A. Condensation and Cloud Formation

• 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)

• 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.

B. Precipitation Formation Processes
 

1) Collision-Coalescence Process
 

• is 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

Summary: Important Elements in the Formation of Raindrops via the "Collision-Coalesence" Process:
 

- relative droplet size

- electric field of the cloud

- cloud thickness

- cloud updrafts

2) 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.

• In the presence of super-cooled cloud droplets (temps less than 0 ^oC), ice crystals grow rapidly.

• 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)

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

• (i.o.w., the air can "hold" more water vapor over the watersurface than over ice surface)
• 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

• the ice crystals may collide w/other ice crystals and stick together to form snowflakes (aggregates of ice crystals), which may reach the ground as snow, or 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

            
The Ice-Crystal Process
 

C. Measuring Precipitation

                
     Standard Rain Guage

• Rain is relatively easy to measure, and any open container having a consistent cross section throughout can be a rain guage

• However, in practice, more sophisticated devices are used so as to measure smaller amounts of rainfall more accurately, and to reduce losses from evaporation

• The "standard rain guage" has a diameter  of about 20 cm (8 inches) at the top.  Once the water is captured, a funnel conducts the rain into a cylindrical measuring  tube that has a cross sectional area only 1/10 that of the receiver.

• As a result, rainfall depth is magnified ten times, which allows for accurate measurements to the nearest 0.025 cm (0.01 inches)

• When rainfall is less than 0.025 cm, it is reported as a trace of precipitation

D. 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)

Cloud Levels

    Basic Cloud Types

Fair Weather Cumulus

Altocumulus

Cirrocumulus

 Cirrus
 

Nimbostratus

E. Fog

• is a cloud in contact with the ground

• the air is saturated

(1) Types:
 

• "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)

                
Advection and Radiation Fog