• a cloud is a dense aggregation of suspended cloud droplets and ice crystals
• a cloud (moisture) droplet is a small (liquid) water droplet that comprises clouds (~0.002cm)

• How Do We Get Precipitation?

1) The Condensation Process:

• a typical cloud droplet is 100X smaller than a typical raindrop - thus, clouds are made up of many, many, many.... small droplets, too small to fall.......clouds are also comprised of ice crystals

• condensation begins on tiny particles called condensation nuclei (bits of dust, salt, & smoke).

• however, the rate of growth of rain drops by this process is too slow ~ 24 hours for growth to medium size rain drop

• thus, other mechanisms are required for the growth of cloud droplets into precipitation-size drops.

(the dominant process for rain formation in warm clouds (temp > 0oC) in tropical latitudes)

collision - bumping together of 2 cloud droplets

coalescence - merging of 2 cloud droplets after collision

• some cloud droplets (those w/larger nuclei) must be larger than others to produce the many collisions that are necessary.

• large drops fall faster than small drops, and thus overtake and collide with smaller drops in their path
• not all collisions result in coalescence
• coalescence is enhanced when the colliding droplets have opposite electrical charges

• the amount of time a droplet spends inside the cloud is also important > strong updrafts & cloud thickness

- relative droplet size

- electric field of the cloud

- cloud thickness

- cloud updrafts

3) Ice-Crystal Process (Bergeron-Findeison Process)

• this is a very important process in mid and high latitudes where clouds extend upwards to where temps are below freezing.

(note - the smaller the amount of pure water, the lower the temperature at which it freezes - since cloud droplets are so small, they require very low temperatures to change them into ice (super-cooled droplets))

Process:

• saturation vapor pressure over water is greater than saturation vapor pressure over ice !!

• thus, a vapor pressure gradient exists between the super cooled water droplets and the ice particles

• and, thus the vapor above the water droplet flows from the water droplet to the ice particle

• where it gets deposited onto the ice particle and increases the size of the ice particle

• meanwhile, in order to replenish the lost water vapor above the the water droplet, some of the water droplet evaporates, & the droplet thus gets smaller

• thus, the ice crystals grow larger at the expense of the water droplets !!

• the growing ice crytals fall downward through the cloud and:

snowflakes - aggregates of ice crystals; whichmay reach the ground as snow, or:

(2) they may melt before reaching the ground, and thus fall as rain.

thus, a lot of the rain in the middle latitudes - even in summer - starts out as snow
 
 

Cloud Types/Classes

Low - ground > 2,000 meters - stratus & cumulus - comprised of water droplets

Middle - 2,000 m > 6,000 m - "alto" - comprised of water droplets & ice crystals

High - 6,000 m > 13,000 m - "cirrus" - comprised of ice crystals

Vertically Developed - surface > 13,000 m - cumulonimbus (t'storm cloud)
 

Fog - cloud in contact with the ground

• the air is saturated

"advection fog" - air migrates in and is cooled to its dew point by an underlying cooler surface - e.g., snow surface, a cool water body, cool ocean current

"evaporation fog" - cool air moves over a warm body if water

"radiation fog" - at night the ground surface is chilled by radiational cooling and the air layer above is cooled to its dew point (clear, calm nights)