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Driving in heatwaves: Why the temperature on your car dashboard is different to BOM

By Kelsie Iorio, Friday January 31, 2020 - 13:18 EDT

Most Australians know the feeling of opening the car door after it has been parked in the sun on a hot day, and the rush of heat that smacks you in the face.

People often look to the temperature display on their car dashboard to find out just how hot it is, and the reading is usually pretty high, especially in summer.

But this temperature is measured differently to the temperature of the air, and it's often not an accurate representation of how hot it actually is.

How cars measure temperature

Cars use a device called a thermistor to measure temperature, by measuring the changes in electrical current when heat is added or removed.

Most thermistors are located , meaning they're fairly close to the road surface, which can affect the measurements.

explains it like this:

"Anyone who has walked on sand on a hot day or felt the heat coming off cement knows that the sun heats the ground hotter than the air. The thermistor in your car is near to the ground, meaning it can pick up the heat that the ground radiates. This means that often, on hot days, the temperature you're seeing in your car is higher than the actual temperature of the air. Your car is picking up a mixed message, the temperature as it's influenced by hot asphalt."

When you Google the current temperature or check the weather app on your phone, you're getting a reading of the average thermal energy in the air.

The Bureau of Meteorology explains that their most commonly used way to measure temperature has been the , but many of these are being replaced with .

Most modern automatic weather stations now use an electronic sensor, which of air temperature.

The difference in measurement methods means that often the reading on your car dashboard is hotter than the one on your phone — but that doesn't mean it's not hot in and around the car.

Knowing the dangers of hot cars

Despite a car's thermistor technically measuring the temperature outside the car, we know how quickly the inside of a car can heat up on a hot day — and how dangerous it is to leave children and animals inside.

But it's not just parked cars that can cause problems, it's important to manage the heat while you're driving too.

Driver Safety Australia managing director Russell White says research on the effects of heat stress on driver vigilance found there was a noticeable impact on driver performance and alertness in increased heat conditions.

"It is always difficult to assess the full extent of this because it may vary from person to person," Mr White said.

"It [the research] showed that even at temperatures well under summer conditions here in Australia, the negative effect of heat stress on a driver's vigilance was statistically significant."

"At 27C, the overall proportion of drivers missing signals was 50 per cent higher and response times were 22 per cent longer than they were at 21C."

Mr White said a Middle Eastern study investigating the relationship between thermal conditions and road safety showed that the level of road accidents increased with the severity of hot weather.

"The majority of crashes occurring under hot weather conditions are those involving only one-person judgement," he explained.

"Incidents such as 'running off the road' were associated with high levels of heat stress."

that crashes involving driver performance–associated factors were increased in association with heatwaves and increasing temperature.

The study included 118,489 motor vehicle crashes, and the estimated risk of crashes increased by 2.9 per cent during heatwave days.

The association was even stronger when restricted to crashes that directly linked to driver performance.

Mr White said drivers needed to factor heatwave conditions in when maintaining their cars, and know the impacts and risks of extreme heat while driving.

"In addition to the hazards and dangers we've seen with the risk of fire, the heat will have impacts on both human performance and vehicle performance," he said.

"Even though modern vehicles have much better cooling systems and are designed handle heat, it does place extra load on the vehicle's systems and tyres.

"Of course, old vehicles may overheat quickly and that can lead to a roadside breakdown."

Mr White said while there was lots to consider while driving in heatwaves, remember to never leave children or pets unattended for any length of time in a car.

"A closed car can heat up to dangerous levels in a very short time," he said.

"Unfortunately, we have seen deaths as a result of people doing this."

How to prep your car for a heatwave

Although the temperature on your car dashboard may not be the exact temperature outside, it's still important to be prepared when driving in heatwave conditions, especially if you're driving long distances.

Mr White said the first thing to do was make sure you kept as cool as you could and stayed hydrated.

"Make sure your car or vehicle is serviced. Heat places your car's cooling systems under an increased load so checking coolant levels, hoses and the general condition of the engine bay is important," he said.

"Tyres also need to be checked for the right pressures. Problems such as tyre blowouts are more common in hot weather."

having a mechanic check over your car battery, coolant, tyres, air conditioner, oil and other fluid levels before hitting the road in a heatwave.

Some ways to be prepared to drive in heatwaves include:
Using a reflective windscreen shield when the car is parked in the sun, and aiming to park in shade if possible
for a couple of minutes, and rather than directly at people in the car
Slipping on a shirt with sleeves, slopping on some sunscreen and sliding on a good pair of sunglasses
Making sure you have an emergency kit in the car with water, first aid equipment, a torch etc
or making sure you have a working phone charger in the car


© ABC 2020

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