These data are produced by a Boltwood cloud sensor, mounted just to the west of House M, which measures the 8-14 micron IR emission from the sky using a single element thermopile detector. The measurements are recorded as a temperature difference between the ambient, as measured by a separate thermistor, and integrated sky temperature in a roughly 90 degree diameter cone. When the sky temperature is more than 25 Celsius degrees colder than the ambient, it indicates clear enough skies for our site testing equipment to operate. The reliability of the measurements is best at night, as solar heating affects the ambient temperature slightly.

The green line separates clear (below the line) from cloudy (above the line) conditions. The yellow line separates cloudy (below) from very cloudy (above) conditions.

A wetness sensor is also integrated. When this sensor detects rain, the red line on the plot above jumps up above the zero mark. The presence of moisture on the sensor makes for erratic sky temperature measurements, so when its raining, the presence of clouds is assumed even though the recorded temperture difference might indicate otherwise.