Formation of Frost

Recently an email was sent to our weather department from a non native northerner. This individual just moved to the state and was wondering exactly how frost is formed and when it forms. I'm assuming he is already tired of having to scrape ice off of his windshield (kinda like the guy below!!).




So lets break things down on exactly why we get frost some days and why we don't get frost on other days.

During the day temperatures are obviously warmer than they are at night, and while the air warms up during the day, objects warm up as well. The ground, cars, street poles, all warm up throughout the day and those objects actually hold their heat after the sun sets. Objects hold heat longer than air does. So frost will form when these objects cool down to temperatures colder than the dew point of the air. Basically what's going on is that the air around the objects are cooling so much that the the water vapor contained inside the air condenses onto whatever object it is near. Normally when this occurs, we get dew, but since temperatures are below freezing (32°F) we have the result of frost. Now of course to get frost we do need some moisture in the atmosphere, so you will usually get frost to form when the air temperature and the dew point are expected to be very close to each other. There are some other factors that do play a role in frost formation but the biggest of those is wind speed. Typically winds need to be 5 mph or less for at least a couple of hours for frost to form. Highs wind speeds can often cause to much evaporation (drying) on surfaces and it doesn't allow ice crystals to form on surfaces before they get blown away.



So, if we go through all of the components to form frost, tonight looks like a pretty good night for it. Now, we may be lacking the moisture a little but the winds will definitely be light enough so there could be some light patchy frost in the morning, so don't forget that scraper.

~KDLT Meteorologist Cody Matz