Moisture, Clouds, & Precipitation Ch. 18 Earth-Space Science Bremen High School Teacher : Aaron McNeely
May 16, 2015
Moisture, Clouds, & PrecipitationCh. 18
Earth-Space Science
Bremen High School
Teacher: Aaron McNeely
Water in the AtmosphereSec 18.1
Water vapor Water in a gas form The source of all clouds, condensation, and
precipitation For weather, water vapor is the most important
gas in the atmosphere Precipitation is any water, solid or liquid, that
falls from the sky Water vapor is 0-4% of atmospheric gases
Water’s Changes of State
Three states of matter: Solid, liquid, and gas
Water can change between these states in earth’s atmosphere, termed the water cycle
Water Cycle
Water constantly moves among the oceans, fresh water bodies, and atmosphere
Planet-wide phenomena powered by the sun
Steps in the Water Cycle General Steps:
Water evaporates from the ocean Water falls as precipitation upon land or ocean (cycle
complete) Water that falls on land becomes run-off or soaks into the
ground (infiltration) Run-off carries water directly back to the ocean in streams
and rivers Groundwater eventually joins lakes and rivers Plants absorb water and release it back into the
atmosphere (transpiration) Cycle complete when land-based water reaches the
ocean
Phenomena of the Water Cycle
Evaporation Precipitation Infiltration Run-off Transpiration
Water Cycle
http://www.metoffice.com/education/images/water_cycle.jpg
Water’s Changes of State
Evaporation—Liquid to gas Condensation—Gas to liquid Melting—Solid to liquid Freezing—Liquid to solid Sublimation—Solid to gas Deposition—Gas to solid
Evaporation
Liquid to a gas Energy required, termed latent heat Evaporation is a cooling process,
removes heat from surroundings
Condensation
Gas to liquid Latent heat is released Ex: Cold beverage, morning car
http://www.2xup-ph.org/album/discovery/condensation.jpg
Melting
Solid to liquid Heat required, used to break bonds
between water molecules Latent heat is the energy source for
weather such as thunderstorms, tornadoes, and hurricanes
http://www.phys.unsw.edu.au/~tonyt/dome%202003/melting%20ice.JPG
Freezing
Liquid to solid Water releases latent heat during
freezing Molecules in water become trapped in
the crystal structure of ice
http://www.bbc.co.uk/bristol/content/weather/2002/a2z/f/freezing_rain.shtml
Sublimation
Solid to gas, skips liquid phase Dry ice (frozen carbon dioxide)
sublimates, also freezer ice cubes can shrink
Dry ice
http://www.nwoutdoorgrrl.com/images/uploads/1953a.jpg
Deposition
Gas to a solid, opposite of sublimation Frost
http://fizyka.phys.put.poznan.pl/~pieransk/Physics%20Around%20Us/Frost%2001.jpg
Water’s Changes of State Diagram
•Red arrows = absorption of latent heat
•Blue arrows = release of latent heat
Humidity
Water vapor in the air Saturation occurs when air holds all
the water vapor that it is able to hold (at a particular temperature and pressure)
Saturated warm air holds more water than cool saturated air
Relative Humidity
A ratio of the air’s water vapor content compared to the amount it could possibly hold
Expressed as a percent 100% is saturated air
Changes in Relative Humidity
When the amount of water vapor in the air is constant: Lowering temperature increases
relative humidity Raising temperature decreases
relative humidity
Relative Humidity Example
Temperature = 20°C
Temperature = 10°C
Saturation = 14g water vapor
Saturation = 7g water vapor
Actual water vapor = 7g
Actual water vapor = 7g
Relative Humidity = 7g/14g = 50%
Relative humidity = 7g/7g = 100%
•Amount air can possibly hold
•Assume no water is taken or added from the parcel of air
•Relative humidity increases just by lowering temperature
Dew Point
A measure of humidity Dew point is the temperature at which a
quantity of air becomes saturated Below dew point, the air’s excess water
vapor condenses as dew, fog, or clouds High dew points indicate moist air, low dew
points indicate dry air (warm air holds more moisture, etc.)
http://static.flickr.com/38/89402458_8dd93eeb91_m.jpg
Dew on a Spider Web
Water Vapor for Saturation
Data: Table 1 on p. 506, Prentice Hall Earth Science
Water Vapor Needed for Saturation0.
1
0.3
0.75 2 3.
5 5 7
10
14
20
26.5
35
47
0
10
20
30
40
50
-40 -22 -4 14 32 41 50 59 68 77 86 95 104
Temperature (F)
Wat
er V
apo
r C
on
ten
t at
S
atu
rati
on
(g
/kg
)
Sling Psychrometer
Wet bulb
•Device to measure relative humidity, uses two thermometers and wet and dry bulbs
Adiabatic Temperature Changes (Sec 18.2)
Adiabatic heating or cooling Compressing or expanding air changes
temperature Compressed air is warmer ,expanded air
is cooler No heat is added or removed
Adiabatic Cooling
Rising air cools due to decrease in pressure
This adiabatic cooling causes clouds to form
Dry & Wet Adiabatic Rates
Rising air cools 10°C every 1000 meters, termed dry adiabatic cooling rate
After saturation, clouds form, cooling rate drops, termed wet adiabatic (5°C for every 1000 meters)
Wet & Dry Adiabatic Lapse Rates
http://geology.csupomona.edu/drjessey/class/Gsc101/adiabatic.gif
Cloud Formation by Adiabatic Cooling
Cooling = 10°C per 1000 m
Cooling = 5°C per 1000 m
Processes That Lift Air (for cloud creation)
Orographic lifting Frontal wedging Convergence Local convection
Orographic Lifting
When air is forced up the sides of mountains As the air rises, adiabatic cooling causes
cloud formation and precipitation Earth’s rainiest locations are often on the
windward sides of mountain ranges Leeward side of mountain range results in
rain shadow desert
Orographic Processes
Windward side
Leeward side
Orographic Clouds
http://home.online.no/~vteigen/orog2.jpg
Where is the air rising?
Take me to your leader.
http://www.lpl.arizona.edu/~jweirich/orographic_cloud.jpg
Frontal Wedging
Warm and cold air masses collide (fronts)
Warm air is forced up over the cooler air
Rising air cools adiabatically creating clouds, precipitation, and storms
Frontal Wedging and Clouds
Convergence
Occurs when air comes together after moving from different directions
Air rises at the collision and cools adiabatically creating clouds and storms
Convergence and Clouds
Convergence often creates storms in Florida
Local Convection
Differences in reflectivity, e.g., asphalt road versus grassy field create areas of warmer and cooler air
Rising warm air creates clouds Sinking cool air is clear Rising air also referred to as thermals Thermals affect birds and airplanes
Localized Convection
http://raanz.org.nz/wiki/uploads/TM/tmfig041.png
Birds and Thermals
Birds, like this condor, often sail using thermals
http://img1.travelblog.org/Photos/6108/22953/t/108896-Condor-sailing-on-thermals-0.jpg
Convective Cells
Convection cells often develop in stable air creating lumpy clouds (cumulus) separated by clear areas
Fair Weather Cumulus
Where is air rising and sinking?
http://webserv.chatsystems.com/~doswell/chasesums/05jun05_01.JPG
Stability
Temperature inversions, air overhead is warmer, creates stability
Warmer air acts as a cap over the cooler air
Surface air can become stagnant and polluted, dangerous air
Condensation Nuclei
Condensation nuclei are small particles around which water can start to condense
Needed for cloud formation Microscopic dust, smoke, ocean salt,
meteoritic material (space)
Tiny Particles
http://vortex.plymouth.edu/precip/dropsizes2.jpg
CloudsSec 18.3
Visible masses of tiny water droplets or ice crystals suspended in the atmosphere
Latin names Classified according to form (shape)
and height
Form and Height
Three Forms: Cirrus Cumulus Stratus
Heights: Low Middle (alto) High
Cirrus
Latin for “curl of hair” High, white, and thin, resemble
feathers or cotton candy Ice crystals
Cirrus Clouds
Cirrus clouds are high, white, and thin
Cumulus
Latin for “a pile” Rounded, lumpy cloud masses,
resemble cauliflower Normally a flat base and lumpy top Water droplets
Cumulus Clouds
http://www.lotc.com.au/images/scenes/sunset6.jpg
Cumulus clouds are
lumpy
Stratus
Latin for “a layer” Flat, layered, sheet-like clouds Extensive, create gray, dismal
conditions Low stratus clouds - water droplets High stratus clouds - ice crystals Often create halos around the sun or
moon
Stratus Clouds (flat)
Stratus clouds form
flat layers
High Clouds
Usually composed of ice crystals Examples:
Cirrus Cirrostratus Cirrocumulus
Cirrostratus
http://hea-www.harvard.edu/hrc.ARCHIVE/2004/2004243.000000-2004243.240000/SpaceWeather/swpod2004/28aug04/Koeman1.jpg
These clouds produced a halo around the sun
Middle Clouds
Middle clouds have the prefix alto in their names Examples:
Altostratus Altocumulus,
Altocumulus (middle-height, lumpy)
http://meteo.astronomie.cz/pic/ac06.jpg
Altostratus (middle, flat)
http://www.mmem.spschools.org/grade5science/weather/altostratus.jpeg
Low Clouds
Low clouds produce local weather such as rain
Prefix nimbo indicates rain Examples:
Stratus Stratocumulus Nimbostratus
Nimbostratus (rainy, low)
http://met.no/met/met_lex/q_u/skyer/nimbostratus/nimbostratus_bilder/nimbostratus1.jpg
Vertical Clouds
Vertical clouds extend through all of these height levels Example:
Cumulonimbus
Cumulonimbus (vertical)
Cumulonimbus clouds often create powerful storms
http://www.meteorologia.it/Fotoatlante/foto/
Cloud Classification
Cloud Summary Table
High Cirrus Cirrostratus Cirrocumulus
Middle Altostratus Altocumulus
Low Fog Stratus
Nimbostratus
Cumulus
Stratocumulus
C u
m u
l o
n I m
b u
s
Fog
When a cloud develops at ground level Results when the ground cools below
dew point Fog condenses in low areas Also can form by evaporation when
cool air moves over a warmer body of water
Fog in San Francisco
http://i1.trekearth.com/photos/19267/100_0032-5.jpg
Mechanisms of Precipitation
Tiny droplets of airborne moisture collect into larger masses
A one million times change in volume Two processes:
Bergeron process Collision-coalescence process
Bergeron Process
Cold clouds Supercooled droplets form ice crystals Fall as precipitation Supercooling occurs when droplets of
water remain in a liquid state even below the normal freezing temperature (0C)
Bergeron Process Diagram
•Ice crystal grow at the expense of cloud droplets
•Eventually the ice crystal becomes large enough to fall as precipitaiton (snow)
Collision-Coalescence Process
Warm clouds Condensation nuclei collect tiny
droplets of vapor Droplets succumb to gravity and fall as
precipitation
Forms of Precipitation
Function of temperature in lower atmosphere
Forms of Precipitation: Rain, Snow, Sleet, Glaze, Hail
Rain and Snow
Rain is drops of water at least 0.5mm in diameter
Snow (ice crystals) will survive on the ground if surface temp is below 39° F (4° C)
Snow can range from tiny crystals to large, fluffy clumps
Sleet & Glaze
Sleet: Small particles of clear ice, fleet forms when tiny water droplets descend through a colder air layer above the earth’s surface
Glaze: Fall of supercooled water droplets, can create clear ice coating on surface objects (ice storms)
Ice Storms
http://www.bellsystemmemorial.com/images/edward_kelly_photos/red_bank_nj_ice_storm_open_wire.jpg
Ice storms can result in power outages
Hail
Solid lumps of ice produced in cumulonimbus clouds
In these clouds, solid particles of ice move vertically and grow by collectiing supercooled droplets
Onion-like internal layers 5-140mm in size
Hailstones
http://www.crh.noaa.gov/arx/images/hail.061201.jpg
Hail Damage
http://externalweb.nmt.edu/reslife/hail/hail%202/Hail%20Storm%20damage%204.jpg
Acid Rain
Precipitation that forms in clouds containing air pollution
Pollution particles act as condensation nuclei
Acid rain can damage forests and stone structures
http://www.terradaily.com/images/forest-acid-rain-bg.jpg
Acid Rain and Stonework
1908 1968