Chapter 18 Page 502

MOISTURE, CLOUDS, AND PRECIPITATION

1. Introduction a. Water vapor

1) Source of condensation and precipitation

2) Most important gas for atmospheric processes

3) Only 0-4% of gasb. Precipitation – any form of water that

falls from a cloud

A. Water in the Atmosphere

2. Water’s Changes of Statea. Allows for the water cycleb. Changes with temperature and pressurec. Energy transfer in the form of heat must happend. Solid to Liquid

1) Melting2) Ice remains 0C until all ice is melted3) Energy is stored in liquid water

e. Evaporation – process of going from a liquid to gasf. Condensation

1) Process of going from gas to liquid2) Causes clouds and fog

g. Sublimation – solid to gash. Deposition

1) Gas directly to solid2) Frost

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Phase Changes

3. Humiditya. Amount of water vapor in the airb. Saturation

1) Can’t hold any more water2) Depends on temperature3) Warm air holds more water

c. Relative Humidity1) Ratio of air’s actual water-vapor

content compared to amount of water vapor air could hold at that temp and pressure

2) Changed bya) Adding or removing water vapor

i. Evaporationii. Precipitation

b) Temperature i. Cooling can make nonsaturated air become saturated ii. Saturated air is cooled which causes condensation = clouds

d. Dew Point 1) Temp at which air needs to be cooled to become saturated 2) If cooled more = dew, fog, clouds 3) Higher dew points – moist air 4) Low dew points – dry air

1. Air Compression and Expansiona. Adiabatic Temperature Change

1) Happens without heat being added or subtracted

2) Happens with air compression and expansion

3) Expands = cools4) Compressed = warms

B. Cloud Formation

b. Expansion and Cooling1) Pressure decreases with altitude2) Rising air expands and cools3) Dew point is reached and clouds

form4) Opposite for sinking air

2. Processes That Lift Air a. Orographic Lifting

1) Caused from elevated terrain (mts.)2) Rainshadow

a) Windward side - air rises, expands, cools, and rains

b) Leeward side – moisture gone, air sinks, compresses, warms = no rain

b. Frontal Wedging1) Front – warm air and cold air collide2) Cold air is denser3) Warm air rises over it4) Air cools, expands, etc.

c. Convergence1) Two air massescollide and are forced up2) Air cools, expands, etc.3) Florida

d. Localized Convective Lifting1) Unequal heating of earth’s surface - Paved parking lot compared to wooded park

2) Warmer air rises, expands, cools, etc.

3) Thermals – parcels of warm rising air

4) Hawks/eagles use these for lift

3. Stability a. Density Differences

1) Warm air – rises; cold air – sinks2) Unstable air – air warmer than

surroundings3) Stable air – remains in original

position

b. Degrees of Stability1) Air is stable when temp decreases

gradually with altitude 2) Temperature inversion - air temp

increase with altitude3) Most stable with temperature

inversiona) Happens on clear nightsb) Ground cools faster than air

c. Stability and Daily Weather 1) Stable air

a) Resists vertical movementb) Wedging or convergence causes

cloudsc) Clouds are widespread and thind) Little to no rain is producede) Dreary overcast days

2) Unstable aira) Frontal wedging or convergence form cloudsb) Towering, thick cloudsc) Thunderstorms and tornadoes

4. Condensationa. Happens to saturated airb. Can be cooled to dew point or

moisture addedc. Types of surfaces

1) Needs something to condense on2) Dew – grass, buildings, trees, etc.

3) Aira) Condensation nuclei – dust, smoke, pollution, etc. b) Condensation nuclei allow for condensation to form

1. Types of cloudsa. Classification is based off height and

shapeb. Cirrus

1) High, white, and thin2) Wispy or feathery appearance3) Could give halo

C. Cloud Types and Precipitation

c. Cumulus1) Rounded individual cloud masses2) Flat base w/rising domes3) Cauliflower structure

d. Stratus1) Sheets or layers that cover entire sky2) No individual cloud units

St. Petersburg, FL

e. High Clouds (3.5 miles +)

1) Cirrus2) Cirrocumulus – fluffy3) Cirrostratus – flat layers4) Thin, white, ice crystals5) No precipitation6) Cirrus clouds followed by

cirrocumulus or cirrostratus may mean approaching storms

f. Middle clouds (1.5 mi – 3.5 miles)

1) Use prefix alto-2) Altocumulus – rounder and denser

than cirrocumulus3) Altostratus

a) White to grayish sheetb) Sun visible as a bright spot

4) Infrequent and very light precipitation

g. Low Clouds1) Stratus

a) Gray and blanket most the skyb) Occasional light precipitation

2) Stratocumulus – long parallel rolls3) Cumulus – fluffy clouds; fair

weather

4) Nimbostratus – dark gray blanketa) Rain cloudsb) Long all day rains

Munising, MI

h. Clouds of Vertical Development 1) Base is low altitude but top is

middle to high2) Caused from unstable air3) Cumulonimbus

a) Heavy rains and thunderstormsb) Hail and tornadoes possiblec) Cauliflower look

2. Fog a. Cloud with base very close to ground b. Caused by Cooling

1) Ground cools air close to ground2) Air reaches dew point3) Becomes denser and accumulates in

low areas

c. Caused by evaporation1) Cool air moves over warm water2) Water evaporates and air becomes saturated3) Rising water vapor hits cold air4) Cools and condenses 5) Rises immediately with air being

warmed from below

Types of Clouds

3. How Precipitation Forms a. Cold Cloud Precipitation – Bergeron Process

1) Air is saturated w/water, which means it is supersaturated w/respect to ice

2) Water can’t coexist with ice crystals3) Extra water becomes ice4) Water continually evaporates5) Crystals grow until heavy enough to fallhttps://www.youtube.com/watch?v=Sd3_rjKQf24

b. Warm Cloud Precipitation1) Collision-coalescence process2) Salt pulls vapor out of the air3) Forms large drops4) Drops move through cloud colliding and coalescing with smaller drops

4. Forms of Precipitationa. Rain – liquid water fallingb. Snow – six-sided light, fluffy crystal c. Sleet

1) Falling clear/translucent ice2) Warm are above freezing air

Sleet Formation

d. Freezing rain1) Supercooled water which

freezes on contact2) Warm air

above freezing air

Dordt CollegeSioux Center, IA

Freezing Rain Formation

Deicing PlanesWater freezes on plane http://www.youtube.com/watch?v=ph8xusY3GTM

e. Hail1) Cumulonibus clouds2) Begin as ice pellets3) Grow by supercooled water4) Updrafts keep hail in airhttps://www.youtube.com/watch?v=DdAGIigvrfg

f. Graupel1) “Soft hail” or “snow pellets”2) Supercooled water freezes on a falling

snowflake