Frost at 35 degrees - how does that happen?

Frost at 35 degrees! How can that be?

We had a chilly start to the day with frost in many countryside locations. Low temperatures overnight were generally in the mid 30s.

Now let's think about this. Frost is essentially ice that forms on objects. But how can there be frost on your lawn if you check your thermometer and see temperatures above freezing? Shouldn TMt it be liquid and not ice?

Yes it should be liquid, IF in fact the air temperature was really above freezing. Here TMs the catch| most of our thermometers are hanging on our deck or porch wall, or if it is an indoor-outdoor thermometer, chances are the sensing bulb runs out the window. Either way, the important point is that the air temperature being measured is actually the temperature of the air a few feet above the ground and NOT right on the ground where the frost actually forms.

Often times our coldest nights are just like last night. Clear skies, light winds, the lower levels of the atmosphere are dry and the air mass is chilly. Once the sun goes down, under these conditions, the temperature drops like a rock as any heat of the day escapes back out into space. The technical term for this is radiational cooling.

With no heating from the sun taking place overnight, this allows colder air to settle in generally undisturbed. Since colder air is more dense (heavier) then warm air, the coldest air settles in closest to the ground, and in fact, right on the ground. If you were to take your thermometer and place it at average lawn height level (on the ground), you would likely find the temperature to be several degrees colder. If you were measuring 35 degrees several feet up you might find the lawn temperature where the coldest air settled to be 31 or 32 degrees, which is cold enough for frost to form.

So, there is nothing magical about frost forming. 32 degrees is still the freezing mark. It all comes down to WHERE is that temperature reading from. Is it several feet off the ground, were most thermometers are located, or right down on the ground?

Now you know how frost can form when your thermometer is reading in the mid 30s.

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