Why do some storms produce snow, while others produce sleet or other forms of wintry precipitation? The type of precipitation that will fall from clouds in any given storm is largely dependent on what is called the vertical temperature profile.
A vertical temperature profile is a series of temperature measurements taken at various levels of the troposphere. Meteorologists send up weather balloons twice each day to detect the air temperatures at different elevations in the troposphere.
During the winter months, access to temperature data reported back from weather balloons helps meteorologists to more accurately forecast what type of precipitation is on the way. Temperature profiles, however, can change completely due to many factors in the troposphere.
For instance, if a low pressure system is moving into an area, cold temperatures at the earth's surface up through 7,000 feet may also be warming. This weather factor is called warm advection.
Warm advection and cold advection have an effect on the type of precipitation that falls and whether it will be frozen. Although vertical temperature profiles are immensely useful in helping meteorologists determine what type of precipitation may fall, temperatures can change so rapidly that forecasts based on these profiles alone can be inaccurate.
Relative humidity also plays a role in the type of precipitation a winter storm will produce. Relative humidity is the ratio of the amount of moisture in the air to the amount of moisture the air can actually hold at a certain temperature.
The relative humidity is often less than 100% between cloud base and the earth's surface but fully saturated at the cloud level. When this occurs, some precipitation that falls evaporates, then cools the surrounding air. This changes the temperature profile that, in turn, may change the type of precipitation that will reach the ground.
Meteorologists stay aware of the effects of both warm and cold advection and of differences in relative humidity at various levels of the troposphere. In borderline situations, slight changes in temperature can mean the difference between sleet, snow, rain or freezing rain.
