I have an older model BMW 525 that “beeps” an alarm every time the temperature outside gets to 38 degrees. Does anyone know why? I could understand bells and whistles for 32 degrees, but what is so special about 38 degrees? Does something happen in Germany when the temperature hits 38?
Thanks for your help. I searched through the owner’s manual, but could not find an explanation there.
I have an Audi, and my car’s computer shows a little snowflake when the temperature is between 23 and 41 deg. F. According to my manual, this is to alert you to a higher potential of frozen road conditions. Anyway, these temperatures convert to -5 and +5 deg. C, and my guess is that BMW just used a different set point (3.33 C, in this case). Hopefully a BMW owner will come along to give a car-specific answer.
38 degrees is the point at which (the car makers consider that)there is a danger of ice on the road - I know that the freezing point of water is supposed to be a few degrees lower, but ice, once formed, can still persist at few degrees above freezing.
Plus there needs tto be a margin of error - if you happen to leave the driveway of your house on the south side of a large hill and zoom off down the road, thinking there’s no risk of ice, you may be unpleasantly surprised when you encounter a region which does not have quite such a favourable microclimate.
At night, the ground is usually colder than the surrounding air so you need a safety margin. The reason the ground is colder is radiative cooling. Everything radiates heat, but opaque things (like the ground) emit much more efficiently than transparent things (like air), so the ground loses heat much faster than the air. Similarly, during the day the ground absorbs sunlight much more efficiently than air, so the asphalt gets much hotter than the air.