Gauging the outside air temperature from inside your car

Why that may not be the most accurate reading
Posted at 11:56 PM, Jul 20, 2017
and last updated 2017-07-21 02:59:12-04

On a hot day you want to get into your car as fast as possible to turn it on and start the air conditioning. Your first thought may be to find out how hot it actually is outside. The easiest way to do that is to check the temperature on your dashboard, which could read at least five degrees cooler or warmer than what the actual air temperature is outside. You may ask yourself why these temperatures are so far off.

"The temperatures that we see in cars a lot of times are not accurate," says Christine Riley, a meteorologist with the National Weather Service in Hanford. 

She says there are several factors that lead to false readings.

"A lot of times a car will be sitting in the sun for a really long period of time," Riley says. 

Or they may be parked over asphalt, which absorbs more heat.

"It's much hotter over asphalt than it is, say over grass," she says.

Even with her knowledge, Riley wasn't exactly sure where the temperature sensor was located on a vehicle. 

"The ambient sensor is right behind the grille," says Daniel Perez, a local automotive technician. Depending on the make and model of your car, it could also be right behind the bumper.

The sensor is protected by panels and that's why it's not usually visible to the eye. Perez says the temperatures could be inaccurate because of the location of the sensors. 

"If you're parked in the direct sunlight, obviously the sensor is going to read higher. If you're parked under a car port, the reading is going to be a little lower," Perez says.

He also says the sensor can pick up heat from the road's surface. 

"Once you start driving the vehicle, the temperature starts dropping, because the fan is pulling air through the grille and through the sensor," he says.

Perez wants people to know that they should not jump to conclusions right after starting the vehicle. Instead, he says to give it about ten minutes of drive time. That will allow for enough airflow through the car, so the sensor can fully update. 

"Every so often, the vehicle's computer will get a reading from the sensor and then send it to the display," he says.

He says that you should also keep in mind that an after-market grille will sometimes skew the temperature sensor.

"The add-on grille partially blocks the airflow and won't give you an updated or actual temperature reading," Perez says. "The original grille was designed for more open space and more airflow."

Overall, Perez and Riley say there are just too many variables that can make the car temperature readings differ greatly from official readings at weather stations across Kern County.

"Those have been sited in a way where they are all equal, so there's nothing really to obstruct the temperature or to sway it and make it either too cool or too warm," Riley says.

If you're going to install your own weather station at home, Riley wants you to know that there are a few criteria you'll want to keep in mind when setting it up.

"Never put it over pavement, make sure you're far away from trees so it's not shaded, and make sure that it's about four to six feet above the ground. Also, make sure it's not near an air conditioner or a chimney, which could add heat to an area," she says.

Riley recommends keeping a close eye on your station. If the surrounding area changes, such as pavement being added nearby, that could sway temperatures several degrees.

"One thing you want to look for are temperatures trends," she says. "You want to see how it's relating to the other weather stations in the area."

Those trends can help you compare your own weather readings to the official ones to make sure you are getting the most accurate numbers.