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Building a Thermoelectric Sensor to Measure
IR Sky Transparency for Cloud, Cirrus and Dust Detection, at Night

A side view of the detector, showing the two aluminum plates and the Peltier device sandwiched between them.

This style of cloud detector was invented by Lou Boyd, of Fairborn Observatory in Arizona. He came up with the idea of running a Peltier device, backwards, to measure the sky temperature. The thermolectric (Peltier device) based cloud detector consists of two aluminum plates sandwiching the thermoelectric device inside. The device functions as a night time cloud detector, because it generates a a measurable voltage, when there is a temperature variation across it's two aluminum plated sides.

Another side view of the detector, showing the two aluminum plates and the Peltier device.

To detect clouds, one plate is positioned facing the night sky, while the other plate faces the ground. Infrared radiation from the ground warms the ground facing side of the detector, while the opposite sky facing side either sees a warm, clouded sky, or a cold clear one. On clear nights, a warm ground, and a cold, cloudless sky, produce a voltage on the device because the two plates have different ambient temperatures.

Heat from fingers generates a rapid rise in temperature on one plate, causing a rapid voltage generation as well.

Under tests, sun shining directly on the cloud detector produced around 60 millivolts of voltage potential.

How the cloud detector works.

Other test results follow:

100% cloudy night = - 0.56 mv

075% cloudy night = - 1.13 mv

050% cloudy night = - 1.14 mv

025% cloudy night = - 1.40 mv

000% cloudy night = - 1.50 mv

Soaking wet detector -1.45 mv

When clouds roll in, the sky temperature increases due to the clouds emission of infrared radiation, causing the temperature variance between the two sides of the detector to decease. In other words, clouds can be detected at night because their presence causes a voltage decrease in comparison to the detectors voltage output during clear skies.

How the cloud detector works.

In order to monitor these voltage variations, the cloud detector's voltages are turned into digital pulses, which are sent to a computer through a serial line. A python program will monitor the pulses and generate a graphical report of the current and recent cloud trends. The report will be available on the network for web based cloud monitoring at night.

How the cloud detector works.

Building the circuitry to turn the voltages into pulses, and the microcontroller to turn the pulses into ascii for transmission to the receiving computer is the next phase.

How the cloud detector works.

The computer will run a Python program to read the ascii from the microcontroller on the cloud detector end of the serial line. Once the values are operated on a log file will be generated and the data set up to be read by the mountain network, so that telescope operators can view the current cloud conditions from the web.

How the cloud detector works.

Bill Gillespie
Kitt Peak National Observatory
Observing Support Office