By David Boughan on Wednesday, November 21, 2007 - 08:36 pm: Edit Post
In past weather dsscussion you explianed how it can sometimes snow when the temp is above freezing and for that matter rain when the temp is below freezing but it seems to me that a 34 degree day in late November will often bring snow while a 34 degree day in January seems to always bring rain or is that just my imagination?
By Eric Koehler on Wednesday, November 21, 2007 - 11:39 pm: Edit Post
I thought it had to do with the humidity?
By SnowFan470 on Thursday, November 22, 2007 - 02:29 am: Edit Post
I think it has to do with the 850 and 1000 mb height temperatures. But, since this is John's site, I'll let him answer this one and see what he has to say!
By John Dee on Thursday, November 22, 2007 - 10:53 am: Edit Post
Well, it has to do with the humidity as well as temperatures throughout the atmosphere that the snow is falling through.
Basically the first thing you need is for the temps to be cold enough to have the water vapor go from it's gaseous state to a solid, called deposition. Once the particle starts to fall, it cannot become a snowflake if it is already a liquid (rain). If there is enough cold air to freeze the drop of rain, you end up with an ice pellet.
Once the flake has formed and is falling, then quite simply you cannot have so much warm air that it melts before reaching the ground. The layer of warmer air can be anywhere between the ground and up to where the snowflake is formed, that does not matter, it just cannot be too thick of a warm layer.
Humidity also plays a role in that if the air is dry enough, then as the snowflake falls and starts to melt, there will also be some evaporation that takes place. That evaporation takes energy, so the air actually cools as it gives up that energy. You can thus have the air cool to the point where it no longer is melting the snowflake.
So I guess the bottom line is as long as the hygrometeor (any form of precip that is falling from the cloud) starts out as a snowflake in the first place and does not encounter enough warm air to completely melt it, you will end up with snow.
I have seen it snow at 42 degrees and rain at 22!