How does frost form?

This week’s weather question comes from Richard Byrd of Columbus. He wants to know how frost is formed.

There are two types of frost: advective frost (from cold air moving into the region – usually behind a cold front)
and radiation frost – which is the one we see more often.

In order for frost to form, we need to have the right conditions.

An overcast sky with lots of clouds is not ideal for frost because the cloud cover acts as a blanket and traps
radiant heat (which is just the heat from the ground rising in this case).

Wind also inhibits frost formation because wind helps mix the air and makes it the same temperature.

This causes the surface to be warmer and higher up to be cooler. In other words, the temperature decreases with
increasing height.

But when we have a cold, calm and clear night (usually following a cold front), it is ideal for fog formation.
This is because the radiant heat isn’t bounded by the cloud blanket and it can rise higher into the atmosphere.

This creates an inversion or the ‘inverse’ of what the atmosphere is ‘normally.’ So now instead of the temperature
decreasing with increasing height, the temperature is increasing with increasing height. In other words, it’s colder near
the surface and warmer aloft – allowing those tiny, thin ice crystals to form on the surface and other spaces.

 

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