LJWorld.com weblogs Lawrence Weather Watch
Below freezing, but melting!
Highs made it to the 20s on Thursday, but a good amount of the snow was melting across the area. It may seem contradictory to say it's freezing outside, yet the snow is melting. It may be odd, but that's the way it happens.
The reason for this lies in two things: (1) Official temperatures are taken 1.5 meters above the surface and can vary from the actual surface temperature. (2) Solar energy largely passes through the air, but heats up objects that it strikes.
On a sunny, cold day with some snow on the ground, the sun's rays aren't warming the air all that much, but as the visible light and UV rays are being absorbed somewhat by the snow, there is a little bit of warming. Snow reflects most of it away, but it does absorb some and makes the very top layer of snow begin to melt (of course, it refreezes at night and gives that "crunchy" layer on top). Other objects like tree trunks can heat up even more and melt the snow right around them. Patches of pavement or a clear spot on the roof can heat up quite a bit in direct sunlight and melt away adjacent snow making the clear patch grow and grow. So when you see puddles all around and water pouring off the roof on a sunny day when the temperature is registering at 21 degrees, it's the air that's below freezing not all the surfaces.
This brings up another interesting thought. Have you ever watched a day game of baseball in the summer on television? Every time it's a hot day game, the camera will pan over to some back-up player who will set the big dial thermometer out on the turf just to get some TV exposure. The camera zooms in to show a temperature of 150 "on the field." The announcers empathetically say "and that's what the players are dealing with on the field today!" However, only the soles of their shoes have to deal with what the ground temperature is. Temperatures are often 20-40 degrees warmer at the ground level out in the sun than what it is "officially" 1.5 meters above the ground. Since our faces are what regulate most of our body's temperature, it's not the lowest 2 feet of air that matters.
Don't get me wrong, I've been to some day baseball games and they get very hot, but not the 145-degree searing heat that the thermometer sitting on the field out in the sun is showing. Absorbtion of solar energy can lead to some big spikes in temperatures. Don't believe me? Next July find a shiny, metal slide and be a kid all over again and slide down it. You'll know all too well about solar heating!