Estimate frost risk from overnight temperature, dew point and clear sky conditions.
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How This Works
Frost risk increases with: lower air temperature, dew point at or below 0°C (ice directly forms), clear skies (radiative cooling — ground can be 2–4°C colder than air), calm wind (wind mixes warmer air, preventing surface cooling). Ground temperature is typically 2–4°C lower than the official screen temperature reading (measured 1.25m above ground) on clear calm nights.
Air frost occurs when the official temperature (measured in a Stevenson screen at 1.25m height) falls below 0°C. Ground frost occurs when the temperature at ground level falls below 0°C — this can happen when air temperature is still 2–4°C above zero on clear calm nights (radiative cooling effect). Ground frost damages plants even though official forecasts may show no frost. Gardeners should monitor both.