Relationship between cold weather and snow

[CosmicEye]

I live in the DC metro area and we recently had snow that lasted all day (none stuck to the roads, booo). But I didnt understand that it was still snowing at well above freezing. I have a digital thermometer outside of my window and it read ~41 degrees F at its highest and never dropped below 32 until around midnight after the snow passed. The %error is only rated at +/- 2 degrees F but will still be above freezing. If I remember right, humidity on weather.com was ~65% as well.

Ive seen this before and even rain when it was in the high 20's. We must have been under a low pressure system which brought all the moisture. Does atm pressure matter that greatly? What causes it to snow above freezing or rain below? I can't think of anything else that I know of that could cause this. Just curious but any help is appreciated.

 

[turbo]

Air is not monolithic. If you have a layer of cold air overlying a lower-level air mass that is a bit warmer/wetter, you can get some pretty impressive snow-falls.

 

[Ryan_m_b]

Above temperatures of -35C snowflakes need ice nucleators to form. These are small particles that water ice can form around. Interestingly research has shown that quite a lot of nucleators are bacteria any it has been speculated that bacteria play a vital (though not clear) role in the hydrological cycle.

 

[CosmicEye]

^ that's a cool fact.

Thanks but I don't think the question is quite answered. How does it still snow well above freezing as I witnessed all that day? Maybe a warm air mass at ground level that is small enough to not melt the snow in time before hitting the ground? I am guessing at this point.

If it helps they were the real fat soft snowflakes that looked like baseballs falling from the sky, but it was snow.

 

[Ryan_m_b]

^ that's a cool fact.

Thanks but I don't think the question is quite answered. How does it still snow well above freezing as I witnessed all that day? Maybe a warm air mass at ground level that is small enough to not melt the snow in time before hitting the ground? I am guessing at this point.

If it helps they were the real fat soft snowflakes that looked like baseballs falling from the sky, but it was snow.

Googling air temperature with altitude brings up a bunch of links (and graphs in pics) that show that you only have to go a few kilometres up for temperature to plummet to -50C. I think you're right in thinking that the snow formed in a much colder layer of air above the warm air on the ground.

 

[rollcast]

^ that's a cool fact.

Thanks but I don't think the question is quite answered. How does it still snow well above freezing as I witnessed all that day? Maybe a warm air mass at ground level that is small enough to not melt the snow in time before hitting the ground? I am guessing at this point.

If it helps they were the real fat soft snowflakes that looked like baseballs falling from the sky, but it was snow.

Seems like you're correct, I checked the science bits in the article I knew and it all seemed solid.

http://www.sciencebits.com/SnowAboveFreezing

 

[CosmicEye]

Thanks guys and that a great link with a great explanation. I am good at guessing haha, but I would not have guessed that the greater cooling from evaporation than conduction of heat kept it frozen.