Frost is the formation of delicate ice crystals on surfaces, and yes, it can occur at an air temperature of 37 degrees Fahrenheit. This happens because the official air temperature reported by weather services is not always representative of the temperature at the ground level. The risk of frost, even with a forecast in the upper 30s, is something gardeners and homeowners must consider.
The Difference Between Air and Surface Temperature
Standard meteorological practice involves measuring air temperature approximately 5 feet (or 1.5 meters) above the ground in a shaded shelter. This measurement provides an official reading, but it often differs significantly from the temperature of the surfaces below it. On clear, calm nights, the air immediately surrounding objects like grass blades, car roofs, and plants cools much faster than the air measured higher up.
The objects on the ground lose heat rapidly, and the air directly in contact with them also cools, becoming denser and sinking. This creates a shallow layer of cold air right near the surface, a condition known as a temperature inversion. The temperature difference between the standard measurement height and the surface can be 5 degrees Fahrenheit or more, meaning a 37°F air temperature can easily correspond to a surface temperature of 32°F or lower. Frost forms when a surface cools to the freezing point (32°F) or below, causing moisture to deposit onto it as ice crystals.
How Radiational Cooling Drops Temperatures
The primary mechanism that allows surface temperatures to fall below the official air temperature is radiational cooling. All objects, including the Earth’s surface, constantly radiate heat energy outward as infrared radiation. On a clear night, this heat travels unimpeded into the cold expanse of space.
The lack of an atmospheric blanket to absorb and re-radiate this energy back toward the ground causes a net loss of heat from the surface. Objects like car windshields and grass blades, which are excellent radiators of heat, cool down much faster than the surrounding air. This rapid heat loss allows their temperature to fall to the freezing point, even if the air several feet above them remains warmer than 32°F. This effect is common on objects insulated from the warmer ground below, such as leaves or elevated surfaces.
Humidity, Wind, and Cloud Cover
Three atmospheric factors significantly influence the likelihood of frost when air temperatures are near 37°F: humidity, wind, and cloud cover. Humidity is measured by the dew point, the temperature at which the air becomes saturated and moisture begins to condense. For frost to form, the surface temperature must drop to the freezing point and also be below the dew point. A high dew point near 32°F increases the frost risk, as less cooling is required for moisture to freeze.
Wind conditions are important; calm or very light winds allow the coldest, densest air to settle directly on the ground. A gentle breeze, however, mixes the colder air layer near the surface with the warmer air slightly higher up, preventing the surface temperature from dropping to freezing. Conversely, cloud cover acts like a blanket, reflecting the outgoing radiated heat back toward the surface. A complete layer of clouds greatly reduces radiational cooling, insulating the ground and making frost highly unlikely at 37°F.
Protecting Plants and Property
When a forecast calls for air temperatures in the mid-to-upper 30s, taking preventative action can protect sensitive plants and property. Covering tender vegetation is the most effective measure, using materials like old bedsheets, blankets, or commercial frost cloths. These covers should extend to the ground to trap the heat radiating from the soil, and the material should not touch the foliage.
Watering the soil in the late afternoon before the cold snap is beneficial, as moist soil holds and releases heat more effectively than dry soil throughout the night. Potted plants should be moved indoors to a garage or porch to shield them from the open air. For property protection, disconnecting and draining outdoor hoses and sprinklers prevents residual water from freezing and potentially bursting pipes or fixtures.