Clouds,Thunderstorms, Fog, Precipitation
General Outline:
I. Cloud formation
II. Definition
III. Classification
a. Shape classes
b. Altitude classes
IV. Thunderstorms and hail
V. Fog types
VI. Precipitation
a. Cold cloud process
b. Warm cloud process
I. Cloud formation
Large mass of air rises, cools to dew point
Condensation (or deposition) occurs on condensation nuclei : salt, dust, soil, pollutants
Elevation at which air becomes saturated is lifting condensation level (flat base of clouds)
II. Definition
A cloud is a mass of suspended water, supercooled water and/or ice particles.
particles 20-50 micrometers
Water is supercooled : liquid below freezing
< -40oC (-40°F): all ice crystals
< -20oC: predominantly ice crystals
-12o to -20o C: mixture of ice and supercooled water
> -12oC (10°F): all water
III. Cloud classification (according to shape and altitude)
a. Shape Classes:
Flat: stratiform
Globular: cumuliform
b. Altitude Classes:
1)High Cloud (cirro-): ice crystal clouds (> 6000 m (20,000 ft.))
CIRRUS, CIRROSTRATUS (halo), CIRROCUMULUS
2)Middle Cloud (alto-) (3000 – 6000 m (10,000 - 20,000 ft.))
ALTOSTRATUS, ALTOCUMULUS
3) Low Cloud (stratus) (<3000 m (10,000 ft))
STRATUS, NIMBOSTRATUS, STRATOCUMULUS
4) Vertically developed clouds (CUMULONIMBUS)
IV. Thunderstorms Convective storm accompanied by lightning.
Parcels of air heat and rise, drawing in air from the environment, creating strong updrafts (40 mph).
Falling precipitation produces downdrafts.
Hail : Hard balls of ice, at least 5 mm across; made of layers of ice.
Hailstones are caught in updraft in thunderstorm cloud; they begin to fall, but are caught again in
updraft. They re-ascend, the outer part freezes
…and this continues
V. Fog
a) Radiation fog: radiational cooling at night
b) Advection fog: warm moist air moves over cold surface
c) Sea Fog: Wet marine layer comes in contact with air over colder ocean water
VI. PRECIPITATION
(rain , snow, sleet, hail)
Process: Large parcel of air rises; Temperature of parcel cools to dew point;Cloud droplets grow large enough to
fall.
Why do cloud droplets need to grow?
How cloud droplets grow:
a. Mid- and High latitudes: “Cold Clouds”
(tops colder than 0°C)
Ice crystals grow by deposition and supercooled water droplets shrink by evaporation.
(All precipitation formed by this method starts as snow).
b. Low latitudes: “Warm Clouds” (tops warmer than -15°C)
Collision and coalescence: big drops collect small drops and drops grow