Abstract
Shade temperatures and humidities in a standard screen and at an arbitrary level of 15 cm in potato crops were continuously recorded during three summers, 1947β49.
In dry sunny periods : (1) the maximum temperature in the crop was from 0Β° to 13Β°F higher than in the screen, and over a period of eleven weeks in 1947 averaged 6Β°F higher (2) the crop minimum was about 2Β°F lower than in the screen (3) over a period of five weeks in 1949 the average daily mean temperature in the crop was 2.2Β°F higher than in the screen, and (4) the average daily temperature range was 8Β°F greater than in the screen.
Wind, wet soil and cloudy weather greatly reduced these contrasts; occasionally the crop minimum was higher than the screen minimum.
By day, humidity was higher in the crop than in the screen, the average difference at the minima being 5 per cent in relative humidity, and 7Β°F in dew point. By night, the dew point in the crop was, on average, 2Β°F lower than in the screen, corresponding to the observed difference in mean minimum air temperatures.
During 1948 and 1949, more precise discontinuous records were taken of temperature, humidity and wind speed in and above potato crops with different densities of foliage in a variety of weather conditions. Temperature and humidity were measured at 10, 20 or 30, and 60 cm; wind at 20, 30 and 200 cm above ground level. In the crop there was only a slight temperature gradient in the early morning and throughout cloudy days, but during sunny days gradients were produced, depending on crop density; it was hottest at 10 cm in an open crop, at 30 cm in a dense crop; in both it was usually coolest at 60 cm when the soil was dry, but over wet soil the lowest temperature was found at 10 cm. Except on cloudy days, temperature inversion took place before sunset whatever the foliage density or moistness of the soil.
Humidity (dew point) was usually greatest at 10 cm, but in a dense crop over dry soil water vapour transpired from the leaves often caused the air at 30 cm to be more humid than that at 10 cm.
Wind affected both temperature and humidity by increasing lapse rates and causing rapid fluctuations within the crop, particularly of humidity. Changes of wind speed within the crop took place every few seconds, the amplitude of the fluctuations depending on the speed of the wind above the crop.