What is the Frost Point?

Mary McMahon

The frost point is a point when the air becomes wholly saturated with water vapor and it begins to precipitate out as frost. Humidity, temperature, and air pressure all play a role in the determination of the frost point and closely related dew point, where droplets of water in the form of dew start to form because the air is so saturated with water. Generally, temperatures must be below freezing for water vapor to develop into frost instead of dew.

The air must be fully saturated with water vapor to precipitate into frost.
The air must be fully saturated with water vapor to precipitate into frost.

Cooler air is less capable of holding evaporated water. When humidity levels are high and temperatures start to drop, the water vapor in the air will start to condense into dew. As long as local conditions are below freezing, the condensed water vapor will develop in the form of frost, rather than dew. It is important to note that while ambient temperatures can be above freezing, there can be pockets of air at or below freezing, causing frost to form in isolated areas.

The frost point is closely related to the dew point.
The frost point is closely related to the dew point.

When the frost point is reached, at locations where saturated air comes into contact with objects, a thin layer of frost will form. The frost crystals should remain intact until temperatures start to rise, melting them. Warmer temperatures will facilitate evaporation, returning the water to the air in the form of vapor and starting the whole cycle over again as humidity levels and temperatures rise.

Numerous devices can be used to measure humidity and this information can be paired with data about temperatures and air pressure to estimate the frost point and the dew point. Computer modeling may do this automatically using data generated by weather stations, providing instant information for meteorologists and other people with an interest in the weather. Tables are also available for hand calculations, including tables to convert between Fahrenheit and Celsius temperature measurements while collating data to determine the frost point.

Weather forecasts may provide information about the frost point for the benefit of the community. Gardeners need to know when frost is forecast and if it is likely that the frost point will be reached, because this can damage plants. Frost can also have an impact on road safety, as frost makes roads less navigable and can increase the risk of accidents. Especially when temperatures are not extremely cold, people may not be aware of the risk of frost on the roads and may fail to take deposits of ice and frost into account while driving.

Frost occurs when temperature drop below 34 degrees Fahrenheit.
Frost occurs when temperature drop below 34 degrees Fahrenheit.

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Discussion Comments


I pay attention to the frost point forecast, but it’s for a different reason than most people do. I think frost is absolutely beautiful, and I love to capture it in photographs.

When the weatherman gives me the estimate of when the frost point will be reached, I set my alarm for an hour after that. If the moon will be out that night, it will be especially beautiful.

If it covers my entire yard, it looks like a million diamonds sparkling in the moonlight. I get up and admire it, and then I go back to bed until sunrise. As soon as the sunlight starts to fall across the frost and make it sparkle, I get my camera and start shooting.


@kylee07drg - I also live in an area that generally stays warm throughout the first half of the fall. I have seen what frost can do to zinnias, and it is so sad, because they could have lived another month!

I thought that mine would be okay, because they were sheltered by trees, and this often keeps them warmer. I guess a pocket of cold air must have drifted into the area, because my zinnias were covered in white crystals one morning in early November.

They looked so beautiful, as if they were frozen in time. Later in the day when the frost evaporated, they turned brown and shriveled up. I should have paid more attention to the frost point!


I have seen some parts of my yard covered in frost while others have only dew. I often wondered how that could be, and after reading this, I now know that it is due to air pockets.

I knew that the areas near the house were likely kept warmer by residual heat radiating off the bricks. However, I could not fathom why an area way out in the yard exposed to the elements would be frost free, when the patch of grass next to it was not.

It blew my mind to walk outside in the morning to find sections of my grass covered in white crystals and others just as green as ever. I’m glad this mystery has been cleared up for me!


My local meteorologist always mentions when there is a chance for frost. In the early fall, this information is very helpful to me, because I have to cover up my plants.

I have several types of flowering bushes that can survive into early November if they are protected at night. Temperatures around here sometimes stay in the eighties until the end of October, but we can have a frosty night thrown into the mix suddenly that will kill everything.

If the forecast mentions that we will likely reach the frost point on a given night, I will go out and cover my flowers with small tarps. I remove them once the temperature gets well above freezing so that they don’t suffocate.

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