Abstract
Dewpoint temperature is the temperature to which a body of air at a constant pressure must be lowered to initiate the condensation of water vapor. Because air temperature never falls below dewpoint temperature and because dewpoint temperature varies far less over the 24-hour cycle than does air temperature, dewpoint temperature is useful in forecasting minimum temperatures in fruit and vegetable fields on nights with radiation freezes. Dewpoint temperature is also important because the difference between air temperature and dewpoint temperature strongly affects the rate of evaporation and the effectiveness of overhead irrigation in freeze protection of crops. This study was designed to determine whether there was a diurnal pattern that might be useful in predicting short-term changes in dewpoint at three agricultural locations in north Florida. Data were studied from nights during the winter half of the year that featured good radiation conditions and frost or dew formation. A well-defined diurnal rhythm was found, with 2 peaks and 2 valleys per 24-hour period. The cycle included a morning rise, a midday slump, an evening recovery, and a nighttime fall. For the Alachua station, which is typical of the three stations studied, the mean change in dewpoint over 47 nights was -3.7ordm;C between 1 hour after sunset and 15 minutes before sunrise. Between 15 minutes before sunrise and 2 hours after sunrise the mean change in dewpoint at Alachua was 5.7ordm;C. This morning rise is believed to be due to evaporation of water that was deposited during the night by frost, dew, distillation, and guttation. The midday slump results when turbulence mixes this new water vapor with drier air above. The evening recovery is due to declining wind speed, which allows the surface air to gain water vapor from evaporation and transpiration, and the nighttime decline is due to the removal of water vapor from the air at instrument level (2 m above the ground) as a result of dew or frost formation. Although the nighttime decline in dewpoint was a pronounced feature, not every night showed a decline, and the extent of decline varied considerably.
