Understanding Why and How It Rains, Snows, Sleets, Or Hails

Clouds can form easier if there are condensation nuclei upon which the droplets can form.  Electrically charged particles are particularly efficient in attracting water vapor and getting the condensation going.  Having nuclei that mimic the shape of an ice crystal can enhance ice formation.  So, dirty air is better for forming clouds, and the right particles can aid in making ice.

If a cloud forms with both water and ice, there is a real possibility of precipitation falling from the cloud.  One interesting question is how can a cloud have both water and ice occurring in the same area?  The answer is that freezing, and we normally think of the temperature at which water freezes as fixed, depends on the water having a flat surface.  If the water is in the form of a droplet, round, the surface tension allows the water to remain liquid well below the normal freezing point, with smaller droplets being able to stay in the liquid state well below the larger droplets.  Very small droplets can remain liquid to about forty degrees below zero.

So, a cloud can form where colder air is mixing with warmer air lite above a cold dome of air, or in air that is chilled, or that is warmed.  This allows both water and ice to occur.  One way this happens is an ice cloud is heated on the top by the sun, which melts some of the ice.  Another way is to have ice form in a water cloud as the sun sets and the temperature drops.  

With both ice and liquid water, along with water vapor, the cloud will become supersaturated for the ice, while it will become unsaturated for liquid water.  The ice crystals will grow.  Normally, air cannot hold enough water vapor for the ice crystals to become heavy enough to fall, but the liquid water evaporates, resupplying the air with water vapor.  This allows the ice crystals to become heavy enough to fall as snow.

As the snow falls it often enters warmer air.  If that happens it could easily melt on the way down.  This melted snow is then known as rain.  If the rain enters colder air on the way down, it can freeze, but cannot reform the six-sided snowflake structure.  So, it will fall as a pellet of ice, sleet.

Sleet can be clear or it can be white and appear to be snow.  If the snowflake melts completely before the freezing occurs a clear ice pellet will form.  If the snowflake partially melts the ice pellet will encase the left over part of the snowflake, and the sleet will be white.

Hail cannot fall from a nonconvective cloud.

In convection, air is forced to rise, either by a hill or mountain, by a dome of cold air, or by a heat source.  As the air rises its temperature decreases, as does the pressure, so the air loses its ability to hold all of its water vapor.  Droplets form, and as the air continues to rise, the droplets grow.  If the air rises high enough the temperature will fall low enough to freeze the droplets, which allows even more growth.  This normally does not produce snow.  The ice particles that form is like sleet, but usually larger than sleet pellets.  When the ice gets large enough, it falls.  Often it melts on the way to the ground and falls as rain.  If it melts and refreezes it is called sleet.  But, it the ice particles are large enough to make it to the ground as ice, without melting along the way, it is hail.

It is possible to have rain from a cloud that does not contain ice, but this is not the norm.  The mechanism for this is that a convective cloud can have droplets heavy enough to fall if they were not being pushed upward by the rising air.  At sunset the thermals can fail taking away the support, and these large droplets can produce a light rain for a few minutes.

Often a cloud can have precipitation fall from it and evaporate before reaching the ground.  This is called virga.

The way cloud seeding works is to introduce a crystal with the same shape and lattice structure and spacing as ice into a cloud cold enough to have ice.  This is done if the ice in sufficient quantity is not naturally forming.  The water vapor condenses on these crystals, hence ice is introduced into the cloud.  This starts the mechanism for producing precipitation.

Silver iodide is the first choice for cloud seding.

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