Foliage exposed to the Sun is rarely at a temperature equal to that of the air around, because evaporation cools the leaves and sunshine heats them. The drier the air, the cooler the leaf of a well-watered plant is, compared to the surrounding air. A comparison of reported temperatures of the leaves of numerous well-watered plants in various parts of the world around midday, and of the adjacent air (T), indicated that the temperature difference between the leaf (T) the ambient air (Ts), (T - Ts), decreases as T rises (1). The difference tends to be positive at low air temperatures but negative in hot conditions. In the Australian study (1), it is about zero at around 30°C. This was subsequently corroborated in a survey of measurements by other people (2).
The topic was later examined by Idso and colleagues (3), who disputed the notion of a fixed temperature of about 30°C, at which leaf and air temperatures tend to be equal. Their work confirmed the decrease of temperature difference as T rises, but showed vegetation temperatures Tv almost always lower than air temperatures, in the case of irrigated alfalfa in Arizona. They estimated temperatures at which Tv equals the air temperature T from relationships between (T - Tv) and T, in turn based on those
The derived temperatures of equality were 11°C during dry winter conditions, and 28°C in a warm, humid summer, i.e. close to the 30°C reported by (1). The differences from (1) may perhaps be due to characteristics of an alfalfa crop and the method used to determine Tv. The crop is essentially three-dimensional, and a radiometer used for measuring Tv ‘sees’ much of the lower, shaded foliage, which is cooler than the surface leaves exposed to the Sun, especially when the Sun is low in winter. This would explain why Tv rarily exceeded T in Idso’s work. The temperature of individual leaves in the midday Sun could be much higher.
Support for this hypothesis comes from the nature of Idso’s linear relationship between the vpd and (T - Tv) for alfalfa. It strikingly resembles the equation for a wet-bulb thermometer, which is based on the assumption that there is no radiation affecting the energy balance of the evaporating surface. In other words, evaporation from an alfalfa crop may be chiefly from shaded leaves. This is a situation quite different from that of the separate fully exposed leaves relevant to Linacre’s papers (1,2).
(1) Linacre, E.T. 1964. A note on a feature of leaf and air temperatures. Agric. Meteor. 1, 66-72.
(2) Linacre, E.T. 1967. Further notes on a feature of leaf and air temperatures. Archiv. Meteor. Geophysik. Bioklim. B 15, 422-36.
(3) Idso, S.B., R.J. Reginato, R.D. Jackson & P.J. Pinter 1981. Foliage and air temperatures. Agric. Meteor. 24, 223-6.