I made this night cloud sensor a few years ago but thought it would be a good project to put on here for others to have a go.
What does it do?
During the day, WD relies on the Solar readings to predict the current conditions. At night, WD has no way of reporting the cloud cover or conditions (unless there’s precipitation) so we tend to rely on the conditions reported from our closest METAR station.
The Night Cloud sensor or detector is used to supply WD with the difference in temperature between the ambient outside air temperature and the temperature inside a glass jar pointed skywards. From this, WD can work out how much cloud is above the station and report the current local conditions at night.
In basic terms, if the temperature between the outside air & Night Cloud sensors is about the same (+/- 0.2°C in my case) then WD will report the sky as overcast.
If the Night Cloud sensor temp is lower that the air temp by more than your setting (1.8°C in my case) then WD will report a clear sky.
This is because overhead cloud will reflect the ambient heat in the air but with a clear sky you are effectively measuring the temperature at higher altitudes, which are colder than the air on the ground.
Working off the difference between these two temperature readings, WD is able to calculate the assumed amount of cloud above the station.
I should add that this is my interpretation of what is happening and I’m happy to be corrected. 
Equipment
I have a Davis VP2 Plus weather station and, at the time I made this cloud sensor, I had just replace my ISS transmitter as the original had stopped working. I managed to get the old ISS going again so decided to use it as the transmitter for an extra temp sensor.
As well as the transmitter I needed:
- Davis 6477 temperature probe with the RJ connector.
- Small glass jar with lid
- Small plant pot tray
- Black packing foam
- Silicon sealant
The first job is to make sure the glass jar is clean and dry on the inside and the lid fits securely.
Then cut the packing foam so it’s a push-fit into the top of the jar (don’t fit it yet though!). If you can’t get hold of black foam, then use white and colour it black. You can also use polystyrene if you can’t get foam, it doesn’t matter as long as it will insulate the temperature sensor from below and is black.
Now make a hole in the centre of the packing foam large enough for the temperature sensor to be pushed through the foam but so that it’s still a tight fit, to hold the sensor in place.
Drill a hole in the lid of the jar and in the centre of plant pot tray, large enough to pass the sensor through the lid & tray. Also make a cut-out on the side of the tray for the cable to exit.
Assembly
Pass the tip of the sensor through the plant pot tray, the lid of the jar and the packing foam so the sensing part is sitting above the foam in clear air.
Push the packing foam into the top of the jar and screw on the lid.
I put two self-tapping screws through plant pot tray and into the lid of the jar to secure it better and give a stable base.
Now use the sealant to seal the lid to the jar and also seal the lid where the sensor cable goes through the hole to stop any water getting in.
You can also use the sealant to glue the plant pot tray to the top of the jar lid if you don’t want to use the self-tapping screws.
Now mount the completed sensor in a location which give it a clear and unobstructed vertical view of the sky and connect it to the transmitter.
Note: I updated this post when I took the sensor apart to clean out the condensation from the glass jar. That enabled me to take extra photos of the assembly so the post is better illustrated.
