Frost is a common weather phenomenon appearing as a delicate layer of ice crystals on surfaces like grass and car windshields. It forms when water vapor in the air transitions directly into a solid state at freezing temperatures.
The Critical Temperature for Frost
Water freezes at 0°C (32°F), which is the fundamental temperature for frost formation. However, frost can appear even when the air temperature measured a few feet above the ground is slightly above this point. This occurs because surfaces cool more rapidly than the surrounding air through radiant cooling. Heat radiates away from these surfaces into the clear night sky, causing their temperature to drop below that of the air. Thus, while your thermometer might show 34°F or 35°F, the surface of your car or lawn could be at or below 32°F, allowing frost to form.
Key Conditions for Frost Formation
Several atmospheric conditions contribute to frost formation. Clear nights are important because clouds trap heat, preventing significant cooling. Without cloud cover, heat radiates freely into space, allowing surface temperatures to plummet. Calm or very light winds are also necessary, as strong winds mix warmer air from above with colder air near the ground, hindering the required temperature drop at the surface.
Moisture in the air is another requirement. This is understood through the “frost point,” the temperature where air becomes saturated with water vapor, leading to direct deposition as ice crystals onto a surface. The frost point is analogous to the dew point but applies specifically when temperatures are below freezing. When both the surface temperature and the air’s frost point are at or below freezing, frost forms. Higher humidity means less cooling is required for the air to reach its frost point.
Protecting Plants and Property from Frost
When frost is anticipated, preventative measures can protect sensitive plants and property. One strategy is to cover vulnerable plants with materials like sheets, blankets, or specialized frost cloths. These coverings trap heat radiating from the ground, providing a protective layer against cold air. Ensure the covering extends to the ground to effectively trap warmer air, and remove it once temperatures rise to prevent overheating.
Watering the soil thoroughly before a frost can also offer protection. Moist soil retains more heat than dry soil, which helps warm the air immediately surrounding plants. For potted plants, bringing them indoors into a garage, shed, or sheltered porch can shield them from freezing temperatures. Mulching around plants further insulates the soil, helping it retain warmth and protect roots from the cold. Checking local weather forecasts for frost advisories allows for timely preparation and protection of outdoor items.