Why does my Prius tell me when it's 37 degress?

<div class='quotetop'>QUOTE(jgills240 @ Mar 2 2007, 07:35 AM) [snapback]399055[/snapback]</div>

I live in San Diego too, and I've seen it a few times this year while driving to nearby Poway along a tree-lined road. It doesn't stay on long, just for a little bit.

 

<div class='quotetop'>QUOTE(Tideland Prius @ Mar 1 2007, 03:21 PM) [snapback]398658[/snapback]</div>

I missed the post before you and you posted while I was posting. No need to be rude.

As for the black ice warning, it's interesting but I don't have a need for it. I can see where it would be useful for people aren't used to colder weather.

 

<div class='quotetop'>QUOTE(FarmGeek @ Mar 1 2007, 03:49 PM) [snapback]398683[/snapback]</div>

The dealer can adjust the temperature display by ±3°C in one degree increments.

 

<div class='quotetop'>QUOTE(FarmGeek @ Mar 1 2007, 12:49 PM) [snapback]398683[/snapback]</div>

Due to the physics involved, I think there must be an insermountable challenge to get an acurate outside temperature reading for a car.

The sensor needs to be shielded any air flow generated by the car's movement to prevent error from wind chill factor that would make it read several degrees low when the car is moving.

but by putting the sensor behind body panels, they cannot help but pick up radiated heat from the sun that comes and goes by day and night.

It's a trade off, and under the circumstances, I think they do OK. I am fairly sure that this is disclosed in the manual that the temperature can be off by up to 4 degrees, so the 5 degrees above freezing warning makes some sense.

much of a challenge to get an acurate reading on the outside of a car.

 

Inanimate objects are not subject to wind chill. If an object is at the same temperature as the air, it will not get colder because the air is moving across it's surface. A person (or a warm blooded animal) "feels' that the air is colder than the measured temperature because the rate of heat loss is higher when it's windy. The windchill temperature is supposed to be the equivalent temperature if the air was still.

 

Re: Why does my Prius tell me when it's 37 degress?

I disagree. From Wikipedia:

"Wet-bulb temperature is measured using a thermometer that has its bulb wrapped in cloth—called a sock—that is kept wet with water via wicking action. Such an instrument is called, not surprisingly, a wet-bulb thermometer. At relative humidities below 100%, water evaporates from the bulb which cools the bulb below ambient temperature."

Seems to me that a wet-bulb thermometer is an inanimate object, yet it's temperature is cooled below ambient temperature by evaporative cooling. Human beings, made mostly of water and apt to perspire, are more susceptible to evaporative cooling than inanimate objects. Thus, the presence of a liquid that can evaporate and the amount of air moving across an object are factors that determine "wind chill", not the presence of life.

The thermometer in my 2007 Prius is at most 1-2 degrees off of the temperature reported on the weather broadcasts. That's good enough for me.

 

Re: Why does my Prius tell me when it's 37 degress?

Where is the temperature sensor located?

 

Re: Why does my Prius tell me when it's 37 degress?

I had no idea it does this - I have a 2007 as well and it's gotten to 37 a couple of times this past month and I've never once seen the warning!

Must be a California thing...nobody expects it to get that cold in AZ!

 

Re: Why does my Prius tell me when it's 37 degress?

LOL

I always got excited when the snowflake popped up when I lived in TX. I could pretend it was winter for a while.

 

Re: Why does my Prius tell me when it's 37 degress?

This is not correct. The only time an error will occur due to wind chill is if the sensor is wet (from rain / snow)...

As the water evaporates, it would cool the sensor below the surrounding air temperature. Once the sensor is dry, the sensor will be at exactly the same temperature as the surrounding air (that does not mean you will have a reading on your dashboard that is exactly correct - just that the tip of the sensor will be at the same temperature as the air... )

The sensor is within the body of the car to prevent it from being damaged... not for wind chill issues.

The sensor does get heated from the engine compartment and the body of the car - as you mention; but only if the car is not moving forward. Once it is moving at a reasonable speed (say 25+ mph), the effects from convection cooling by the wind will far outweigh the radiant heat from the engine / body.

 

Re: Why does my Prius tell me when it's 37 degress?

Around here the snowflake icon stays lit constantly from December through March. This morning the icon was lit and the temperature was -6F. No kidding there might be ice! I don't find that feature to be particularly useful, but I suppose it might be for drivers who are not used to the cold.

 

Re: Why does my Prius tell me when it's 37 degress?

Actually I believe you are both incorrect.

Wind chill is an effect on your body, not a temperature sensor.

Here is a defination from:Wind Chill Factor

"A phenomenon called the Wind Chill factor makes us feel colder in winter than the air temperature really is. This is due to the interaction of air temperature and wind on the human body that is already giving off heat. Both temperature and wind cause heat loss from body surfaces. A combination of cold and wind makes a body feel colder than the actual temperature."

 

Re: Why does my Prius tell me when it's 37 degress?

You are right - the effect that i was describing is not called wind chill - as Wind Chill only describes a heat transfer process which is only associated with wind and not any other factors.

What I was describing related to the evaporation of water (if any) on the sensor body. That evaporation results in a temporary cooler temperature within the sensor than in the surrounding air.

Of course, in dealing with cars, the effect that I describe is amplified by the wind blowing over the water / sensor combination. And this is where I brought in the term Wind Chill more as a visualization than as a technical term.