Clouds
Learning Objectives
Lifting
Condensation Nuclei
- water resists forming drop in atmosphere, rather condenses onto a surface
- very small particles stay suspended in the air
- .1 - 10 microns in size
- composed of:
salt: this is important even 100's of miles inland
dust: usually clay sized sedimentary particles
volcanic ash
pollutants: smoke, acid particulate
Hydrophobic particles: resist having condensate form on them
composed of covalent molecules such as oil and gas
Hygroscopic particles: attract condensate
composed of ionic particles such as salts and acids, also dust and ash
Cloud Droplets: condensation forms water droplets, these very small- 20 microns (see handout)
Clouds
classified by both altitude and appearance
cloud base: bottom of cloud
cloud top: point at which air stopped rising because it has become stable
Altitude
High Clouds:
base (bottom) at 23,000 ft [7000 m ]
made up of ice crystals
Middle Clouds:
base between 6600 and 23,000 ft [2000 and 7000 m]
composed of supercooled water or ice or both
Low Clouds:
base from surface to 6600 ft [2000 m]
composed of water
Appearance
Cumuliform Clouds: Cumulus Clouds
clouds that exhibit vertical development
puffy cottony white
usually have a somewhat flat bottom and are characterized by vertical development
can produce weather
Stratus Clouds:
layered sheet like look
caused by uplift of whole region of air (front)
usually produce weather or predict weather is coming
Cirrus Clouds:
wispy
not weather producers
Cloud Prefixes:
nimbus: rain producer
alto: altitude (usually refers to 12,000 - 20,000 ft)
others: see text
Can combine cloud names (see handout)
Common Clouds:
cirrus: high wispy clouds, not weather producers, formed from ice crystals
cirrocumulus: small puffy and high, “mackerel sky”
cirrostratus: high sheet like, no real “shape” visible, can indicate an approaching front “colors” sky white
altocumulus: puffy and usually small, middle altitude resembles waves
altostratus: “gray sky” at middle altitude, sun dimly visible, no real cloud “shape” visible
moves in ahead of front
stratus: low sheets, “gray sky”, differs from altostratus in that sun not visible
uniform-little to no cloud shape distinguishable and may have a drizzle produced
nimbostratus: stratus cloud with steady rain or snow falling from it, usually darker “gray” then a plain stratus
usually associated with a frontal system
cumulus: cottony and puffy
cumulus humilis: fair weather cumulus, blue sky with white clouds
stratocumulus: thick concentration of cumulus clouds, “piled up” look with very little blue sky or they can be isolated flat clouds without the "popcorn" appearance
cumulonimbus: EXTREME VERTICAL DEVELOPMENT: thunderheads-can get VERY high
strong up and down drafts
anvil cloud:top of cumulonimbus is at bottom of stratosphere and spreads outward rather than into stratosphere
mammatus cloud: cold air sinking in blobs under the anvil-looks like an udder
lenticular: formed from horizontal air moving on top and bottom
some VERY cool lenticular cloud pictures
contrails: air pulled into jet engine and heated, when heated air exhausted into colder air condensation happens
long contrails=moist air at altitude
short contrails= dry air at altitude
Fog: often called a cloud on the ground caused by air cooling past dew point, condensation
Radiation Fog: also called
ground fog calm night, ground cools rapidly and air next to ground cooled as well, condensation occurs, fog produced
common near marshes in fall and spring
Valley Fog:
cool air sinks in valleys, condensation and fog occur
Advection Fog:
warm air flows over cold surface, common over large water bodies
In San Francisco: cold ocean, warm air
also common where 2 ocean currents meet
Sea Smoke
cold air blows over warmer ocean, steam rising from ponds, pools ect in fall
Upslope Fog
common in West, warm air moves up slope of Rockies cools past dew point, fog
fog does not “burn off” rather the sun heats the air and water that had condensed into drops goes back to a vapor
PRECIPITATION:
when cloud drops become big enough that they can no longer remain suspended they fall to Earth as precipitation
drops become big enough through:
Collision and Coalescence
cloud droplets collide and the adhesive property of water causes them to merge, strength of updrafts determine size of drop
Bergeron Process
ice crystals attract supercooled water, when too heavy the ice falls and if lower atmosphere is warm then ice melts into raindrop... much precipitation starts this way
Types of Precipitation:
Drizzle: .2 - .5 mm diameter
Rain: larger than .2mm in diameter
Virga: rain evaporates before it hits the ground
Snow: water molecules have formed ice crystal, shape of crystal depends on temperature
what is received on ground depends on temp profile through atmosphere
10 in of snow melts to 1 in of rain
Sleet: snow melts to water drops and then refreezes, hits groud as an ice particle
Freezing Rain: snow melts to water drops, the surface is cold so when drop its ground it deposits ice (freezes), usually referred to as an ice storm
rime ice: ice coating formed when supercooled fog deposits on a surface
stratus clouds usually produce lots of snow and rain
hail: product of a thunderstorm
Cloud Seeding:
method of encouraging large cloud drops to form- this encourages precipitation
silver iodide acts as condensation nuclei
