LECTURE 12 | PRECIPITATION
• Cloud droplets are very small and can easily be kept aloft by weak updrafts
• Cloud droplets fall so slowly that it may take days for it to reach the surface
o it would most likely evaporate before reaching the surface
• Typical droplet is between 10 - 50 μm
o Fall speed is 1 cm/sec (0.02 mph) to 25 cm/sec (0.5 mph)
• A typical rain droplet is 1000 μm
o Fall speed is 650 cm/sec (~15 mph)
• So, droplets must somehow grow big enough that gravity overcomes the effects
of the updrafts...
Cloud & Rain Droplets
• Highly Curved Drops Require Supersaturation to offset evaporation
• “Solute” Effect (material dissolved in water) reduces the amount of
• Curvature Effect is approximately balanced by “Solute” effect to Enable
Nucleation at or near 100% Relative Humidity
• Hygroscopic (water attracting) aerosols enable heterogeneous nucleation closer
• Homogeneous nucleation (no aerosols) require large relative humidities--not
Growth by Condensation
• Initially cloud droplets can grow by condensation of water vapor.
• There is limited water vapor and a lot of condensation nuclei competing for the
• Thus, none will grow very big.
• Other processes are needed for further growth...
Growth in Warm Clouds
• Most clouds in the tropics and some in the middle latitudes are warm clouds -
those with temperatures above freezing.
• In these clouds, the collision-coalescence process causes precipitation.
Collision and Coalescence • Larger drops collect smaller drops & grow
• Must collide and coalesce to grow.
Growth in Cold and Cool Clouds
• Some middle latitude clouds have temperatures below freezing.
• Cold and Cool clouds may consist of ice crystals, supercooled droplets, or a
combination of the two.
• The coexistance of ice and supercooled water droplets is essential for
precipitation formation. The process is called the Bergeron Process.
Supercooled Water droplets & Ice Nuclei
• Supercooled water droplets: water below the melting point but still in liquid form
• Just as condensation nuclei are needed for water to condense, ice nuclei (IN) are
needed for water to freeze.
• IN are less common than CN-must be six-sided structures
• A material’s ability to act as an ice nuclei is temperature dependent.
• Most are effective at temperatures between -4 oC and