## Abstract

The non-linear temperature profile which obtains in the lowest 20 or 30 m over heated ground allows convection plumes, once formed, to maintain themselves in existence without any preferred heat supply. These plumes should form in general as elongated strips oriented downwind, their average section becoming more nearly circular as the negative Richardson number $|$Ri$|$, increases. The heat flux H from a large area, by the agency of plumes of this type, is shown to satisfy the theoretical law (Priestley 1954) of free convection, H$\propto \rho $c$_{p}\Big(\frac{g}{T}\Big)^{\frac{1}{2}}$z$^{2}\left|\frac{\partial T}{\partial z}+\Gamma \right|^{\frac{3}{2}}$, with a constant of proportionality which conforms to the observed value (T is the temperature at height z and $\Gamma $ the adiabatic lapse rate). The observed validity of the above law in conditions of quite appreciable wind is at the same time explained. The value - Ri = 0$\cdot $011 is deduced from theory as indicative of that at which free convection should begin to compete with forced convection as an effective agent of heat transfer, and it should become dominant at a value not many times greater. This supports the indications from earlier experiments. Various other lines of evidence confirm the reality of the plume models.