Orlando – It’s that hot time of year when pictures of car temperature readings flood social media. The pictures often show extreme heat outside the car, but are these readings accurate?
The answer may surprise you, but most of the time they’re not.
Cars are equipped with thermistors, not actual thermometers that have mercury in them.
When mercury inside a thermometer gets hotter, it expands (increases in size) by an amount that’s directly related to the temperature.
A thermistor works differently by measuring temperature by detecting the change in electrical current that temperature can cause.
So what’s the big deal, right?
When placing a thermometer in your yard for an accurate reading of the air temperature, the ideal location is away from a building about 4 to 6 feet above the ground. It should sit above dirt or grass, not pavement.
Also, you want the device to have its own shelter to prevent direct and indirect sunlight from impacting the reading. The shelter also has to have good airflow to give a more accurate reading.
Keep in mind if the thermometer is in the shade of a tree, it will be a little cooler. If it’s too close to a building or pavement the reading will be warmer. These things have a tendency to absorb and retain heat longer than the ground does.
So, back to the car thermistor.
The placement is very close to the pavement usually in the front of the car behind the grill. This location exposes the thermistor to re-radiated heat from the surface the car is on. Keep in mind pavement is often dark which absorbs a lot of heat if in direct sunshine throughout the day versus a shaded spot or grass. Not to mention this device is surrounded by the car, which is a hunk of metal.
During the day, the sun heats the pavement and the car more than the air. The result is the thermistor reads a hotter temperature than the actual air. For example, this picture is of the dashboard temperature taken at 3:30 p.m. that read 100 degrees.
Compared to the actual observed temperature of 96 degrees at the same time, it reads a few degrees hotter.
While this might not seem like that great of a difference, readings can be greater too. Sometimes they display over 10 to 15 degrees hotter than it really is.
Even in the evening, the heat from the road will cause the reading to be a little warmer, because it takes a while for the asphalt to let go of that built-up heat.
At night and on a cloudy day, the dashboard temperature reading will be a little closer to the actual air temperature due to the lack of direct sunlight. After it storms there will be a distinct drop in temperature from the rain cooling the surface down quickly.
Despite the difference, it’s still fun to see what different dashboards across Central Florida have to say about the heat.