery /veil like clouds made up of ice crystals.

• Clear sunny days

• Don’t produce rain … but are usu. Ahead of rain producing clouds.

• High AltitudesLink to chart: http://educ.queensu.ca/~science/main/concept/biol/b03/G08LABR8.gif

Forms of Precipitation

• Rain = liquid precip. Forms in nimbostrat. or cumulonibus clouds

• Snow = ice cystals• Hail = round, solid pieces of ice more than 5 mm

in diameter. Ice is tossed up & down in cumulonimbus clouds and get coated w/ water that freezes layers of ice.

• Sleet = Ice particles less than 5mm in diamter. Rain freezes as it falls.

• Freezing Rain = rain freezes after hitting surfaces.