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Since winter will soon be upon us, I have a question about winter morning temperatures that I've observed. I've noticed that on cold days, the temperature always seems to drop a degree or two as soon as the sun comes up before it starts rising again. Why is that?
Doug C.
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On a calm and clear night we see the temperature drop first after sunset and then at sunrise, it drops right after the sun comes up. When the sun's rays are hitting us here at the earth's surface, the air heats from the ground up. The ground gets warmer and then the air gets warmer. We need that direct light from the sun to heat the ground.
As the sun rises and the edge of its disk comes above the horizon, our temperature still drops. It is dropping because the light from the sun isn't direct, and there is a lag of time for that ground to start heating up. As that sun rises, it may take a few minutes for the warm air to become warmer and then reach the thermometer which can be around 5 feet above the ground.
The incoming solar energy when the sun rises doesn't have enough strength to reverse the heat loss from the overnight. Thus, the surface is still losing heat at sunrise and for a little while afterwards. When the sun's energy becomes more direct, we start to heat up and the temperature will rise.
We use the radiation budget for this. When we have radiation coming into the earth and exceeding what is being taken out, such as sun during the day, we heat up. At night, more radiation is going away then coming in, we cool off. During the morning hours, the sun starts depositing energy, once the energy starts to exceed what is lost, we will heat up. But at sunrise with the sun low in the sky, it can be up to an hour after the sun comes up.
Answered by KSL Meteorologist Dina Freedman.
