Use of Dewpoints in Determining Frost Potential

The ability of the air to hold water vapor is dependent upon its temperature. The higher the air temperature, the greater the amount of water vapor that the air can potentially hold. The dewpoint temperature is defined as the temperature at which water vapor in the air will condense (into dew, fog, or clouds) if cooled (at constant pressure). When the air temperature falls to the dewpoint, the air is said to be saturated, as it cannot hold any more water vapor. Any excess moisture is condensed out into liquid or ice. Relative humidity is defined as the amount of water vapor that is actually in the air divided by the amount that could be potentially held by the air at that temperature level.

Many of you may have a sensor that gives wet bulb temperature, which is not the same as dewpoint temperature. The wet bulb temperature is the temperature to which air can be cooled by liquid water evaporation (the rate of cooling is dependent upon how much water vapor is already in the air). This is normally measured by moving air across a thermometer bulb or sensor covered with a wetted wick (usually with either a fan or by a hand-held sling). Given the wet bulb temperature, one can also obtain the dewpoint temperature and relative humidity. For reference, the dewpoint temperature at a given time will always be less than or equal to the wet bulb temperature, which will always be less than or equal to the air temperature. The only time all three of these temperatures can be equal is at 100% relative humidity.

A common rule of thumb in frost control strategy assumes the minimum temperature on a given morning will approach the dewpoint temperature taken the prior evening. This "rule" has some scientific merit for the most common spring frost scenario; clear, calm conditions with cold air drainage and vertical temperature stratification. In this situation, there may be little transport of moisture in the air in or out of a general area, so the dewpoint temperature taken the prior evening will be indicative of the moisture in the air during the following several hours. When the air temperature does fall to the dewpoint temperature, two things happen. First, water vapor condenses out of the air and gives off heat (from the phase change of water vapor to liquid water). Secondly, some of the vapor may condense in the air itself, forming fog. This in turn reduces the amount of heat lost by the ground to space. Both of these combine to prevent the air temperature from falling much further. From past experience, you may have noticed that the minimum temperature during one of these events commonly remains a few degrees above the dewpoint temperature. This is probably because the air temperature is taken at 5 feet above the surface, and under these calm, clear conditions, it is warmer than the temperature at or near the surface, which is relatively cooler and has reached the dewpoint.

Some important notes of caution with this rule of thumb. First, it is _only_ helpful under the circumstances described above (relatively clear, calm conditions). Freezing temperatures experienced under cloudy, windy conditions would be much more complicated to determine (based on the temperature of the airmass upstream, etc.). Secondly, with the clear, calm frost scenario, it is also possible for the dewpoint temperature to drop a few degrees during the nighttime hours, as moisture is condensed out of the air and into dew or frost, resulting in minimum temperatures somewhat lower than those estimated the evening before. The best strategy is still to closely monitor conditions throughout the event, by occasional reading of thermometers and/or dewpoint/wet bulb sensors in the field or by electronic frost alarms.