Why We Mix

So it should seem surprising to most people that in Boston last night around midnight, the temperature was 22 degrees. Even as I write this at 8:30 AM, my outside temperature in West Roxbury is 30 degrees, but I am getting all sleet right now. So how and why does this happen?

This reminds me of an argument I once had with my good friend Ryan Hansen. Hansen claimed that it HAD to be 32 degrees to snow no matter what. If it was any warmer, snow just couldn't exist. I, of course, disagreed and tried to explain to Hansen that it in fact can snow if the ground temperature is above 32. This is usually when you see those big almost fake looking snowflakes with very high water content. It is the result of cold air sitting right over our heads and precipitation falling very fast, so fast that it doesn't have time to melt before it reaches ground level. Of course, this snow doesn't stick to much of anything except grass and cars BUT it does prove that it can snow when it is above 32 degrees.

I bring up this story because sleet/ice and that magenta coloring that you see on your radars today works in a very similar way, just backwards. (On a side note, I love how mixing is depicted on the radar! Its like a ball of uncertainty coming towards you!!) Anyways, despite us having very cold air in place at the surface, you need to remember that our atmosphere is quite complex and more importantly, it continues up in the air for many miles! As I explained to Hansen, just because our surface temperature is cold enough for snow does not mean that we will get snow. The reason is somewhat complex, but in layman's terms, its because the air about 2000ft above us is too warm and is in fact above freezing.

This particular storm affecting us today is VERY large. Many people saw the satellite images of it yesterday and heard horror stories from the midwest of epic snowfall totals. This storm is so large that it began to suck up some very warm air from the Gulf of Mexico. That warm air ran into our firmly entrenched cold air (cold air that has been pretty much the result of a stationary high pressure system over Iceland all winter!). Our cold air actually won the battle, but as we all know, warm air rises and despite losing the battle at ground level, the warm air has risen above the cold air and now has the "high ground" which any battlefield nerd will tell you is the most important thing in a fight!

The result of this process is what you see today, sleet and ice. The air above us warms falling precipitation so much that by the time it hits the cold air at the surface, it doesn't have time to freeze back up. The result is sleet, which is basically just a half assed snow flake. Ice is much more dangerous. Ice happens when the falling precipitation makes it all the way down to the surface as just a cold rain drop but then freezes immediately on impact. This usually happens when there is a bigger difference in surface temp and low level atmosphere temp. I don't think Boston will have a problem with ice today because our surface temp will continue to warm up but other areas away from the influence of the water might have an icing issue.

Ok, so now you know why it sleets and ices. By the way, despite my logical explanation to Hansen, he still didn't believe me. Anyone who has ever had an argument with Hansen knows that he doesn't give up easily.