Weather question

[ticka1]

This is partialy in regards to Sparky's upcoming Ice Storm event. My question is what conditions in the atmosphere cause ice storms - rather than snowstorms or rain? And how does sleet figure into this scenario too?

If anyone can answer - I would like to know. Also, this is a question I get from my 9 year old too.....

[jeffl]

The temperature profile of the atmosphere determines the kind of precipitation one gets during an event.

The two easiest are rain and snow.

If the air column from the ground to about 5,000ft is above freezing then rain will fall. The snow will melt on the way to the ground in the large above freezing level.

If the entire air column is below 32 F then snow will fall all the way to the ground.

The problem is with sleet and freezing rain.

To get freezing rain you need a cold surface layer then a warm layer and then the typical decrease above the warm layer. The surface cold layer can vary with depth from only a few hundred feet to around 2,000 ft thick. In this situation the snow melts through the warm layer and then does not have time to re-freeze before it reaches the ground. The result is liquid striking the ground and then freezing on contact with roads, trees, power lines ect. producing a glaze of ice.

For sleet you have nearly the same setup as freezing rain, but the difference is that the surface cold layer is much deeper say around 2000 to 4000 feet thick. This allows enough of a cold layer for the melted snow to re-freeze into small pellets before they reach the surface.

Freezing rain and sleet are most common north of a warm frontal boundary where warm moist air is flowing NNE ahead of a low pressure system and overruning the cold air at the surface.

Forecasting P-type is many times difficult because you can never be positive what kind of profile you will be dealing with during a precip event. Mnay times the profile changes during the event and that is when you get rain changing to snow or freezing rain changing to rain ect. As I said before rain and snow are straight forward, but freezing rain and sleet are much more difficult.

You can visit this link for a graphic explantation:
http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/cld/prcp/zr/fcst/fcst.rxml