Air
contains moisture also known as water vapor.
The water
vapor content of air can be expressed in two different ways, relative humidity
and dew point.
Relative
humidity relates the actual water vapor present in the air to that which could
be present.
It is the
temperature that largely determines the maximum amount of water vapor that the
air can hold.
Warm air
can hold more water vapor than cold air.
Air with
100% relative humidity is said to be saturated.
Air with
less than 100% is unsaturated.
Dew point
is the temperature to which air must be cooled to become saturated by the water
already present in the air.
When water
vapor condenses on large objects, like leaves, windshields, or airplanes, it
will form dew.
When it
condenses on microscopic particles such as salt dust or combustion condensation
nuclei, it will form clouds or fog.
If the
temperature and dew point spread is small and decreasing, condensation is about
to occur.
If the
temperature is above freezing, fog or low clouds will be most likely to
develop.
The growth
rate of precipitation is enhanced by upward currents.
Cloud
particles collide and merge in to a larger drop in the more rapid growth
process.
This
process produces larger precipitation particles and does so more rapidly than
the simple condensation growth process.
Upward
currents also support larger drops.
If wet snow
is encountered at your flight altitude, then the temperature is above freezing
at your present altitude.
Since
melting snow has been encountered, the freezing level must be at a higher
altitude.
The
presence of ice pellets at the surface is evidence that there is freezing rain
at higher altitude.
Rain
falling through colder air may become super cooled and freeze on impact.
Or it may
freeze during its descent falling as ice pellets.