On Frosts and Freezes

It’s getting to be that time of year again. Although the average date for the first freeze in Austin occurs in late November, the National Weather Service (NWS) has observed freezing temperatures as early as October 28. Keep in mind that the “official” minimum temperature is observed about six feet above the ground in a NWS instrument shelter. The difference between the air temperature in such a shelter and the temperature of an exposed plant near the ground can be significant, especially on a calm, clear night. Your plants could be in an environment that is 5 to 10 degrees (or more) colder than the “official” minimum. Because cold air is dense, it will tend to sink. As a result, this difference could be even larger if you happen to live in a valley or other low- lying region. For this reason, experienced gardeners will modify the official NWS minimum temperature forecast for their specific landscape or garden.

Be alert for the combination of 1) dry air (the dew point temperature below the mid thirties), 2) calm wind (a nighttime wind forecast of less than 10 miles per hour), and 3) clear sky. With this combination, overnight cooling can be significant. The minimum temperature can easily be thirty degrees colder than the observed afternoon maximum temperature. Thus even with a daytime high temperature of 65 or 70 degrees, if we have conditions of dry air, calm wind and clear sky, an official overnight minimum of 40 degrees or colder is very likely and frost or freeze damage in an unprotected garden is likely. (Editor)