Frost is the deposit of ice crystals on a surface, which requires the surface itself to be at or below 32°F (0°C). Yes, it can absolutely frost when the air temperature is 36 degrees Fahrenheit. The temperature reported in a weather forecast is the official air temperature, measured several feet above the ground, typically between four and six feet high. This official air temperature often remains above freezing even while objects at ground level cool significantly below the freezing point.
Understanding Air Temperature Versus Surface Temperature
The difference between the air temperature and the temperature on a surface is due to radiative cooling. Objects exposed to the open night sky constantly radiate heat outward into space. This heat loss is particularly efficient on clear nights, causing the surface temperature of things like grass blades, car windshields, and leaves to drop rapidly.
Surfaces with low thermal mass, such as grass and leaves, can cool several degrees lower than the surrounding air. The temperature right at ground level is commonly five to ten degrees colder than the air temperature measured a few feet higher. This localized cooling means that when the air temperature is 36°F, an exposed leaf surface can easily fall to 32°F or lower, creating the necessary conditions for frost formation.
The Role of the Dew Point
Frost formation requires two conditions: a surface temperature at or below 32°F and sufficient moisture in the air. The dew point is the temperature at which air becomes fully saturated with water vapor. If the surface temperature drops below the dew point, excess water vapor condenses.
For hoar frost (pure white, feathery frost) to form, the dew point must also be at or below 32°F. Water vapor then changes directly into ice crystals on the sub-freezing surface. If the surface temperature drops below freezing but the dew point is above 32°F, dew forms first and then freezes into a solid coating of ice, sometimes called frozen dew.
Atmospheric Conditions That Encourage Frost Formation
Certain atmospheric conditions maximize radiative cooling, allowing frost formation even at 36°F. Clear skies are a factor because clouds act as an insulating blanket, reflecting heat back toward the ground. Without cloud cover, heat escapes unimpeded into the atmosphere.
Calm or very light winds are also necessary for frost to develop. Wind mixes warmer air from above with colder air near the surface, preventing the temperature drop at ground level. Cold air is denser than warm air, causing it to sink and pool in low-lying areas, such as valleys. This phenomenon, known as cold air drainage, makes these topographical features prone to frost.
Simple Strategies to Protect Vulnerable Plants
Gardeners can take several actions to protect sensitive plants when the forecast calls for temperatures in the mid-30s.
Watering the Soil
One effective strategy is to thoroughly water the soil around plants before sunset. Wet soil retains heat much more effectively than dry soil. This stored warmth radiates upward overnight, slightly warming the air immediately surrounding the foliage.
Covering Plants
Covering plants is an effective method, as the cover traps heat radiating from the ground and protects the surface from heat loss. Use materials like old bed sheets, blankets, or commercial frost cloths. Ensure the cover extends all the way to the ground to maximize insulation. Avoid using plastic directly on foliage, as it can transfer cold and damage the leaves where it touches.
Moving Containers
Potted plants and small containers should be moved closer to a warm structure, such as a house wall, or entirely indoors, like into a garage or shed.
Using Thermal Mass
If an exposed plant is too large to cover, placing a few gallons of water in dark containers near the base of the plant can help. The water absorbs heat during the day and slowly releases it throughout the night.