MFD changed for no reason

What was the outside temperature? Was it 37 degrees by any chance?

The Prius gives a freeze warning when the outside temperature drops to 37. It's a hold-over from the snowflake on the earlier Prius.

Tom

 

Because that's the way it works. Why? Who knows. Why 37 degrees? No one knows that either, unless we get deep inside the engineering minds at Toyota.

Tom

 

Maybe because when it is 37 degrees F a couple of feet above the road (where the outside temp sensor is located), then the road itself will be close to 32 degrees F?

 

That is around 3 degrees Celsius. At least all Toyotas sold in Europe have a freeze warning below this temperature. So for me, this makes sense for Prius, too.

Ogo

 

Exactly. It's just set a few degrees above freezing. How they picked the exact offset is a mystery.

Tom

 

37 degrees, because as temperatures rise, all ice does not disappear instantly just because the air has risen above 32.
If you get in your car for your morning commute and the air temperature is 33 degrees, wouldn't you like that warning? 37 is a safety cutoff.

I love my 'snowflake.' :wub:
I consider it my 'going skiing happy-light.' :biggrin1:
That's when I see it the most.

 

It doesn't work that way with the new Prius (post snowflake). You get the warning when the temperature drops to 37. You can drive all day at 33 and not get a warning.

Tom

 

I recall this experiment, except we used joules.

I believe the 37 degrees C isn't a random temperature but is a danger zone temperature for black ice. I'm also guessing Toyota responded to the whining babies who complained about the light being on all winter so they dropped it from later models but there is still an MFD response. Search the snowflake if you want to read some whiny baby posts.

 

Latent heat of fusion. It's 60 calories per gram.

Tom

 

37 degrees C ???

Mannnnn that's some real nasty black ice if it can survive in 98 degrees F.

Or do thermometers work backwards when they are upside-down there in the Southern Hemisphere?

 

Keep in mind that bridges and overpasses can easily radiate heat away at night and be considerably colder than the surrounding air. With the temperature close to freezing, a little water vapor can condense and freeze on the surface of the road. 37 degrees F seems a reasonable point for a warning.

Outer space (and the upper atmosphere) is very cold...so on clear nights there is considerable radiant heat transfer from objects exposed to the sky, and they grow colder than the surrounding air. Dew and frost result. The driving force for this radiant transfer is the difference between each of the absolute temperatures raised to the 4th power if memory serves.

There is also the condition where the road is already cold and the air temperature warms.

 

My 1987 BMW gave me a visual and audio warning at some temperature near 36 F. My 2002 Audi A4 does so, but at a higher temperature (maybe 41F).

As the old sign "bridge freezes before road surface" reminds us, radiation cooling can make a portion of the road cold enough to condense moisture and make ice while the air temperature is still well above freezing.

Interestingly enough, the Audi turns OFF the continuing visual indicator (it looks like a snowflake) when it gets cold enough (low 20s F, if I recall). I don't know whether the thinking is that it is less slippery down there, or just that your are unlikely to be unaware.

 

I kinda like the snowflake. Not that I've seen it yet this year, but it makes more sense to have a continuous, unobtrusive warning than a quick flash.

 

37 degrees is the temperature universally decided upon where the production of ice on the vehicle or road could occur, depending on relative humidity and speed of the vehicle. Most review mirrors that have the integrated outside temp display will begin to flash a caution to the driver at 37 degrees for this purpose...