What is Frost? Frost is a thin layer of ice on a surface. It begins as water vapor (water in gas form) in the air at an above freezing temperature. It becomes frost when the water vapor meets a surface that is below freezing. The water vapor condenses, freezes, and becomes solid.
There are different types of frost, but the frost we’re most familiar with and is most common is hoarfrost, or radiation frost. Hoar just means ‘old age’ and when frost hits plants, it looks like it turned gray haired overnight!
Frost mostly occurs on cloudless cold clear nights with little to no wind, especially if it had been warmer with clouds and humidity the previous day. Clouds are a blanket which holds the heat in the air next to the earth. When it’s a clear night, the heat from the earth rises. Cold air can become trapped against the ground when there’s no wind to cart it off. Under these certain conditions objects cool below the frost point of the surrounding air, which is below freezing.
Low lying areas like valleys, hollows (hollers), or at low points in a hilly landscape are more prone to frost. This is because the cold air sinks and lays low to the ground in these areas.
How can frost form when it’s not below freezing outside? The temperature has to be at or below freezing for frost to form. One reason it may not seem like it is because cold air sinks and the temperature can be lower at ground level than up in the air. If you are using a temperature reading from a weather station, temperatures are recorded 5′ above ground level! This is why frost can happen on a night when it ‘officially’ isn’t below freezing.
However frost can form on house and car roofs. This is because they retain heat and at night they rapidly loose this heat until it reaches freezing point. Then the water vapor on the roof freezes.
Gardener’s Observation: Frost often escapes plants underneath trees that have their leaves still on. This is because the tree helps trap heat from the earth, raising the temperature or dew point.
Dew Point is something to know about if you want to predict a frost. At night, the temperature of the air drops and can do it quickly or over a large range. Cold air cannot hold as much moisture as warm air. So where does the extra moisture go? The air basically sweats out the extra water vapor and it condenses into a solid, known as dew. This temperature is the point in which dew forms. Dew point changes all the time based on temperature and relative humidity and it is calculated by using those values.
- A light frost happens when the temperature drops to below 32 degrees and the dew point is low enough to where ice forms.
Here are some atmospheric observation tips you can use to predict if there will be frost
- If at sunset, the temperature is close to freezing, in the right conditions there could be frost. Note: Temperature is recorded at 5′ above ground level. So you need to know the temperature of your garden at ground level!
- If the dew point is below 40 degrees at sunset, frost is possible.
- If the night is cloudy, frost is unlikely.
- If there are mild breezes at night, they will mix the warm air with the cooler air near groundlevel, making frost unlikely.
- A calm windless night allows cool air to collect near the ground, which makes frost possible.
- Strong, gusty winds at night can also push away warm air, increasing the likeliness of frost.
- If the air near the ground is warm and moist and there’s the right atmospheric conditions (no wind, cloudless, cold night), there could be frost.
- The more moisture in the air at night, the less likelihood that frost will occur that night. GARDENERS TIP! This is why farmers often irrigate crops when frost is predicted. This is because they are raising the dew point to decrease the chance of frost. You can also use this trick!
Here are some weather conditions as to when frost or freeze warning may be given:
- A hard freeze is a period of at least four hours of air temperatures that are below 25 degrees.
- “A Freeze Watch is issued when there is a potential for significant, widespread freezing temperatures within the next 24-36 hours. A Freeze Watch is issued in the spring at the start of the growing season when it is late enough to cause damage to new plants and crops.”
- “A Freeze Warning is issued when significant, widespread freezing temperatures are expected. A Freeze Warning is issued in the spring at the start of the growing season when it is late enough to cause damage to new plants and crops.”
- “A Frost Advisory is issued when the minimum temperature is forecast to be 33 to 36 degrees on clear and calm nights during the growing season. A Frost Advisory is issued in the spring at the start of the growing season (when it is late enough to cause damage to new plants and crops).”
- (Source National Weather Service)
- Read more definitions of Watches, Warnings, and Advisories
These are some general parameters in which these warnings can be issued:
- A frost advisory is issued if the wind speed is below 10 mph and the air temperature is between 33-36 degrees Fahrenheit.
- A frost/freeze warning is issued when winds are below 10 mph and the air temperature is below 32 degrees.
- A freeze warning is made when winds are above 10 mph and the temperature is below 32 degrees.
Why is all this important? Well as a gardener you know in the springtime if you plant too early, you can loose your plants. If you wait too long, you miss precious growing time. You can obviously just watch the weather and for any frost advisories being issued. However, there’s nothing like getting an advisory at 7pm at night, and covering up plants until midnight having to use flashlights to see. Being able to predict any chance of frost ahead of time is useful skill.
Understanding why and when frost forms can make your brain spin, but if you watch the weather and know what kinds of conditions frost forms under, you’ll be an excellent predictor in no time! If not, you can always just hope the weather forecast is accurate (or not!) in its predictions.