Wx Savvy | Winter

How and Why Does Frost Form?

Frost forms when water vapor (water in the air that we can't see) deposits itself onto an object as an ice crystal. This is called deposition as it goes from a gas to a solid. This differs from glaze or rime ice which goes from a gas to a liquid and then to a solid.

In order for frost to form, the object the water vapor freezes onto must be below freezing. Part of the reason some objects are more likely to have frost than others is because different objects retain and release heat in at different rates. The result is that objects will have different temperatures (for more on this check out specific heat or heat capacity- it's the technical term for what I described). If this confuses you, just remember the "temperature" on TV is the AIR temperature and not the temperature of an object.

So While it may be 34°F outside, the grass might be 28°F. This can explain why sometimes there is frost eventhough the morning low was 34°F. Also, the temperature is measured at 5' above ground typically and the denser cold air will be lowest to the ground. And yet another reason why you may see frost when the TV weather guy said the low was 34° is because you don't live at the airport. Often daily high and low temps are measured at the airport. It can be considerably cooler by as much as 10°F to 15°F in the valleys because the denser cold air will sink into the valley.

(Above left is frost on glass and to the right is the more fluffy frost with tree like growths)

Frost is a fuzzy or feathery layer of ice crystals. The terms frost, hoar frost and white frost are basically the same, but I found that the AMS glossary refers to white frost as a heavy layer of frost.

One simple way to forecast frost is to look for a clear night with calm winds and a forecasted low below freezing. Although it's not a guarantee due to several other factors, these are the big factors that clue you in to the possibility of frost.