ENVIR SCI 1A03 February 10, 2014
Precipitation and Snow
What is Precipitation?
• Any form of water that falls from a cloud and reaches from the ground.
• Either liquid or solid water
• Driven by the same processes as the same processes of cloud formation
• Can occur very quickly (within a few hours of cloud formation)
How does Precipitation form?
• In warm clouds (greater than 0 deg): water droplets condense.
• Air mass cools at MALR as it rises within the cloud
• The droplets fall because the of the updraft and gravity imbalance.
o As the mass of the droplet increases, the pull of gravity outweighs
• In cold clouds: ice crystals form and grow
o Initially at the base of the cloud, the water cools (supercool)
extremely fast, so much so that it does not actually freeze and
eventually they coalesce and transfers and grows the ice crystals
• Collision and Coalescence
o Constant speed achieved when air resistance equals gravity:
o As the larger droplets are falling, they will accumulate the smaller
droplets and fuse with the larger droplets and thus grown and then
reach the ground surface
• Cold Clouds: the base of cloud to top of cloud and below 0 deg.
• Cool Clouds: base is just above 0 deg. And the top is below.