The heat load on a leaf exposed to full sunlight is very high. In fact, a leaf with an effective thickness of water of 300 µm would warm up by 100°C every minute if all available solar energy were absorbed and no heat was lost. However, this enormous heat load is dissipated by the emission of long-wave radiation, by sensible (or perceptible) heat loss, and by evaporative (or latent) heat loss (see textbook Figure 9.14).

Air circulation around the leaf removes heat from the leaf surfaces if the temperature of the leaf is higher than that of the air; this phenomenon is called sensible heat loss.

Evaporative heat loss occurs because the evaporation of water requires energy (see textbook Chapter 4). Thus, as water evaporates from a leaf it withdraws heat from the leaf and cools it. The human body is cooled by the same principle, through perspiration.

Sensible heat loss and evaporative heat loss are the most important processes in the regulation of leaf temperature, and the ratio of the two is called the Bowen ratio (Campbell 1977):

This concept was developed by Ira S. Bowen (1898–1978), an American astrophysicist. When the evaporation rate is low, because water supply is limited, the Bowen ratio tends to be high. Thus, the Bowen ratio is about 10 for deserts, 2–6 for semi-arid regions, 0.4 to 0.8 for temperate forests and grasslands, 0.2 for tropical rain forests and 0.1 for tropical oceans (Nobel 1999)

In well-watered crops, transpiration (see textbook Chapter 4), and hence water evaporation from the leaf, is high, so the Bowen ratio is low. On the other hand, in some cacti, stomata closure prevents evaporative cooling; all the heat is dissipated by sensible heat loss, and the Bowen ratio is infinite. Plants with very high Bowen ratios conserve water but have to endure very high leaf temperatures in order to maintain a sufficient temperature gradient between the leaf and the air. Slow growth is usually correlated with these adaptations.

Very low Bowen ratios can be measured in a lawn on a relatively still day. In these conditions there is no sensible heat loss, because the air around the leaf is at the same temperature as the leaf; the Bowen ratio therefore approaches zero, and heat dissipation is due mostly to evaporative heat loss.

In other cases, such as cotton leaves in the afternoon, water loss from stomatal transpiration cools the leaf below the air temperature by evaporative heat loss (see textbook Chapter 20). In that case, there is sensible heat gain rather than heat loss, and the Bowen ratio becomes negative.

One can calculate the evapotranspiration rate for an entire canopy using measurements of the Bowen ratio, net incident radiation, the heat loss from the soil, and the gradients in temperature and water vapor concentration above the canopy (Ibanez and Castellvi 2000).