Moisture, Clouds and

PrecipitationChapter 18

H2O exists in atmosphere in all

three states of matter…

Solid:

snow

hail

ice

Liquid: rain and cloud

droplets

Gas:invisible H2O

vapor

When it comes to understandingatmospheric processes, watervapor is the most importantgas in the atmosphere.

H2O may change from one state to another:

The process of changing state requires that energy is transferred in the form of heatheat

Energy is absorbed or released during state change

What is latent heat?

What is Melting?• from solid ice to liquid H2O

What is Freezing?

• liquid H2O to solid ice

What is Evaporation? • from liquid H2O to H2O vapor

What is Condensation? from H2O vapor to liquid H2O

What is Sublimation? change from solid to H2O vapor

Example: Dry ice

What is Deposition? change directly from H2O vapor to a

solid

Warmer the air, the more H2O vapor it can hold

• What is Humidity?water vapor in the air

• What is Relative humidity?the % of moisture the air can hold relative to the amount it can hold at a certain temperature

Relative humidity=specific humidity X 100 capacity (saturated)

Psychrometer: instruments used to measure relative humidity

• Works on principle that evaporation causes cooling

• 2 thermometers…wet-bulb and dry-bulb

• Readings show how dry the air is

What is Dew point? the temperature at which water vapor condenses

• Dew, clouds, and fog forms• If dew point is below

freezing, frost will form

What are Clouds? simply high fog, mist,

or haze• Form when air above surface

cools below dew point

• Clouds are classified on the basis of their form and height

-horizontal air movement = layers

-vertical air movement = piles

•Shape depends on air movement that forms it:

What determines cloud shapes?

• Temperature above freezing – clouds drop water

• Temperature below freezing – clouds drop snow crystals

What are the three main cloud

types?

CIRRUS: - thin, feathery, made of ice crystals

- form at high altitudes

- seen when weather is fair, but can mean rain or snow

- “a curl of hair”

STRATUS• Low sheets or layers; gray and smooth

• Block out the sun

• Associated with rain and drizzle

• “a layer”

CUMULUS• Piled in thick, puffy masses

• Usually mean fair weather

• “a pile”

Other cloud types:

Cirrostratus

Stratocumulus

Cirrocumulus

Altocumulus

Nimbostratus

Cumulonimbus: large cloud that produce

LIGHTNING, THUNDER, HEAVY SHOWERS =

Thunderstorms

What is Precipitation?

• Water that falls from the atmosphere to the earth

• Occurs when cloud droplets grow into drops heavy enough to fall to Earth

• Type of precipitation that reaches Earth’s surface depends on the _______________in the lower few kilometers in the atmosphere

Condensation nuclei: suspended particles that provide the necessary

surfaces for cloud forming condensation.

What are the FORMS OF PRECIPITATION?

• Drizzle: fine drops….less than 0.5mm diameter

• Rain drops: larger… 0.5mm to 5mm diameter

• Snow: falls in clumps of six-sided crystals

• Sleet: pellets of ice that fall to the ground when raindrops fall through freezing air

for precipitation to form, cloud droplets must grow in volume by roughly one million times!

• Hail:

irregular balls or lumps made of layers of ice

forms in cumulonimbus clouds-

What is a Rain gauge?

instrument used to measure the

amount of rainfall

What is Acid precipitation? acid drops that fall to the ground

- contain nitrate and sulfate particles that come from burning

fuels, volcanoes and cars

http://www.epa.gov/acidrain/site_students/acid_anim.html