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.