Air Temperature and Humidity

Air temperature shows unstable profiles during the day and stable profiles during the night. Therefore, canopy temperature is generally higher than air’s during the day and lower at night. Heat transfer between the crop and the atmosphere is sustained by turbulence and increases with wind speed, i.e. as aerodynamic resistance decreases. In situations of unstable atmosphere, turbulence is enhanced (by added thermal turbulence) while in a stable condition, turbulence is reduced. The atmosphere water vapor content can be expressed by different variables (vapor pressure, relative humidity, vapor pressure deficit, mixing ratio, and vapor density). The flow of water vapor (equivalent to energy spent as latent heat) between the crop and the atmosphere is directly proportional to the vapor pressure difference and inversely proportional to the sum of the canopy and aerodynamic resistances.