Cloud Sensor

A simple thermoelectric infrared sky temperature sensor is mounted on the north west corner of dome. The voltage from the sensor (irvolts) and the case temperature (ovolts) are measured and then a relationship is derived to compute the aproximate cloud cover.

The field of view is about 60 degrees. And therefore does not give an all sky view. The sensor only responds to low level warm clouds and not cirrus.

The voltages are measured using the Test points.

Software

The command taux server command will read the voltages and do the conversion.

Calibration

The infrared sensor puts out a voltage depending on the difference between the case temperature and the sky temperature. The sensor voltage is denoted irvolts and the case temperature is related to ovolts. I determine the "cloud" calibration by observing the range of irvolts and ovolts over as wide a range of conditions as possible. Here is a graph taken over early 2023.

The upper envelope is the clearest sky. The bottom represents cloud. The amplifier can only produce positive signals and there unfortunately seems to be a negative bias, which I will remove by installing a resistor in the future. However we can still continue. By assuming the "clear" value is the red in the graph and "cloudy" is 0 irvolts. We come up with the crazy formula that: 

double ircloud=1.8065-.075*ovolts;
double irclear=1.22504+.192*ovolts;
double cloud=(irclear-irvolts)/(irclear-ircloud);
where cloud ranges from 0 --> 1.0. The formula does a good job for clear skies and lousy for really cloudy ones. In other words if cloud > 0.2 don't bother.

Pictures

Picture of the cloud sensor on the dome.

Device Index