• The temperature of the air that precipitation falls through is what determines its form.

• In fact, precipitation always starts out within the cloud as either liquid drops or snow crystals.
• It is the temperature and winds beneath the cloud that will determine whether this precipitation will change into one of many forms that eventually hit the earth's surface.

• Meterologists define rain as liquid water drops that have a diameter of atleast .5 millimeters.  Drops smaller than this are considered drizzle.

• Drizzle is defined as precipitation that consists of fine uniform liquid drops, which are less than .5 millimeters in diameter.  Drizzle usually falls from low level clouds, called Stratus.  It is a mist that falls out of lower clouds when there are not sufficient updrafts to keep the smaller water droplets up within the cloud.

• It can also form as larger raindrops fall into dry air and evaporate, becoming smaller until they are less than .5mm in diameter.

Below is a diagram that meterologists use to show the vertical structure of the atmosphere.  This is very helpful in determining what form precipitation will take on as it falls through a layer with specific temperature and wind profiles.  This profile was made by a balloon that carries sensors up into the atmosphere.  This diagram is called a Skew T Log P Diagram and is also known as a "Sounding".

The sounding below is a typical rain sounding.  The air is moist, and this is indicated by the dashed black line showing that the dewpoint temperature and actual air temperature are almost the same throughout the lower levels of the atmosphere*.  [The dewpoint temperature is the temperature at which water vapor will condense and the air will be saturated, that is to say it will hold all the water vapor that it can at that particular temperature.  Any extra water vapor will be condensed into liquid water and thus, we have clouds & precipitation!]  Notice that the maroontemperature line is above freezing at levels beneath the cloud layer, this will cause any preciptation to melt and fall as rain.  It may help to picture the precipation following the maroon line all the way to the surface, thus encountering a changing environmental temperature as it falls.
 
 

*If the layers beneath the cloud were drier, (if the black dashed line were farther to the left of the maroon line), then the falling precipitation may evaporate totally before hitting the surface.  When this happens it is called Virga, sometimes virga can be seen as a shaft or darker area of precipitation that seems to be hanging overhead.