Abstract:
As the world warms, it will be tempting to relate the biological
responses of terrestrial animals to air temperature. But air
temperature typically plays a lesser role in the heat exchange of
those animals than does radiant heat. Under radiant load, animals
can gain heat even when body surface temperature exceeds air
temperature. However, animals can buffer the impacts of radiant heat
exposure: burrows and other refuges may block solar radiant heat
fully, but trees and agricultural shelters provide only partial relief. For
animals that can do so effectively, evaporative cooling will be used to
dissipate body heat. Evaporative cooling is dependent directly on the
water vapour pressure difference between the body surface and
immediate surroundings, but only indirectly on relative humidity. High
relative humidity at high air temperature implies a high water vapour
pressure, but evaporation into air with 100% relative humidity is not
impossible. Evaporation is enhanced by wind, but the wind speed
reported by meteorological services is not that experienced by
animals; instead, the wind, air temperature, humidity and radiation
experienced is that of the animal’s microclimate. In this Commentary,
we discuss how microclimate should be quantified to ensure accurate
assessment of an animal’s thermal environment.We propose that the
microclimate metric of dry heat load to which the biological responses
of animals should be related is black-globe temperature measured on
or near the animal, and not air temperature. Finally, when analysing
those responses, the metric of humidity should be water vapour
pressure, not relative humidity.
