We’ve all gotten into our cars on a hot summer day and looked at our dashboard to see just how hot it actually is outside. The temperature reading in your car however, can be very inaccurate when it comes to properly representing the air temperature.

To understand why car thermometers can be so unreliable, let's start by looking at how official temperature measurements are taken.

For temperature readings to be accurate, a thermometer should be placed five to six feet above the ground, and away from direct sunlight. Typical thermometers absorb solar radiation at a higher rate than the air around it, meaning that if the thermometer is placed in direct sunlight you will get a higher temperature reading than the actual air temperature. Additionally, the thermometer should be far enough away from buildings where any extra heat given off by the structure does not affect the temperature reading. 

Many official observing sites have shields over their thermometers to help block direct sunlight from hitting the thermometer while still allowing air to still move in from either side. 

So now that we know how temperature should be measured, let’s get back to those car thermometers. 

First off, most cars do not use stereotypical thermometers, they use thermistors. Thermistors are a small electric thermometer that measures the change in electrical current as heat is added or taken away. The thermistor then uses that change in current to determine the temperature. Normally, thermistors are very accurate, so what is the issue? 

The issue is where the thermistor is placed within the car. The thermistor is usually placed behind the front grille of a car, meaning that not only can it be impacted by the heat of the car's engine, but it is also close enough to the ground to be impacted by the hot asphalt beneath it. 

The dark pavement absorbs solar radiation better than the air around it, meaning that on a sunny day, the road temperature can be significantly higher than the air around it. That heat is emitted back into the air directly above the pavement, where it can be picked up by the car thermistor. 

For these reasons, the temperature reading on your car dashboard during the summer is often higher than the actual air temperature. That reading can be more accurate at night or on cloudy days when the difference between the road temperature and the air temperature is not as great, and the thermistor is not in direct sunlight. These same principles can be applied to bank thermometers that you might see while driving around town. 

In conclusion, the next time you are looking for an accurate temperature reading, your car dashboard might not be the best option. The official temperature reading in our viewing area is taken at the Youngstown-Warren Regional Airport, and you can find those hourly observations here (An example of one of those observations is below)