PrecipitationChapter 7

Objectives

• Differentiate several ways that precipitation is formed

• Distinguish between several types of precipitation

General concept

• There are several forces acting on a water droplet or ice crystal in a cloud– Winds

– Atmospheric stability

– Gravity

– Drag (friction)

• When a droplet reaches a certain critical mass the force of gravity will exceed the other forces and precipitation will fall

• Rain drops are 100X larger than cloud droplets

Formation of precipitation

1. Condensation and deposition

2. Collision and coalescence

3. The Bergeron process

Warm air

Cold air

Condensation and deposition

• As long as the rate of condensation exceeds the rate of evaporation water will be deposited and accumulate on condensation nuclei

• Droplets grow very slowly

• Rarely produces rain drops

Collision and coalescence

• As water droplets fall through a cloud, the larger droplets fall faster than the smaller ones, thus they collide

• If the droplets coalesce a larger drop is formed

• If it gets big enough rain will fall

Collision efficiency

A drop must be larger than the other drops to be an efficient collider

If the drop is too big it will be less efficient, because it creates high pressure that pushes small drops out of the way

Cold and cool clouds

• Cold < 00C throughout • Cool Part is < 00C

Part is > 00C

The Bergeron processVapor pressure over ice is less than vapor

pressure over water of the same temperature

Thus, water molecules move from water to ice and freeze on the ice

If the crystals get large enough they will fall

If they fall through cold air they will stay frozen and it will snow

If they fall through warm air they will melt and it will rain

Growth of ice crystals

• Riming – super-cooled water freezes onto ice crystals

• Aggregation – Ice crystals join together to form large snow flakes. Happens more readily if ice crystals have a thin coating of liquid

Types of precipitation

snow

rain

graupelsleet

freezing rainhail

Rain

Can originate as liquid drops or ice. Ice melts as it falls through warmer air

Shape of rain drops

Snow• Ice crystals grow in a cloud. Variety of

shapes depending on temperature

Graupel

• Ice crystals grow by riming

• Contain air pockets

• Has a spongy texture

Sleet• Ice crystals melt as they fall through warn air

aloft. Then water drops freeze as they fall through cold air below and hit the surface as ice pellets

Freezing Rain

• Ice crystals fall through warm air aloft and melt, Then liquid water falls through air at nearly 00C and freezes when it hits the cold (00C)ground

Hail

• Strong updrafts in a cloud force ice particles upward

• As they move up and down in the cloud hail stones accumulate layers of ice

• Eventually they get too heavy and fall

Distribution of precipitation in the US

Why?

Distribution of snowfall in the US

Why?

Any questions?

Have a great day!