sleet when above freezing

[zwerob]

I know for freezing rain and sleet, you have to have a warm air layer overtaking freezing temperatures above the  ground. The flakes above melt to rain drops and freeze into ice pellets or freezing rain depending on the layer of warm air. W/O freezing  temps above ground level to freeze rain into ice pellets/sleet, how does precipitation come down as sleet? Can someone go into detail about what fronts are involved (warm/occluded/stationary/cold), wind direction (Sn unlikely with SE winds in this region according to Bolaris) and how this affect the type of precipitation? Links to visuals would be cool. Thanks, Rob

[famartin]

I know for freezing rain and sleet, you have to have a warm air layer overtaking freezing temperatures above the  ground. The flakes above melt to rain drops and freeze into ice pellets or freezing rain depending on the layer of warm air. W/O freezing  temps above ground level to freeze rain into ice pellets/sleet, how does precipitation come down as sleet? Can someone go into detail about what fronts are involved (warm/occluded/stationary/cold), wind direction (Sn unlikely with SE winds in this region according to Bolaris) and how this affect the type of precipitation? Links to visuals would be cool. Thanks, Rob

 

First, its quite possible to have a cold layer sandwiched between two warm layers... i.e., a melting layer, a re-freezing layer, and then above freezing right at the ground. 

 

Second, a very dry layer below the warm layer can evaporationally cool raindrops below freezing, causing them to re-freeze. 

 

Sleet definitely can and has happened on southeast winds.  I'd say winds with a significant west to northwest component are the least likely to result in sleet, but I don't have facts and figures to back me up.  Just experience.  Usually, anything pellety and frozen that you see with west to northwest winds is actually graupel, not sleet.

[irishbri74]

Usually,

Warm fronts are the boundary responsible for mixed precip events, as warm air surging into the cold air rises. The colder, denser air stays below it. Not always, but a majority of events.

[kdupeswx]

Freezing rain falls frozen, melts (in a warm layer) and refreezes on contact with the surface. The bottom cold layer is too shallow to freeze before making contact with the ground. Sleet falls falls frozen, melts, and refreezes before making contact with the ground. The cold layer is larger than that of freezing rain, which is why it has time to freeze again before it hits the ground. Upper air soundings (skew-t's) are a great way to tell what kind of precip will fall.