Estimating cloud type from pyranometer observations

In this paper the authors evaluate an inexpensive and automatable method to estimate cloud type at a given location during daylight hours using the ti me series of irradiance from a pyranometer. The motivation for this investi gation is to provide ground-based estimates of cloud type at locations wher e there are no human observations of sky condition. A pyranometer naturally measures the effect of intervening clouds along the solar beam path to the sensor. Because a daily time series of irradiance is nonstationary, it is appropriately scaled to yield a stationary time series. From the latter, th e standard deviation and ratio of observed irradiance to clear-sky irradian ce derived from a 21-min moving window are related to one of the following cloud types or conditions: cirrus, cumulus, cirrus and cumulus, stratus, pr ecipitation or fog, no clouds, and other clouds. Comparisons with human obs ervations at the Department of Energy Atmospheric Radiation Measurement Cal ibration and Radiation Testbed site in northern Oklahoma show that the pyra nometer method and human observations are in agreement about 45% of the tim e. Many of the differences can be attributed to two factors: 1) the pyranom eter method is weighted toward clouds crossing the sun's path, while the hu man observer can view clouds over the entire sky, and 2) the presence of ae rosols causes the pyranometer to overestimate the occurrence of cirrus and cirrus plus cumulus. When attenuation of the solar beam by aerosols is negl igible or can be accounted for, the pyranometer method should be especially useful for cloud-type assessment where no other sky observations are avail able.