FAQ of Winter Weather
In this section we will highlight your questions concerning meteorology, forecasting, and atmospheric properties. Just write to me at Phil.Schumacher@noaa.gov to ask your question and I'll answer you in this forum. Just to kick things off I've listed some responses to frequently asked questions we receive at our office.
How can it rain when the temperature is below freezing?
All precipitation starts out as snow high in the atmosphere. Occasionally, as weather systems move over the area, a warm layer of air aloft can move in causing the snow to melt into raindrops as it falls through this layer. At the same time a shallow layer of very cold air with temperatures below freezing can exist over the area. The rain can remain in liquid state as it falls through this shallow, cold layer because it does not have time to freeze. However objects on the ground will be below freezing so when the rain hits them it freezes upon impact.
What causes sleet?
Sleet is simply frozen raindrops. Sleet develops much the same way as freezing rain where snow produced high in the atmosphere melts into rain as it falls through a warm layer. However with sleet, a much deeper layer of cold air with temperatures below freezing exists near the surface. Rain falling through this deep layer of cold air has enough time to freeze into solid pellets of ice we call sleet.
I've often heard sometimes it can be too cold to snow. Is this true?
To some extent this is true. Generally, the amount of water vapor in the air (liquid water in its gaseous state) is related to the temperature. When the air temperature drops to its dewpoint temperature there is net condensation and water vapor condenses onto condensation nuclei (microscopic dust particles, salt particles, pollution). However when the atmosphere is very cold there is not as much water vapor available to condensation nuclei so super-cooled cloud drops cannot grow as quickly as with a warmer air mass. Often when temperatures are near or just below freezing snowfall can be heavy and be composed of a high liquid content. As temperatures get colder the snowfall is much lighter and is more fluffy as the amount of available water vapor decreases. Finally as temperatures drop to near zero, the atmosphere becomes "starved" of water vapor: it becomes too cold to snow!
