On the temperature-humidity correlation and similarity

The correlation coefficient between temperature and specific humidity (r(Tq)) is considered. A brief literature review as well as experimental data reveals that there is evidence that the eddy diffusivities for heat and water vapour are not equal if |r(Tq)| not equal 1. Several conditions under which this can occur are discussed. It is concluded that if non-local effects are of the same order as local effects T and q will behave in a non-similar fashion. Moreover, it is shown that if |r(Tq)| = 1, temperature and specific humidity both will obey Monin-Obukhov similarity. In addition, based on that derivation, a new method is presented to determine the Bowen ratio from the T - q correlation. A first test of this new Bowen ratio regression method is presented. It is recommended to use r(Tq) as a diagnostic tool to judge whether T - q similarity holds.