Crozet Weather Almanac: July 2012


by Heidi Sonen and Roscoe Shaw

Thermometers come standard in most cars these days and we take them for granted.  But are they accurate? Yes and no is the answer, depending on several factors.

Before about 1997, nobody had a car thermometer. Nobody, that is, except our old (and I mean really old) professor of meteorology, Dr. Blackadar.  He had a Mercedes Benz into which he had carefully installed a thermometer. On clear, calm nights, Dr. Blackadar would take students out in his Mercedes to find “cold pools.” Sometimes, he could find a valley that was 15 degrees cooler than in town.

Sometimes, the cold pools of air were so small and fragile that driving the car through on a deserted rural road would stir the air up and ruin the cold pool. The temperature might be 40 degrees on top of the hill and drop to 25 at the bottom. But when you turned around and drove back through, the newly stirred air was up to 30.

The field trips were great fun for meteorology nerds like us, but Heidi and I mostly slept through his boundary-layer meteorology class. As for Dr. Blackadar, he still reports to work and is as nice as ever well into his 90’s.

Now that most of us have a car thermometer, you have to know when to believe them.  Some are located in the front bumper, some in the rear bumper and some under the car or on the mirrors.

The key to accuracy is to measure the temperature of the air and not the temperature of your bumper. That requires air flow. If you are parked in the hot sun and your car is black like mine, the readings will start off way too high.  For example, on a hot, sunny day at noon, my car might read 104 as I pull out of the driveway, drop to 99 when I pass Crozet Pizza and settle at a fairly accurate 96 when I pull onto I-64. If you stop at a red light on the asphalt, you will start reading too high again. Heidi’s car, which is a much lighter shade, cools to the correct temperature more quickly and is generally more accurate.

The weaker the sun and the more air flow, the more accurate the reading.  At night, your thermometer will be correct almost as soon as you start driving.

In summary, your car thermometer is only accurate when you have a nice steady air flow on it and it has had time to adjust to the temperature of the air. The errors will always be too hot since I can’t think of anything that would make it read too cool. As long as you are moving steadily, it will probably be pretty close.

July Recap

On June 29 at 10 p.m., we were clobbered by a squall line that turned Crozet into Asplundh heaven. Firewood prices fell in half overnight. Then, when very few people had power or air conditioning, the high temperatures averaged 99 degrees for the first week of July culminating with 104 on the 8th. We were the luckiest folks in town. Our tree damage was minor, power came back right away and we left town on summer vacation and missed all the heat.

After that, the rest of the month was actually quite nice. Temperatures were normal and rainfall was light but amazingly consistent. Measureable rain fell an amazing 18 days at our house even though the monthly total was a below-normal 3.52 inches. The steady rains have kept the landscape green.

July Rainfall Totals

  • Crozet  3.52”
  • White Hall  1.88”
  • Greenwood  3.33”
  • Nellysford  2.34”
  • Afton Summit  2.70”
  • Waynesboro  4.13”
  • Charlottesville Airport 3.48”
  • U.Va.  2.80”