Eli5: how does frost/ice on the ground not melt, even after the temperature isn’t in the minus numbers
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The ground below it may remain at freezing. The air can become warmer but ice has a much higher heat capacity than air so it takes a while for the ice to heat up enough to melt. Frost often melts when sun shines directly on it rather than by heating from the air. You will notice the frost remains in shaded areas for much longer.
It takes time for the ground to warm up. The air temp may be above freezing, but if the ground has been frozen for some time it will take time for it to warm up again. If the ground is below freezing it will keep the ice/snow frozen
Insulation works both ways. In the winter, you want the inside of your house to remain warm and not need the furnace as much, so you stuff the walls with insulation to make that transfer slower. Since all the insulation is doing is slowing down the transfer of heat, it also helps keep your house cooler in the summer meaning you need your air conditioner less often.
Snow is actually a great insulator. Actually, air is a great insulator if it isn’t allowed to flow like when trapped between the panes of double-paned glass, and snow does a great job at trapping air. This means that the snow on top melts relatively quickly, but it keeps the snow underneath colder. The deeper the snow, the more insulation is above it and the longer it will take to melt.
There are 2 other effects at play, other comments have already talked about the ground temperature. The ground is also insulated and also a tooooon of thermal mass that changes very slowly. But the other effect is a double edged-sword. The energy of fusion is described as the energy it takes to change state to/from solid/liquid. Normally, if you heat up H2O, the amount of energy you put in goes into a linear increase in temperature. But when it comes time to melt, that energy no longer heats up the substance, it all goes into breaking the tight bonds that make it a solid.
This slows the speed of melting for 2 reasons. First is obvious, it takes a lot of energy to melt ice. If you had one kilogram of ice at 0⁰C, it would take 330kJ to completely melt it. If you added another 330kJ after it was melted, it would rise to 80⁰C which is enough for near instant burns. So clearly melting is a slow, energy costly process.
The second reason is because melting actually cools the surroundings. The melting ice can steal energy from the snow underneath to continue the phase change leaving the snow underneath slightly cooler. It’s why evaporation keeps you cool. The water changing phase from liquid to gas steals some of the energy from your body cooling it down. Even though your body isn’t at 100⁰C, it can still lend heat energy to the evaporation process.
Cold air sinks and temperature gauges are a few meters off the ground. The official temperature can be higher than then surface temperature.
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