Why Frost Can Form on Your Car Even When It’s Above Freezing
It is the time of year when you need to keep your ice scraper handy. The first frost of the season is set to impact millions of Americans this weekend, diving as far south as central Florida. You may wonder why you wake up to a frosty car windshield even when the temperatures bottom out well above the 40-degree mark. Here is a closer look at how frost develops even above the freezing level, as well as what you should expect in the days ahead as the first cold blast of the season pushes down from Canada.
Why Ice Forms on Windshields Even in the Absence of Freezing Temperatures
Forecasters are cautioning residents of the South to be prepared for some frosty mornings in the coming days. This may seem confusing when looking at the forecast and seeing overnight lows that remain in the 40s for many areas.
Meteorologists explain that most forecasts detail the expected air temperature at about 6 feet off the ground. This means that frost may form in areas farther from the surface of the warm ground, including across car windshields.
The windshield begins to give off heat after the sun sets. Clouds acting as a blanket then work to slow down the cooling and mitigate the development of frost. However, this all changes when the skies are clear. Without the clouds working as insulation, the glass of the windshield cools at a faster rate. This process is known as radiational cooling.
A car windshield can drop to 32 degrees while the air surrounding it remains at 40 degrees. Breezes in the area can also slow the cooling process. This means that if the glass is colder than the air around it, the breeze will push warm air across the windshield. The movement of the warm air across the windshield then works to prevent frost from forming if the winds are strong enough. Conversely, nights with little to no wind are more likely to result in a frosty windshield in the morning as the cold air collects closer to the surface of the ground.
You can see this phenomenon in action by opening the top freezer of a refrigerator on a humid day and observing the movement of the mist. The condensation is naturally pulled to the bottom rather than the top. This is because cold and dense air gravitates toward the lowest level rather than rising to the top like warm air. Another example of this is how basements are typically always cooler than the upper levels of a home.
Going back to the car, the moisture in the air will lead to the formation of ice crystals on a car windshield once the temperature of the surface of the glass dips below freezing. As such, you may wake up to a frosty windshield even when the air temperature would suggest otherwise.
Where to Expect Frost This Weekend
A large swath of the U.S. east of the Rockies is going to be under the umbrella of a frost or hard freeze threat this weekend. The coldest temperatures will be released from Canada and end up as far south as the lower Mississippi and Tennessee valleys by Saturday morning. The core of this chill will shift to the east by early Sunday morning.
Most of the Appalachians, the Piedmont region of Virginia, and the Carolinas will all be in the zone for frost by the end of the weekend. The forecast for Atlanta is calling for overnight lows to settle in the mid 30s. The suburbs of the Peach City could see readings approach the freezing mark. The same goes for cities such as Raleigh.
In addition to having your ice scraper ready to go, this is also a good time to bring vulnerable potted plants into the garage. You can also consider covering the plants with an insulated blanket before the sun goes down to add another layer of protection.
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