Temperature Inversion Data Can Help Growers Determine Frost Responses

A new article by Mark Battany, Water Management and Biometeorology Advisor UCCE for San Luis Obispo, measuring temperature inversions can help California avocado growers determine if and when to use wind machines for frost events.

In California, growers typically rely on water (sprinklers) or wind machines to protect frost-sensitive crops during cold temperatures. Because water is an expensive option, the use of wind machines is growing. Wind machines effectively help to “stir up” the air and mix the warmer air (which rises) with the colder air that is found near the ground and the crop. This condition — warm air up high, cold air down low — is known as a temperature inversion. They usually occur on cloudless nights with little to no wind.

In comparison, under advective conditions (when a large mass of cold air moves into a region and is accompanied by winds) the use of wind machines is not advised because the colder air tends to be aloft, rather than close to the ground and the crop.

Therefore, measuring temperature inversion can be useful for growers to a) determine whether wind machines are suitable for their site and b) whether to use a wind machine for a particular frost event. To measure temperature inversion, you need to measure the upper air temperature. Battany recommends a triangulated steel meteorological tower on a concrete pad with guy wires that can withstand wind and hold a sensor. The other option is a slender flexible mast, which is inexpensive and easy to install, but it’s worth noting these can only hold very small temperature sensors and may break under extreme winds.

The data collected by the temperature sensor at 5 feet and 30 feet will indicate inversions to growers. For example, during the day the temperatures at 5 feet will be warmer than the temperatures at 30 feet but reverse at night. When a strong inversion is indicated, wind machines will be effective at warming the crop area by mixing the air. The data also can indicate wind conditions — if the wind speed is low, inversion tends to be strong.

For more information, read “Temperature Inversion Data Helps Guide Frost Responses.”