Measured radiative flux divergence near the ground at night



Nocturnal radiative flux divergence profiles in the lowest few metres have been directly measured and typical cases are presented in graphical form.

Comparison of flux divergence calculated from radiation charts with measured divergence between 0·5 m and 1·5 m for a wide range of wind speeds and cloud conditions has shown the measured value to be up to 2·8 times larger than the calculated one. This discrepancy and others are attributed to the presence of haze.

Since radiative cooling rates were generally found to be higher than actual rates, development of the nocturnal inversion near the surface is shown to be predominantly caused by radiation, with eddy cooling supplanted in the lowest layers by eddy warming.

Periods of radiative warming sometimes associated with long-period (40 min) air-temperature fluctuations were found. A possible explanation is given for these and for fog formation in airlayers not in direct contact with the surface.

Comparisons are given between radiative ground temperatures and those obtained from standard grass thermometers.