Winter Precipitation Types
Types of Winter Precipitation
In a winter storm affecting North Carolina, it is common to see multiple precipitation types. There are many reasons for this, but it is largely due to the interaction between cold, low level air trapped by the Appalachian Mountains with warm, higher level moist air being transported into the state from the Gulf of Mexico and/or the Atlantic Ocean. The different types of precipitation seen in winter weather are described below, and the figures to the right illustrate the atmospheric thermal structure that accompanies it.
Snow
Precipitation begins as snow and if it doesn’t encounter any layers of air that are above freezing before reaching the ground, it will fall to the surface in the form of snow.
Notice in the figure to the right that the blue 32°F line is never crossed, which means that the atmospheric temperature (dashed green line) remains below freezing from the cloud to the ground. This allows the snowflake to fall without melting.
Sleet
If the snowflakes encounter a shallow layer of warm, above freezing air with below freezing air through the rest of the column, then it will partially melt and refreeze into a sleet pellet before reaching the ground.
Notice in the figure to the right that the dashed green line representing the atmospheric temperature briefly crosses the blue 32°F line, which indicates that part of the atmosphere is above freezing and causes the snowflake to partially melt before refreezing into a sleet pellet in the subfreezing layer extending to the surface.
Freezing Rain
If the snow falls into a deep layer of above freezing air with only a shallow layer of below freezing air located at the surface (which is often the case with cold air damming), then the liquid droplets will freeze on contact with any objects (such as trees, power lines, and roads) at the surface that are below freezing.
Notice the difference in the figure to the right: the dashed green line representing the atmospheric temperature crosses the blue 32°F line and remains on the above freezing side through a large depth of the atmosphere. As a result, the snow flake completely melts before it encounters the thin, below freezing layer at the surface.
Cold Rain
If precipitation falls through a large depth of the atmosphere frozen but encounters a relatively deep layer of above freezing air at the surface, then the frozen precipitation may completely melt and have no opportunity to refreeze. Thus, what traveled almost the entire trip to the ground as snow may end up falling as a cold rain instead of anything frozen or freezing!
Liquid Equivalency
When determining the amount of snow that will fall, the atmospheric temperature profile (as illustrated above) must be considered. The more warm layers encountered in a profile, the more the snow melts, and this makes for a lower snow to liquid equivalency. If some of the snow is melting into liquid, then the snowfall is not as efficient as one where no snow melts on its journey to the ground. This plays a crucial role in determining how much snow accumulation will occur at a particular location during a snow event.
The ratio compares inches of snow to inches of liquid precipitation. On average, 10 inches of snow is equivalent to one inch of liquid precipitation, but this is all dependent on the temperature of the entire atmosphere, as the charts above illustrate.
Physical Processes Governing Precipitation Type
Evaporative cooling: When precipitation falls into a very dry layer of air it will evaporate. Evaporation is a cooling process, meaning it removes heat from the air which allows the temperature to drop. This is an important process for NC winter weather, as often times temperatures at the onset of precipitation are above freezing but through evaporative cooling, fall below freezing. The rate and amount of cooling that occurs is directly related to the amount of dry air present, and the rate of precipitation.
Latent heat release: Freezing rain is just rain until it makes contact with an object at the surface. At that point, if the object’s temperature is below freezing, the rain will undergo a phase change from liquid to ice. This phase change releases heat, called latent heat, to the surrounding air. If enough evaporative cooling from dry air doesn’t occur to offset this process, eventually the surface temperature will rise above freezing and anything that falls will remain in the liquid form.