Frost forms when the temperature of a surface drops to or below 32°F (0°C). While the air temperature might be slightly higher, the surface itself needs to reach this critical point for ice crystals to form.
Understanding Frost Formation
Several factors contribute to frost formation:
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Temperature: The primary factor is the surface temperature reaching 32°F (0°C) or lower. This can occur even if the air temperature remains slightly above freezing due to radiative cooling on clear, calm nights. Cold air sinks to ground level, leading to lower temperatures near the surface. (Weather.gov)
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Moisture: Sufficient moisture in the air is necessary for frost to form. This moisture deposits as ice crystals on surfaces below freezing. (Garden City Harvest)
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Clear Skies and Calm Winds: Clear skies allow for greater radiative cooling, and calm winds prevent the mixing of warmer air from higher altitudes. (Weareiowa)
Different Frost Types: The intensity of the frost can vary, ranging from light frost to hard freezes. A light frost occurs when the temperature is at or just below 32°F (0°C), while a hard freeze involves significantly lower temperatures. (The Old Farmer's Almanac)
Some sources mention frost forming at temperatures as high as 36°F (2°C), but this is less common and depends on specific microclimatic conditions. (ABC27) The key takeaway is that frost requires a surface temperature at or below 32°F (0°C). (Weather.gov).
