Dewpoint is a powerful forecasting tool. The amount of moisture in the air and the percent of saturation of the air influence temperature patterns. The dewpoint can be used to forecast low temperatures. The low will rarely fall far below the observed dewpoint value in the evening (unless a front brings in a different air mass). Once the temperature drops to the dewpoint, latent heat must be released to the air for the condensation process to take effect. This addition of heat offsets some or all of further cooling. This is especially true for dewpoints above 55 degrees. If the dewpoint happens to be much lower than the temperature, the air will cool off much more rapidly at night than if the dewpoint was closer to the temperature in the evening. This is why dry regions such as the high plains and desert regions have such large differences between the high and low temperature. Moist areas, such as regions near the coast tend to have a smaller spread between the high and low temperature. If the air has a high moisture content, some longwave radiation emitted by the earth's surface will be radiated back toward the surface. Water vapor is a greenhouse gas. This is another reason humid (especially warm / humid) places have much warmer lows than dry locations at about the same elevation and latitude. If dew is very heavy in the morning, the temperature will not rise as quickly when compared to situations when there is no dew. This is because as dew evaporates, it cools the surrounding air (absorbs latent heat). Eventually the sunshine will overpower the cooling produced by evaporation, but the temperature will be cooler than a location that did not experience any dew. Transpiration from trees also absorbs latent heat. This is a primary reason why dry areas at the same altitude and latitude as moist locations tend to have warmer high temperatures. |