HORTICULTURE
Frost or Freeze: What is the difference.
I am trying to train
myself to stop saying frost when I mean freeze. They are not always the same thing. A
frost is when we get a visible frost. A freeze is when the air temperature drops below
freezing. Sometimes we get frost when the temperatures are above freezing and we often
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 most people use is the relative humidity.
The relative humidity measures how much water is in the air compared to the maximum amount of vapor the air can hold. When the air cannot hold any more water it is saturated and the relative humidity is 100%. Warm air can hold more water than cooler air. So during the day and night the relative humidity changes as the temperature rises and falls. If the temperature falls far 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 surfaces such our cars, the roofs of houses or lawns and we have 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%. So 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 are freezing temperatures. If the dew point is below freezing then a frost becomes more likely. Dry air heats and cools much more quickly than humid air. Water vapor in the air acts as a heat reservoir absorbing heat.
If a dry air mass moves into the region 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. If we have clear calm conditions the ground cools rapidly at night by radiating heat away to the open sky. As the ground cools, it cools the air next to it. If it is windy, then this cool air is mixed with warmer air above and the warm air warms the ground. In calm conditions, the ground continues to cool. The ground is actually 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 have a water soaked black appearance as they die. This is also called a black frost, a freeze without a frost.
