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.
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.
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.