Measuring cloud cover and brightness temperature with a ground-based thermal infrared camera

Thermal infrared cameras can be used to monitor clouds and the sky at high spatial and temporal resolutions. In particular, this study shows that, without the need for any external data, cloud cover can be retrieved both day and night over a field of view extending to zenith angles of similar to 80 degrees. Zenith clear sky temperatures are estimated for cloud cover up to 80%. During periods of 50% cloud cover or more the cloud-base brightness temperatures (CBBTs) can be calculated to an accuracy of +/-1 K. These calculations are made possible by using a new parameterization for the variation of sky brightness temperature with zenith angle. Both clear and cloudy conditions are found to follow this simple empirical equation more closely than the widely used parameterization of Unsworth and Monteith. A simple, angle-dependent threshold system based on cloud transmittance can then be used to retrieve cloud cover, and clear sky temperature and CBBT are calculated using the two parameters resulting from the fitting process.