Thermometer Placement
By Heidi Sonen & Roscoe Shaw
Our streak of eight straight months of below normal temperatures came to an end. October got off to a hot start with six days in the 80’s but the second half of the month was cold. We had some frost and a cold second half, but the month still finished half a degree above the long-term normal of 58 degrees.
Temperature reports are so common that we take them for granted, but it is actually difficult to measure accurately. Official records require a rather sophisticated instrument shelter and careful calibration. The tough part is to measure the temperature of the actual air. Do it incorrectly and you will measure the temperature of the side of your house or the sun-heated metal around the thermometer.
Many different types of thermometer exist and most are quite accurate. It’s the location that usually messes things up. Ideally, the thermometer should always be shaded and about four to six feet above a grassy surface with plenty of airflow. This is harder than it seems to accomplish, especially if you want to read it from the house.
If you stick the instrument on a window or very close to the house, usually you get some artificial heat that causes error. These days, you can get a cheap ($15-$50), wireless thermometer that allows you to correctly place the instrument in one place and the readout in your kitchen or wherever you wish. This seems straightforward, but I’ve had a fair amount of trouble getting the wireless to work all the time.
Almost all the location problems cause the readings to be too high. If you get too close to a structure, then you get excess heat. If you get in direct sunlight, then you are measuring the temperature of the thermometer more than the air. Being above grass or vegetation is best but bricks or asphalt will emit heat. The best place is inside a “Stevenson screen”. That is a large, ventilated, white box specifically designed to block the sun’s rays but allow air to flow through freely. Unfortunately, these will set you back $150 to $900 and they look a little weird in the back yard.
Heidi and I set thermometers up for friends all the time. Usually, we get the best spot we can that combines convenience, readability, and accuracy. Mostly, that is on the shaded, north side of the house. We have at least five thermometers at home. Two are wireless and in perfect locations, but they sometimes don’t work. We have two others attached to the house and they get ambient window heat but they are easy to read and always work. Our “official” one works great, but you have to go outside to read it. There is no easy, perfect solution.
These days, I often find it simpler to push a single button on my iPhone to get the latest temperature from Old Trail or Pollack Vineyard off of Wunderground.com. Those sites are reliable, but many other amateur stations are not. Airport readings are accurate 99.9 percent of the time. SHD or CHO are the closest airports with official readings.
October Review
October was a remarkably normal month for weather. Temperatures were just a touch above normal and the rainfall of 3.54” was just a smidgen below normal. Snow tried to fall momentarily on the 24th and frost came the next morning. Leaf color peaked right around Halloween, also perfectly normal
Rainfall Totals…
- Normal 4.00”
- Old Trail 4.71”
- Wintergreen 4.26”
- Univ. of VA 3.74”
- Waynesboro 3.59”
- Crozet 3.54”
- CHO Airport 3.52”
- Greenwood 2.01”
- Nellysford 2.72”
