|Annotated plot for 2013-05-30.|
With no clouds to reflect back thermal radiation the sky temperature for a clear sky is very low, typically < -10°C. This results in a large difference (> 20°C) between ambient and sky temperatures.
With the presence of clouds most thermal radiation is not lost to space but reflected back to Earth and the sensor. The difference between ambient and sky temperatures is small (< 10°C). With a uniform cloud coverage the variation in the difference is small.
A sky with partial cloud cover has an intermediate difference in ambient and sky temperatures. The cumulus clouds resulted in a large variation of the temperature difference, caused by the more sensitive central viewing area observing alternately clear sky and cloud. Complete coverage by a thin layer of cloud is expected to give an intermediate temperature difference but low variability.
When the sensor is wet with rain the sky temperature is measured incorrectly since the water is not transparent to long-wave infra-red radiation and matches the ambient temperature. As wet weather implies cloud this is not a major problem although I have concerns over the time taken for the sensor to dry off.
The formation of dew is observed in the evening as the ambient temperature falls. The difference between ambient and sky temperature is reduced to less than 10°C.
Future workEarly operation shows that dew is a problem and can indicate cloud when the sky is actually clear. I have considered creative solutions such as painting the top of the box white and the bottom black in order to minimise heat lost to the sky and maximise heat absorbed from the ground. I expect this will only delay the onset when dew becomes a problem, not eliminate it. The only viable solution is to heat the sensor slightly (~2°C) above ambient, which means moving away from battery-powered operation. If a cable is used for power then wireless communications makes little sense, power-over-ethernet seems the way forward. A heated sensor will also reduce the drying time after rainfall.
I still have some concerns regarding waterproofing around the sensor. I have ordered the MLX90614ESF-DCH-000-TU-ND which features a 12° field of view. This narrower view is obtained by fitting a refractive germanium lens above the sensor. The TO-39 package has been modified to be taller than standard to accommodate the lens. The additional height should mean that the top of the sensor is above the cable gland. Water should not collect so easily and it should be possible to use silicone sealant without accidentally coating the sensor window.
Previously I tried pointing the sensor downwards and using a metal surface as a mirror to view the sky. The experiment was not successful and I think this was in part due to the wide field of view of the sensor (90°) preventing a clear upward view without also observing the ground or enclosure. It will be worthwhile repeating the experiment with the narrow field of view sensor.
I only have basic plotting routines at present. The goal is to produce a cloud cover index, varying between 0 indicating no cloud and 1 indicating complete cloud cover. The index will be used as the basis for sending alerts.