Most of us have cars that have a thermometer, but it’s not a standard mercury thermometer. It is a thermistor. So, you may be asking what is a thermistor? As it turns out it is one of the most commonly used devices to measure temperature as it is small, cheap to make, and they are fairly accurate. It is like a mercury thermometer, but instead of mercury contracting or expanding with temperature change, the thermistor measures temperature by a change in electrical current when heat is added or subtracted.
So, if a thermistor is fairly accurate, why is the car thermometer always wrong? It comes down to where it is located on the car and they are usually placed in the front of the car behind the grill. This location allows the car to pick up re-radiated heat from the ground. We have all walked in the sand or felt the heat from parking lots on a hot day, well, that is what your car thermometer is picking up on the, the re-radiated heat. Therefore, during warm and sunny days your car thermometer has a reading well above what the temperature is. This is also common problem on things like bank signs and billboards as they are usually close to parking lots or other objects that are great absorbers of heat.
It also turns out it isn’t all that accurate in winter time as it has a hard time within a couple degrees above and below freezing. So, it won’t be able to help you figure out if there is black ice or if the roads are just wet.
However, there are times when the car thermometer is more accurate and that is during the night, cloudy days, or at high speeds. At night the thermistor isn’t picking up as much re-radiated heat from the ground as the sun is not out. During cloudy days the ground doesn’t heat up as much due to the clouds blocking incoming solar radiation. While you are traveling at high speeds, more air can pass by the thermistor allowing a more accurate reading of the ambient air.
Overall, the car thermometer is a fun little thing to have, but if you want an accurate measurement of temperature for where you are at, you will have to look elsewhere.
How is temperature measured? Temperature is just a measurement of how fast or slow gas molecules are moving. Therefore, the faster the gas molecules move the more the mercury expands in a mercury thermometer. And, the slower the molecules travel, the mercury will contract.