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Chapter 18 Page 502
A. Water in the Atmosphere
1.
Introduction
a. Water vapor
1) Source of condensation and precipitation
2) Most important gas for atmospheric
processes
3) Only 0-4% of gas
b. Precipitation – any form of water that falls
from a cloud
2. Water’s Changes of State
a. Allows for the water cycle
b. Changes with temperature and pressure
c. Energy transfer in the form of heat must
happen
d. Solid to Liquid
1) Melting
2) Ice remains 0C until all ice is melted
3) Energy is stored in liquid water
e. Evaporation – process of going from a liquid
to gas
f. Condensation
1) Process of going from gas to liquid
2) Causes clouds and fog
g. Sublimation – solid to gas
h. Deposition
1) Gas directly to solid
2) Frost
Phase Changes
3. Humidity
a. Amount of water vapor in the air
b. Saturation
1) Can’t hold any more water
2) Depends on temperature
3) Warm air holds more water
c. Relative Humidity
1) Ratio of air’s actual water-vapor content
compared to amount of water vapor air
could hold at that temp and pressure
2) Changed by
a) Adding or removing water vapor
i. Evaporation
ii. 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
B. Cloud Formation
1.
Air Compression and Expansion
a. Adiabatic Temperature Change
1) Happens without heat being added or
subtracted
2) Happens with air compression and
expansion
3) Expands = cools
4) Compressed = warms
b. Expansion and Cooling
1) Pressure decreases with altitude
2) Rising air expands and cools
3) Dew point is reached and clouds form
4) 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 Wedging
1) Front – warm air and cold air collide
2) Cold air is denser
3) Warm air rises over it
4) Air expands, cools, reaches dew point, etc.
c. Convergence
1) Two air masses
collide and are
forced up
2) Air expands,
cools, etc.
3) Florida
d. Localized Convective Lifting
1) 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 – sinks
2) Unstable air – air warmer than
surroundings
3) Stable air – remains in original position
b. Degrees of Stability
1) Air is stable when temp decreases
gradually with altitude
2) Temperature inversion - air temp increase
with altitude
3) Most stable with temperature inversion
a) Happens on clear nights
b) Ground cools faster than air
c. Stability and Daily Weather
1) Stable air
a) Resists vertical movement
b) Wedging or convergence causes clouds
c) Clouds are widespread and thin
d) Little to no rain is produced
e) Dreary overcast days
2) Unstable air
a) Frontal wedging or
convergence form clouds
b) Towering, thick
clouds
c) Thunderstorms and
tornadoes
4. Condensation
a. Happens to saturated air
b. Can be cooled to dew point or moisture
added
c. Types of surfaces
1) Needs something to condense on
2) Dew – grass, buildings, trees, etc.
3) Air
a) Condensation nuclei – dust, smoke,
pollution, etc.
b) Condensation nuclei
allow for condensation to
form
C. Cloud Types and Precipitation
1.
Types of clouds
a. Classification is based off height and shape
b. Cirrus
1) High, white, and thin
2) Wispy or feathery appearance
3) Could give halo
Cirrus Clouds


White and wispy
Made of ice crystals
Pictured Rocks, UP
c. Cumulus
1) Rounded individual cloud
masses
2) Flat base w/rising domes
3) Cauliflower structure
d. Stratus
1) Sheets or layers that cover entire sky
2) No individual cloud units
St. Petersburg, FL
Cumulus
Mackinac Island
Stratus
Ouachita, AK
e. High Clouds (3.5 miles +)
1) Cirrus
2) Cirrocumulus – fluffy
3) Cirrostratus – flat layers
4) Thin, white, ice crystals
5) No precipitation
6) Cirrus clouds followed by cirrocumulus or
cirrostratus may mean approaching storms
f. Middle clouds (1.5 mi – 3.5 miles)
1) Use prefix alto2) Altocumulus – rounder and denser than
cirrocumulus
3) Altostratus
a) White to grayish sheet
b) Sun visible as a bright spot
4) Infrequent and very light precipitation
g. Low Clouds
1) Stratus
a) Gray and blanket most the sky
b) Occasional light precipitation
2) Stratocumulus – long parallel rolls
3) Cumulus – fluffy clouds; fair weather
4) Nimbostratus – dark gray blanket
a) Rain clouds
b) Long all day rains
Munising, MI
h. Clouds of Vertical Development
1) Base is low altitude but top is middle to
high
2) Caused from unstable air
3) Cumulonimbus
a) Heavy rains and thunderstorms
b) Hail and tornadoes possible
c) Cauliflower look
Sioux City, IA

Cauliflower top
2. Fog
a. Cloud with base very close to ground
b. Caused by Cooling
1) Ground cools air close to ground
2) Air reaches dew point
3) Becomes denser and accumulates in low
areas
Sioux City, IA
Normal
Fog
c. Caused by evaporation
1) Cool air moves over warm water
2) Water evaporates and air becomes
saturated
3) Rising water vapor hits cold air
4) Cools and condenses
5) Rises immediately with air being warmed
from below
http://www.weather.com/news/weather/video/phenomenon-that-looks-like-a-fog-tsunami
Wall Cloud

https://www.youtube.com/watch?v=QQBo3w9bTEY
Squall Line


Produce a shelf cloud
https://www.youtube.com/watch?v=ylack7IG2YA
SCUD Cloud

Stratocumulus Under Deck
 Result from high humidity near
storm
 Can be very low to the ground
 NOT an indication of severe
weather
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 crystals
3) Extra water becomes ice
4) Water continually evaporates
5) Crystals grow until heavy enough to fall
https://www.youtube.com/watch?v=Sd3_rjKQf24
b. Warm Cloud Precipitation
1) Collision-coalescence process
2) Salt pulls vapor out of the air
3) Forms large drops
4) Drops move through cloud
colliding and coalescing with
smaller drops
4. Forms of Precipitation
a. Rain – liquid water falling
b. Snow – six-sided light, fluffy crystal
c. Sleet
1) Falling clear/translucent ice
2) Warm air above freezing air
Sleet Formation
Dordt College
Sioux Center, IA
d. Freezing rain
1) Supercooled
water which freezes
on contact
2) Warm air above
freezing air
Freezing Rain Formation
Freezing Rain
Lansing, IL

Deicing Planes
 Water freezes on plane

http://www.youtube.com/watch?v=ph8xusY3GTM
e. Hail
1) Cumulonimbus clouds
2) Begin as ice pellets
3) Grow by supercooled water
4) Updrafts keep hail in air
https://www.youtube.com/watch?v=DdAGIigvrfg
https://www.youtube.com/watch?v=Zz-9zhXuLN0
f. Graupel
1) “Soft hail” or “snow pellets”
2) Supercooled water freezes on a falling snowflake