Science Course with Ryan Morse: Why are car thermometers frequently inaccurate?

Beyond The Forecast

(WFRV) – We all have gotten in our cars on a hot, sunny day in the summer and glanced over at the temperature displayed on the dashboard. A lot of the time the temperature reading is way too hot.

Why is this the case?

First off, cars don’t use thermometers. Instead, they use what is called thermistors.

Thermometers use liquid mercury inside a glass tube to expand or contract to a certain value as the air temperatures rises or falls. Meanwhile, thermistors measure the change in electrical current from heat added or removed.

The problem does not lie with the thermistors themselves, which are mostly very accurate.

The real problem is where the thermistor is located within the car. Many automakers place the thermistor just behind the grille of the car in the front, including on our StormTracker.

At this height, the thermistor can be influenced by the hotter temperatures of asphalt, and road surfaces. Temperatures can also be off based on whether you park in the shade or in the sun.

Experiment outside on a hot day (>85°)

Steps:

  1. Take a thermometer and measure the temperature just above the road after 5 minutes
  2. The National Weather Service standards to take temperature are 4 to 6 feet off the ground. Place the thermometer 4 to 6 feet off the ground for 10 minutes
  3. Measure the temperature off the ground

Notice the difference between the two temperatures. The one near the ground was much hotter with the effect of the road surface.

Car temperature readings are not always drastically off. Once you start driving in the summertime heat, the thermistor tends to cool down the temperatures.

For nighttime and cloudy days, the ground surface doesn’t vary much from the air temperature in the summer. This means car temperature readings perform a little better in these conditions.

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