Temperature Differences

Measuring the temperature of the air seems like a simple thing, but in fact getting an accurate measure is actually not so trivial. For meteorological/cimatological purposes we of course want to keep the sensor out of the sun (because we're interested in the temperature of the air, not the temperature of the thermometer). This is done by enclosing the thermometer (or temperature sensor) in some kind of enclosure. But you want to keep the air moving through the enclosure, otherwise the mere fact that there is an enclosure will impact the temperature of the air inside.

So, there are basically two approaches used. First, have a fan move air from outside the enclosure across the thermometer and then out, or design you enclosure so that air can move freely through. The fan approach is termed aspirated, the second is termed passive. Usually there is not not much difference, but conditions this week, with brilliant sunshine, a still solid snow cover and not much wind are producing small but consistent differences. The graphic above shows the high temperatures on Wednesday. As you would expect, aspirated sensors run a little bit cooler.