For the past ten days I’ve been collecting data from three sets of temperature sensors located in different places around the yard. There’s the sensor in the Rainwise weather station at the top of the dog yard gate, a collection of sensors out behind the house under the oil tank, and a set of sensors under a collection of yogurt containers on top of a foundation post on the west side of the house.
It’s not easy to get accurate temperature readings. You need to site the sensors where they’ll get a good reading (between 4 and 6 feet off the ground, out in the open and away from buildings and trees), keep the sensors from getting heated by the sun, and keep them dry both from rain and snow, as well as from condensation inside an enclosure. I’ve got the last one figured out, but siting and solar radiation are proving to be big challenges.
The plot below shows the hourly average temperature readings for all three sets of sensors over the last ten days since I added the west sensor.
The sensor atop the dog yard gate (the red line) is well sited in terms of it’s distance from large objects like buildings and trees, but it’s too high off the ground. It’s enclosed in a Gill multi-plate radiation shield, which is effective at reducing the effect of solar radiation when the wind is blowing. Compared with other temperature sensors in the region, this sensor is commonly several degrees warmer during the middle of the day, and I think this is because the shield isn’t keeping the sensor cool enough. We do seem to get less wind than in other places, and I think this is why the shield isn’t working as well as it should be. The sensor’s location away from everything does allow it to reach accurate minimum temperatures at night.
The sensor cluster behind the house is effectively shielded from the sun because it’s very close to the north side of the house, and even when the sun is in the north (in Fairbanks, the sun comes pretty close to circling the sky in summer) there are trees behind the house that keep it shaded. But it’s much too close to the house, and the location is far more sheltered than is appropriate. The moderating effect of being so close to the house reduces the diurnal temperature range, clipping the highs and lows compared to the data from the dog yard sensor.
On the west side of the house, I’ve got three sensors sitting on top of a foundation post (a telephone pole driven into the ground). There are several layers of yogurt container on top of the sensors, both to protect them from rain, but also in an attempt to reduce solar heating under the containers. The radiation shielding appears to be almost as effective as the commercial Gill shield over the dog yard sensors (the high temperature peak on the graph is very similar between the two), but something is keeping the temperature from dropping at night. The low temperatures from the west sensors are more than 5 degrees warmer than the dog yard sensor. I suspect the sensors aren’t high enough off the ground, and that the foundation post may be absorbing a lot of heat during the day and keeping the sensors artificially warm at night.
My plan is to place the west sensors inside the Stevenson shield pictured at the top of this post. I’ll raise it to between 4 and 6 feet in the air, and see how the temperatures compare with the dog yard sensors. I’m also working on a solar powered aspiration system in case the Stevenson screen doesn’t have enough of an effect on the high temperatures on sunny days. I haven’t quite worked it out yet, but the idea is to put a small computer fan on the top of a short piece of 4” plastic pipe that contain the sensors. When the sun is shining, the solar panel drives the fan, which pulls air up through the pipe and over the sensors. We’ll see if it’s needed in the next few days.