Clouds don't always produce precipitation. We know this by experience. The cloud particles must grow large enough so that gravity is stronger than the force exerted by the cloud updraft.
Needed high relative humidity and CCN.
Curvature effect: when air is saturated with respect to a flat surface, it is unsaturated over a curved surface. The smaller the droplet the bigger the effect - smaller droplets are easier to evaporate. Reason most CCN are greater than 0.2 microns.
Solute Effect. a droplet containing a hygroscopic nucleus can be in equilibrium with its environment when the relative humidity is lower than 100%. The dissolving of salt reduces the relative humidity necessary for the onset of condensations. At some point, the droplet gets dilute and its like not having salt, so growth slows down.
It takes a couple of DAYS for droplets to get to raindrop size by condensation alone. How to create a raindrop?
1) COLLISION-COALESCENCE
2) ICE-CRYSTAL PROCESS
Important in warm clouds (cloud temperature is greater than 0C)
First off we need enough water vapor in the atmosphere. Given that, growth by collision and coalescence is determined by:
1. relative droplet size
2. electric charge of droplets and electric field in the cloud
3. the cloud thickness
4. the updrafts of the cloud
Stratus produce steady rain. Cumulonimbus - big rain drops and heavy rain.
Extremely important in cold clouds - midlatitude
Supercooled water - liquid water but at a temperature below freezing (-30C)
ICE NUCLEI
1. Deposition Nuclei
2. Freezing Nuclei
3. Contact Nuclei
Ice crystals might collide with supercooled droplets, which freeze on contact and stick together - ACCRETION or RIMING - forms a grauple particle.
Ice a liquid water together in a cloud - ice crystals grow at the expense of the surrounding water droplets. The saturation vapor pressure over a water surface is greater than that over an ice surface at the same subfreezing temperature. (Bergeron Process)
Cloud seeding - used to disperse supercooled fogs.
Cirrus clouds may also seed lower layer clouds (Fall streaks - Mares tails).
In cloud clouds, the snow flake may melt on its way down to the surface (Virga, melting layer, snow darker as it scatters more).
Precipitation falling out the base of a cloud can be modified by the atmospheric conditions encountered beneath the cloud.
Rain
Drizzle, smaller drops (.5 mm) more uniform in size
Snow - Crystals have six sides, common forms are Plate, Column and Dendrite. Shape depends on the temperature actual vapor pressure the crystals grow in. Dendrite structures are preferred in the -12 to -16C temperature range. Maximum difference in vapor pressure between ice and water.
Snow flakes are aggregates of ice crystals.
Freezing level much higher in summer than winter. Fall streaks
Good insulator, dry snow sounds crunchy.
Virga - rain falling out of a cloud by evaporating before reaching the surface
Vertical Temperature Profiles for snow, sleet, freezing rain and rain
Hail - accretion accumulation of supercooled liquid droplets on a frozen particle:
hail cross section - clear and opaque shells : water doesn't freeze quickly enough then air bubbles become trapped causing the opaque shells - this occurs if the cloud is very cold. If the water freezes slowly, the air escapes and the ice looks clear.