Already for some time one could notice a strong influence of sunlight on temperature readings. This is true for most “cheap” weather stations and has to do with the internal temperature of the outdoor housings used and the like. For instance, the central unit that is applied here originates from Alecto and is common to most of their current units. It consists of a Stevenson screen with an internal case where the sensors and associated electronics are housed. The Stevenson screen would be great if it were not so small! There is hardly any air ventilation possible. Obviously, this warms up in the sun.
The solution to the problem is clear: a larger Stevenson screen. And actually, most of the other sensors and associated electronics need also not be in. Such devices are for sale albeit at relatively elevated prices, see e.g. the Ambient Weather model SRS100LX. It even features a solar power driven fan: when there is sufficient irradiation, the fan will force the air to flow inside the screen.
Interestingly, many weather station owners discard of their hardware after a while, usually because it is no longer functioning. This opens the opportunity to acquire another – empty – Stevenson screen that one can tinker to ones needs.
The circular shaped solar panel, the low friction electro motor and the fins were obtained from a cheap solar fan to wear on one’s hat or so. They are of the order of 5$. The solar panel is glued to the top of the screen and the screen itself is held by a zinc protected iron frame. The motor is glued to the frame and the fins are cut to size so that they freely rotate within the frame.
As a temperature one may use a water-proof, pre-wired DS18B20 tempeature sensor with 1-wire inferface, This device readily couples to the Raspberry Pi controller. Problem might be its metal shield. This could under circumstances lead to too low temperature readings. We’ll have to see. So far, the readings were significantly below previously measured values and also below those of neighboring stations. A week of experience may tell more …
Added 18 November 2018:
The DS18B20 temperature sensor was not a success. It gave way too low temperatures compared to weather stations in the neighborhood. Another option was the AM2315 integrated temperature/humidity sensor. It appeared to just fit the housing that was made and air movement produced by the fan would certainly pass by. So far, temperatures stayed in line with those in the neighborhood without excessive overheating and that was the whole idea.