AUSTIN, Texas — While summer may be technically winding to a close, we are far from out of the brutal Texas summers. Temperatures soar every single day into the upper 90s and low 100s, and we have surpassed 42 days of triple digits so far this year in Austin. But if you’ve been looking at your car thermometer each time you get in your car, you see triple digits every single day! Sometimes your car thermometer can report temperatures that are well into the 110 range or even, on those really awful days, near 120 degrees! So, this may have you asking, why was our hottest temperature this year only 107 degrees?
For that answer, we need to first find the car’s thermometer. On most cars, it’s on the front of the car, under the bumper, near the grille, or sometimes under the side view mirror. This is because the thermometer can get the best temperature reading if there is constant airflow around it. This prevents any uneven heating or cooling and can feel the temperature before it is warmed by the engine heat or any other interference. While driving, the car’s thermometer is honestly decently accurate. The problems arise when the car is sitting still.
On a dark asphalt parking lot, the black pavement will absorb an enormous amount of heat from the sunlight, radiating that heat back into the air just above the ground, right at the height where your car’s ambient thermometer will read. This means your thermometer is getting some incorrect readings based on the heat released from the hot pavement. Only when you start driving again does the temperature drop back down to what it actually is outside.
But it is still technically accurate for that parking lot. It really is around 15-20 degrees warmer in the air on top of a dark parking lot than on a lighter colored pavement. Why is the number from the National Weather Service so different? So why do we prefer that temperature over the one in our cars?
Thermometers, like the one at Austin-Bergstrom International Airport and Camp Mabry, all have a very specific set of requirements that need to be “meteorologically accurate”. They must fulfill the list of requirements, such as distance from buildings, distance from concrete or pavements, at least a certain distance from any trees, etc. If they are not, then the data could be skewed by artificial means rather than legitimate ambient temperatures. Such as, if a thermometer is right next to an A/C unit or dryer vent, it will produce a crazy high temperature reading; thus, the reasoning for being distant from buildings.
Your car thermometer doesn’t follow nearly the same number of requirements for “accurate temperature readings”. But that’s not what you need it for. You need to know, “How hot is it right outside my car?” “Do I need to worry about ice accumulating on my windshield when my car thermometer says “30” but the nearby airport says 34?” The car’s thermometer gives you hyper-local temperatures around your vehicle, which are extremely helpful to us all as we get about our daily routine.
But these temperatures we see inside our cars are still not accurate. The National Weather Service doesn’t need the information about heat just outside your car; it doesn’t need to know if one parking lot is hotter than the forest nearby. They need to know the temperature of the majority of the area. By obeying each of the instrumentation requirements, you can create a level playing field for forecast models to use the data to help forecast.
On the days when we hit 107 degrees, my car thermometer read 118 degrees in the station’s dark parking lot. Truly a miserable day. So next time you see that in-car thermometer go crazy and you think, “Gee, that’s so much hotter than my weather app”, you’d be right. But that’s not what the weather app is forecasting or showing you information for.