Water in the Atmosphere

Chapter 18

H2O exists in atmosphere in all

three states of matter…

Solid:

snow

hail

ice

Liquid: rain and cloud

droplets

Gas:invisible H2O

vapor

H2O may change from one state to

another:

Melting:• from solid ice to liquid H2O

Freezing:

• liquid H2O to solid ice

Evaporation: • from liquid H2O to H2O vapor

Condensation: from H2O vapor to liquid H2O

Sublimation: change from solid to H2O vapor

Example: Dry ice

•a measure of the energy in molecules

•the more energy the molecules have, the hotter it feels

isotherm: lines on a map that connect places that have the same temperature

thermometer: instrument that measures temperature

•alcohol and mercury thermometers used

(mercury more accurate)

water freezes at: 0ºC or 32 ºF

water boils at: 100ºC or 212ºF

Water vapor enters the atmosphere from the

evaporation of water from oceans, lakes, marshes and

glaciers

Warmer the air, the more H2O vapor it can hold

• Humidity:water vapor in the air

• 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

Dew point: the temperature at which water vapor condenses

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

freezing, frost will form

Clouds: simply high fogs, mist, or haze

• Form when air above surface cools below dew point

• Shape depends on air movement that forms it:

-horizontal air movement = layers

-vertical air movement = piles

• Temperature above freezing – clouds drop water

• Temperature below freezing – clouds drop snow crystals

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

STRATUS• Low sheets or layers; gray and smooth

• Block out the sun

• Associated with rain and drizzle

CUMULUS• Piled in thick, puffy masses

• Formed by vertically rising air currents

• Usually mean fair weather

Other cloud types:

Cirrostratus

Stratocumulus

Cirrocumulus

Altocumulus

Nimbostratus

Cumulonimbus: large cloud that produce

LIGHTNING, THUNDER, HEAVY SHOWERS =

Thunderstorms

Precipitation:• Water that falls from the

atmosphere to the earth

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

FORMS OF PRECIPITATION

• Drizzle: fine drops, very close together, fall slowly ….less than 0.5mm diameter

• Rain drops: larger, farther apart, fall faster… 0.5mm to 5mm diameter

• Snow: falls in clumps of six-sided crystals

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

• Hail:

irregular balls or lumps made of layers of ice

forms in cumulonimbus clouds-

Acid rain: acid drops that fall to the ground

- contain nitrate and sulfate particles that come from burning

fuels, volcanoes and cars

• Cloud seeding: method to cause an increase in precipitation

Rain gauge: instrument used to measure the

amount of rainfall

Condensation nuclei: suspended particles that provide the

necessary surfaces for cloud forming condensation.