Posted July 23, 2006, 1:07 PM.

You may have noticed bank signs and other billboards flashing basically the same thing.

And yet, the WeatherCurrents station in Temecula recorded only 111°F as the high temperature yesterday. It was the highest ever recorded there. Other locations, from Riverside to Hemet to Lake Elsinore to Fallbrook to Sun City, all came in between 109°F to 114°F. Most of them were records as well. And yet, the numbers you saw were higher. Why?

Because I've received several messages about this recently from you, through the contact form, I thought I'd try to address this.

The thermometer in your car contains basically the same ingredient as a sophisticated weather station: a thermistor. These are highly accurate sensors that produce electricity based on ambient temperature.

The major difference is the environment surrounding the temperature sensor. When we place our weather station sensors, we go to pains to avoid re-radiated heat. So our sensors are solar-shielded and located at least ten feet away from trees, fences, buildings, concrete and asphalt. We also take care not to locate sensors in stagnant air corridors.

Often this means a sensor out on a lawn or dirt, on a post five feet above the ground.

This allows us to meet National Weather Service standards, and the National Weather Service receives data from many of our locations on an hourly basis.

Car thermometers obviously don't operate in this kind of environment. Neither do those bank signs and billboards.

In particular, a car is a very difficult environment for a temperature sensor. That the sensors are located as well as they are are a testament to careful consideration on the part of the engineers. There is engine heat to worry about, plus re-radiated heat from the concrete and asphalt roads, paint, tires, etc.

It's also very possible to locate a weather station sensor poorly. I've seen many of these sensors incorrectly mounted on roofs. Think about the re-radiated heat a roof gives off, and what that must do to distort readings.

So think of your car temperature sensor as a ballpark figure. It's accurate for its environment, but that is it. On cooler days, it may only be different from more carefully placed weather station sensors by a degree or two. But on days like yesterday, an excess of re-radiated heat likely exaggerates those differences.

-- John Toman, WeatherCurrents