THE DEWPOINT TEMPERATURE
One of the important weather factors that growers should pay attention to in the frost forecast is the dew point temperature. The dew point temperature is the temperature to which the air must be cooled before dew or frost begins to form. The dew point temperature is also a measure of the amount of water vapor in the current air mass. The higher the dew point, the more water vapor there is in the air. Air holds water in various amounts in the invisible gas state, called water vapor. The amount of water vapor in the air, and therefore its dew point, varies with the different air masses that move in and out of the eastern Washington. Therefore, knowing the dew point gives an idea of how moist or dry the air is.
At night, the temperature of the air cools down and often reaches its dew point temperature, so the water vapor in the air changes to a visible liquid (dew) or solid (frost). During this change in state, from vapor to a liquid or to frost, a large amount of heat is released. It is this release of heat during the change in state from a gas to a liquid or solid that is important for frost protection. If the dew point is high, meaning more water vapor in the air and dew forming while temperatures are fairly warm, then the heat released slows down the normal fall in temperature at night and keeps the air temperature from getting too cold. On the other hand, if the dewpoint is low, meaning dry air and dew or frost not forming until temperatures get colder, then heat from the change in state is not released until much later at night, or possibly not at all if the air is so dry that dew or frost do not form. In this case the air temperature can drop rapidly and reach dangerous levels.
Usually, the dew point temperature runs in the 30s and 40s in the air masses that cover eastern Washington in the spring. This means that on most nights, when temperatures are falling toward the low 30s, dew will form and slow the temperature fall, keeping temperatures either above critical, or at least from falling so low as to make frost protection difficult. Occasionally, very dry air invades eastern Washington in the spring, resulting in dew points in the teens and low 20s. These dew point values are considered dangerously low. When dew points are this low, the dew point temperature will not be reached so little or no heat will be added to the air. Temperatures will drop steadily and rapidly and can reach critically cold levels. Protection is usually more difficult and must be started well before critical temperatures are reached, due to the fast drop in temperature. Fortunately, low dew point cases are fairly rare and usually occur early in the frost season, when buds are hardier.
Very often the dew point temperature is nearly the same over a fairly large area and usually changes only slowly with time, especially when the dew point is in its normal range. This means, for example, that the dew point reported at Yakima airport in the early evening will likely be representative of much of the Yakima valley and will stay about the same or fall slightly during the night. When the air is very dry, with low dew point temperatures, there is often more variability and fluctuation in the dew point over the area. During these very dry cases, growers may need to look at several stations over a period of a few hours to assess a representative dew point for the area.
In sum, growers should pay attention to the dewpoint temperature. Dewpoints in the 30s to low 40s are normal and while not ensuring against freezing temperatures, they usually mean temperatures will not fall extremely fast or reach critically low levels and frost protection measures will usually be successful. Dew points running in the teens and low 20s are critical and could mean a long, cold night with possible difficulty in keeping temperatures above critical levels. Clearwest gives the current dewpoint temperature for several sites in eastern Washington in their daily frost forecast. Growers can also select the “Current Washington Weather” button on our web site to get the latest hourly weather, temperature and dew points values (shown under the DP heading) from reporting stations in eastern Washington