It is a reality, and it happened Yesterday!! Yesterday morning, It was snowing like crazy, when my thermometer was well above freezing (around 4 degrees C).
This seems against everything we learned in science classes when we were taught that snow melts when the temperature rises above Zero C (or 32 F). How then did we witness snow?
The naive answer is that snow start melting indeed but it did not have enough time to completely melt before it reached the ground. That is partially true, but this is not what happened yesterday!! Yesterday the upper atmosphere was cold and wet, so snow formed. The lower layers were dry with temperature above freezing. The catch here is DRY, while snow is following, it started melting. Now water of course, has a good tendency to evaporate, The evaporation is falling snow took place at higher rates than normal due to the dryness of the lower levels of the atmosphere. To transfer melting snow from liquid state to vapor state, energy is required. This energy is taken from snow crystals themselves, so the energy within the snow flake becomes less, and henceforth, its temperature goes down. What are we having here:-
The atmosphere is warm enough to cause snow to melt; the atmosphere is dry enough to cause evaporation of melting snow; energy is required for evaporation,. This energy is taken from snow crystals causing them to cool down. If the evaporation rate is just right, the temperature of the snow crystal drops back to zero and the crystal maintains its freezing state. The crystal is at Zero temperature, and the atmosphere is above zero, therefore, snow keeps falling because it is still at zero temperature regardless of the fact that its surrounding is really above zero.
Summary, when the atmosphere is above freezing and the wind is dry, snow flake can keep falling. There is a formula that tells you for at what temperature snow can still fall at what humidity level. In fact, and based on such formula, snow can still fall at temperature of as high as 8 degrees when the humidity is 20% . Interesting, isn't
(picture on the left is taken last Tuesday)
