PREDICTION OF DAILY MEAN VAPOR DENSITY FROM DAILY MINIMUM AIR-TEMPERATURE

Vapor density data are seldom available despite the fact that they are required for computing a wide range of agroclimatological factors. In this paper we examine the use of daily minimum temperature as a predictor of daily mean vapor density. The prediction is based on the assumptions that the daily minimum temperature approaches dew point temperature, and that the diurnal variation in vapor density is small. Analysis of data from three north Australian agrometeorological stations showed that vapor densities could vary significantly during the course of a day, sometimes exceeding 10 gm-3 at the arid inland site during the summer months. Minimum air temperatures at the arid site also failed to reach dew point on most occasions, the exceptions in thc summer months usually being associated with rain periods. This was not the case at the humid coastal sites, where there was no systematic departure of minimum air temperature from dew point. Minimum air temperature therefore proved a good predictor of daily mean vapor density at the more humid coastal sites, accounting for 75-90% of the variation in daily mean vapor density. By contrast, it accounted for only 50% of the variation in daily mean vapor density at the arid inland site. It was only by using humidity measurements to account for the departure of minimum air temperature from dew point temperature that reasonable estimates of daily mean vapor density (accounting for 80% of the variation) were obtained. Minimum air temperature is therefore not recommended for routine use as a predictor of daily mean vapor density at arid inland locations.