This transcript was edited for clarity and length.
Bill Burton: It's time for us to take a look at the Science Behind the Forecast as I'm joined by WAVE 3 meteorologist Tawana Andrew. Good morning, Tawana.
Tawana Andrew: Good morning. Today's topic is a little bit chilly, let's say.
BB: It's October, but it's still fairly warm. We've been in the 70s for a while. It's going to get colder eventually, though. So today we're talking about frost. Tell us about frost.
TA: Typically for the Louisville area, our first fall frost occurs on Oct. 25. We typically see our first fall freeze happening closer to Nov. 4. So for frost to happen, you need temperatures at or below 36 degrees. For a freeze, you need 32 or lower. You can have frost forming, though, with temperatures a little bit warmer, it just depends on where you are.
The National Weather Service defines frost as the formation of thin ice crystals on the ground or other surfaces in the form of scales, needles, feathers or fans. It could even come in frost flowers.
BB: Frost flowers? I think I've found a new obsession. I'm going to spend the next few months trying to hunt down frost flowers.
TA: They are rare, but once you do see them, they're absolutely beautiful.
Frost, that thin layer of ice that forms on a solid surface, can only occur on a surface that is below freezing. And a lot of people tend to think that once frost is happening, that water vapor is going from gas to liquid to ice, but actually it is deposition that occurs. So it's going straight from water vapor into the ice form when frost occurs on the surface. Of course, it has to be colder than the dew point of the air surrounding it for that moisture to stick and freeze. But if there's too much moisture, if the atmosphere is too humid, that water vapor may condense into a liquid before freezing, and that technically results in icing instead of frost.
To tell the difference, frost has more of that feathery, crystal-like appearance. You will see those amazing designs on your windshield, but if it's frozen dew, which is the icing, then you end up with more of that transparent, round, kind of hard look to it. It looks more like that sheet of ice that can form in your freezer. Not so much the beautiful frost designs that we like to see. There's a couple of things that you need for frost to form. One, clear skies, because that allows the heat to escape through radiational cooling, allows temperatures to drop a little bit better and faster. You need the freezing temperatures. You need that super cooled air dropping to the dew point or lower, and then you can have that frost forming on a variety of objects. The other thing you need is calm winds. The light winds allow that thin layer of super cooled air to form near the ground. And keep in mind that layer of air can be about 10 degrees colder close to the ground than what the actual temperature is five feet above the ground. So you can have, let's say your phone is telling you it's like 40 degrees outside, but that layer of air is closer to 30 to 36 and that is why you end up with that frost in this situation, even if the air temperature is a little bit warmer.
BB: So many varieties of frost. Who knew? Well, we know now thanks to this edition of Science Behind the Forecast with WAVE 3 meteorologist Tawana Andrew. Thanks for the knowledge, Tawana.
TA: Of course.
