This is the time of year when the weather forecasters turn up the talk about night temps that can dip down into frost territory. Either way, it’s a crucial time for gardeners who are itching to plant outdoors. Plenty of the big-box stores already have annuals for sale and that’s a dangerous thing for anxious gardeners. There’s a good chance that these plants will succumb to a frost or a freeze before May 15.
There’s a subtle difference between a frost and a freeze. You can have a freeze without frost and vice versa. Here’s why: A freeze occurs when the air temperature drops below 32 F. Sometimes we get frost (a deposit of ice crystals) when it’s above freezing and we can have a freeze without frost. It all has to do with the amount of water in the air. There are two different ways to measure humidity, the amount of water vapor in the air. The one that weathermen (and women) use is “relative humidity.”
Warm air holds more water than cool air. The relative humidity changes throughout the day as the temperature rises and falls. If the temperature drops low enough, the amount of water in the air is more than it can hold at that temperature. The air is saturated and water vapor in the air condenses as water on cars, lawns, sidewalks--and voila--dew.
The dew point measures the absolute amount of water in the air. It is the temperature at which the air is saturated and the relative humidity is 100%. For a given volume of air, with a set amount of water vapor in it, the relative humidity varies with the temperature but the dew point is always the same.
What does that have to do with frosts and freezes? It all has to do with the dew point. If the dew point is much above freezing, a frost is unlikely. The higher the dew point is above freezing the less likely we’ll have freezing temps. If the dew point is below freezing then a frost becomes more likely.
If a dry air mass moves into the region at this time of year in the Chicago area, a freeze is likely. Dry air has a low dew point and a low relative humidity. The dry air warms quickly during the day but also cools quickly at night. When there are clear calm conditions, the ground cools rapidly at night, losing heat to the open sky. As the ground cools, the air next to it also cools. On windy nights, the cool air mixes with warmer air above and the warm air helps heat the ground. On calm nights, the ground continues to cool and can be colder than the air.
Water condenses on the ground and other surfaces as dew. If the dew point is near freezing, the water vapor condenses as ice, freezing as frost. So the air can be above freezing and the surface of your car is colder than freezing causing a frost even thought the air temperature is above freezing. That is how we get a frost without a freeze. If the dew point is much below freezing then we can get freezing temperatures cold enough to freeze plants without any frost. When frozen plants thaw, they are blackened and die. This is sometimes called a black frost, a freeze without a frost.
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