How Precipitation Forms: The Bergeron Process pp

Why do some clouds produce rain and others don’t? How Precipitation Forms Bergeron process: Collision-coalescence process

Recall… All weather occurs in the troposphere Temperatures in the upper troposphere can be -50°C Clouds contain: –Water vapor (gas) –cloud droplets (liquid) –Ice crystals (solid) Water vapor needs a SURFACE to stick to (condensation nuclei)

At temperatures above 0°C water droplets exist Rain won’t form (fog might) At temperatures between 0 and -10°C SUPER-COOLED WATER exists Rain won’t form At temps. between -10 and -20 super-cooled water and ice crystals At temps below -20°C only ice crystals exist (cirrus clouds) Rain won’t form

Cloud Droplets are VERY small. They fall VERY slow. They evaporate Before they reach the ground How big does it have to get to reach the ground as rain?

Rain begins in the UPPER tropopause As snowflakes!

Fact #1 Super-cooled liquid Cloud droplets do NOT freeze at 0°C Pure water has to cool to -40°C to freeze! But…. Clouds droplets will freeze on contact when it impacts another object Ex: freezing rain An ice crystal will act as a freezing nuclei for cloud droplets to ‘stick to’

Fact #2: Clouds need a source of water to form. H 2 O molecules are more tightly bonded (held) in ice crystals than in water droplets because they are a solid. So… H 2 O molecules in water droplets will evaporate (escape) easier to form clouds

Bergeron Process Ice crystals collect water molecules and grow larger while cloud droplets evaporate and get smaller.

Forces Acting on Cloud Droplets and Raindrops

Equato r In the tropics, The temperature at the tropopause are too warm to form precipitation by the for the Bergeron process By what method does precipitation form in the tropics?

The Collision- Coalescence Process

Large raindrops reach maximum size (5mm) and maximum velocity (20 mph) Then they break-up into smaller drops

Bell work: TRUE / FALSE 1. Weather occurs in the troposphere and stratosphere. 2. In the atmosphere, water droplets freeze at 0° C 3. At temps. of -20°C, clouds are made of only ice crystals. 4. A tiny cloud droplet can act as a ‘freezing nuclei’ for other cloud droplets to stick to. 5. According to the Bergeron process, ice crystal grow larger while cloud droplets get smaller. 6. As the diameter of the falling cloud droplet INCREASES, the velocity DECREASES. False True False True False