Beware! Your thermometer may not be giving you a reliable reading on these bright, sunny early spring days. I get calls and emails when there is something remarkable going on; like the first taste of genuine spring weather weâ??ve experienced the last few days. Believe me, I appreciate the information I get. However, this is worst time of the year for â??sun-aidedâ?? temperature readings.
Meteorologists measure the temperature of the air. That is why thermometers are placed in shelters to shade them from the sun. Obviously, it does get warmer on a sunny day than a cloudy day. However, the sun only heats the air to a limited extent compared to a solid surface like a thermometer. There are clear guidelines for a weather â??instrumentâ?? shelter. They are painted white; the shelter has a venting system to allow the free flow of air and it must be placed away from buildings that might add heat. Also, a shelter should be located over a grassy surface to avoid heat contamination from pavement, etc.
Now we come to the issue of this post. I know from experience that the phenomenon of â??sun-aidâ?? reaches its maximum extent for the year during the early spring. Hereâ?? why: the increasing sun angle of late March shines on the remaining snow cover. The light reflected off the snow is a source of heat that will give your thermometer or temperature sensor an inflated reading. For instance, yesterday my high temperature went to 49.5 degreesâ??weâ??ll call it 50. The National Weather Service (obviously sheltered properly) three miles from my house had a high of only 39.
I have my sensor placed under the railing plate of my deck sheltered from the sun on the north side of the deck. Yet, my reading was 10 degrees higher than the NWS. This can only be explained by the reflection off the snow adding extra heat to my sensor. The greatest discrepancy between my temperature readings and the NWS readings occur precisely at this time of the year (By the way, at night, my lows are very close to the NWS lows all year.). My daytime readings match the NWSâ??s best during late fall when the sun angle is the lowest.So, please keep on notifying me of your highs, lows, rain, snow and other meteorological phenomenon. But please do not be offended if I fail to mention your sun-aided high temperature, especially in the early spring.