Why car and bank thermometers give an inaccurate representation of summer heat

When you turn on your car in the middle of a hot summer day and take a look at the air temperature before pulling out of your driveway, it may catch you by surprise. You might notice that the reading on your car seems a bit too hot, even for the oppressive heat outside.

This is because of where the sensor to measure the ambient air temperature is placed. For example, in the ABC 17 Stormtracker 360, the sensor is in the front of the car, sitting just behind the grille.

Most cars will have a similar placement of the ambient temperature sensor, either in the very front of the vehicle or along the front edges. The problem with this is that for cars that are parked outdoors, they are typically parked on darker surfaces that are better at absorbing incoming solar radiation, like concrete and asphalt. As they absorb more of this radiation, they begin to emit their own longwave radiation. This drives up the temperature of those surfaces at a faster rate and, in turn, heats the air that is in close contact with those surfaces.

The car's thermometer sits close enough to the ground that it experiences this added heat from the surface, which gives a biased representation of the true ambient air temperature at a given time. The other factor to consider is that these vehicles themselves are not only more efficient absorbers of incoming solar radiation and efficient emitters of longwave radiation, but they also contain plenty of mechanical features that make the car run, which heat the vehicle over time.

Even after you start driving and notice that the temperature is beginning to drop due to better air circulation, the ambient air temperature reading will still usually sit 5-10 degrees above the actual air temperature.

A similar, although not as excessive, problem occurs with temperature readings you may see when passing by a bank. This especially happens on sunnier days, and it is because these thermometers are typically not set up to the standards of what the National Oceanic and Atmospheric Administration uses to officially report air temperature across the nation.

These stations are usually placed away from urban areas, buildings, and concrete so as not to experience any of that added radiation emission from structures that are more efficient at absorbing incoming radiation and more effectively heating up the surrounding air.

The thermometer itself is usually placed a couple of feet above the ground, and this is done to avoid the measurement device capturing a biased measurement of the temperature of the surface on which the station has been placed.

The thermometer is placed inside a white shield, otherwise known as a Stevenson screen, and this is done to ensure that the device itself has proper airflow, all while not absorbing any of that incoming solar radiation that would otherwise increase its internal temperature and the temperature of its surroundings.

Since the shield is white, it also has a high albedo, so it is better at reflecting the majority of the incoming solar radiation into the atmosphere rather than absorbing it. All of this ensures that the most accurate representation of air temperature is being reported for meteorologists to analyze across the nation.