RECENT communications1,2 have directed attention to the importance to the biologist of the peculiar microclimate on the ground and among ground vegetation. A thin hot layer of air covers the surface of bare earth in sunshine, and among vegetation it is lifted to a height which depends upon the height and density of the vegetation. Measurement of the temperature of this air is difficult: the temperatureprofile is steep and is likely to be disturbed by the ventilation of an aspiration thermometer. When using an aspiration thermocouple over bare earth, I have found that the galvanometer reading rises initially to a high value and then falls, suggesting that hot air is being withdrawn and replaced by colder.