[WayneZat]

I may never see this again in Tucson so I have to ask why. Last night I got a chance to drive in the rain/snow on the way home. Started out at 39 degrees on the rear view thermometer in the rain. As the temperature dropped to 36 and below the thermometer showed ICE for a few miles. After a few miles the temp went to 32 and the rain was now snow. Does the thermometer show ICE at around 36 to warn you on the possibility of black ice? That's the only reason I can come up with since I know at 32 and below it would generally be ice. At 36 the road still appeared to be wet and I know that's often when there is a black ice problem, at 32 I noticed the snow was sticking on the road.

Well, I may never see this again unless I decide to move. Just wanted to know why.

 

[cr8gster]

Actually, I think its 37 degrees that this starts. Yes, it is a warning for dropping temps to help you be prepared for black ice/freezing roadways.

 

[Clark20ry]

Because 37 degrees is freezing and that makes ice.....

 

[schulte]

Because 37 degrees is freezing and that makes ice.....

37 degrees in the front of your engine compartment could mean 31 degrees on the asphalt.

 

[DangerousDuramax]

Because 37 degrees is freezing and that makes ice.....

Did the law of physics change while we weren't looking?

 

[partsguy662]

Because 37 degrees is freezing and that makes ice.....

):h You gotta love folks from Arizona...

 

[Unit453]

That chit was funny....


I dont care who you are....


I see ICE so often on my mirror, I'm not sure if its even capable of showing double digits...

 

[WayneZat]

OK, I didn't make the comment. The only thing I know about the freezing point of water is that water shrinks as it cools to 36 degrees then a significant amount of latent heat is released as it continues to cool as it expands at it's normal freezing point of 32.

New question, where is the sensor in the engine compartment? I always through it was in all that electronic stuff in the rear view mirror. Makes sense to be in the front of the engine compartment somewhere in front of the cooling systems, etc. Never could figure how it could be calibrated to read temperatures through the windshield.

 

[Unit453]

From what I read and hear, its in the mass air sensor or MAF. Its thrown off by running the winter cover...

Of course, I could be wrong...

 

[Turbotug]

I think my mirror shows triple digits as often as double, would be nice if it could flash "HOT AS HELL" though.:weld:



psst Clark, water freezes at 32*f. I know 70* feels like it's freezing, but it's not. I was in Telluride all weekend, it is freezing there.

Just another Zonie checking in.....

 

[Unit453]

I'd pay to get it out of single digits...

Back on topic....

I've seen these mirror displays go from bright to dim and back again....

 

[DangerousDuramax]

OK, I didn't make the comment. The only thing I know about the freezing point of water is that water shrinks as it cools to 36 degrees then a significant amount of latent heat is released as it continues to cool as it expands at it's normal freezing point of 32.

New question, where is the sensor in the engine compartment? I always through it was in all that electronic stuff in the rear view mirror. Makes sense to be in the front of the engine compartment somewhere in front of the cooling systems, etc. Never could figure how it could be calibrated to read temperatures through the windshield.

Depending on which truck you have its either on the left side or right side behind the grill. LB7 is on the left and LLY is on the right. Dont know where they mounted it on the LBZ. For the indicator to warn that there could be ice it has to take not only the temperature into account but barometric pressure and humidity. It may show at 37* or you might not see it well below 32*.

 

[DEWFPO]

You can get frost at 36F. I've seen it many, many, many times.

DEWFPO

 

[schulte]

You can get frost at 36F. I've seen it many, many, many times.

DEWFPO

I don't mean to come off as an a**hole, but there's no way frost happens at 36 degrees. Water freezes at 32 degrees, plain and simple. The only variance in the freezing point of water would be pressure; you could raise the freezing point of water by decreasing the atmospheric pressure, but this wouldn't be even measurable until you got down to almost a 100% vacuum.

 

[irjpr]

I have seen "ice" on my mirror display well in the -20's.... Usually the temp is within 5 degrees or so of actual, but I don't rely on that thing to tell me whats up outside....

 

[DEWFPO]

I can understand your scepticism, I don't blame you..... but, I've seen frost form on grass, plants, decks, anything outside (as long as there is no cover over it like an overhang) as high as 36F, many, many, times..... and no I don't drink(well, not that much).

I didn't say solid ice........ but frost. The weatherman explained it well once, but I can't remember the science behind it now. It's a hard freeze when the temps hit less than 32F.

Maybe some scientist on here can explain it.

DEWFPO

 

[SSonnentag]

When temperatures are officially recorded for hourly weather observations and climate reports, they are measured at a height of between 1.25 and 2 metres (4.1 and 6.6 ft) above the ground in special shelters, called Stevenson screens. (The shelter is named after the father of writer and poet Robert Louis Stevenson.) Meteorologists call this temperature the "surface temperature," and it is what is reported on the radio and TV (and internet and newspapers, reports, etc.). The distinction is important for the following reason.
During clear and calm nights, the temperature at the ground or some surface near the ground can become much cooler than the "surface temperature". The radiation of heat away from the ground is the cause of this drop. The coldest air, therefore, forms near the ground, and being heavier than the air above it remains there.
If we were to make measurements of temperature from the surface to the height of the official "surface temperature" measurement every few centimetres or inches, we would find the air temperature increases as we move upward from the ground. Meteorologists call this a surface temperature inversion.
Since cold air is heavy air, in the absence of wind, the coldest air will remain nearest the ground, thus allowing surface temperatures to continue to fall. Thus, under such conditions -- clear and calm nights -- the ground temperature may fall below the freezing point while the temperature measured officially at was still above freezing. This is particularly common in the autumn and winter when nights are long allowing more time for cooling to occur.
Now frost is a covering of ice crystals on the surface produced by the depositing of water vapor to a surface cooler than 0° C (32° F). The deposition occurs when the temperature of the surface falls below the frost point. Similarly, dew forms when the air or surface temperature falls below the dew point temperature. (Note that the water vapor goes directly from gas to ice. Therefore, frost is not frozen dew.)
Thus, if the temperature on the ground or an object such as a bush or a car windshield near the ground falls below the frost point, frost crystals may form. But the measured "surface temperature" may still be above freezing.
This is the most common way in which frost may form when the official surface temperature is still above the freezing point.

 

[The Rookie]

I think my mirror shows triple digits as often as double, would be nice if it could flash "HOT AS HELL" though.:weld:




Now thats funny. I could use the "HOT AS HELL" here in Texas.

 

[Clark20ry]

Did the law of physics change while we weren't looking?

Ok ok our freeze warnings here in Phoenix are at 37 degrees.

 

[cit1991]

When the air is cold enough (36 to 38, depending on who you ask), if it's a clear night, radiation of heat to space can reduce the radiating surface (like a road) temperature to 32 or below, causing black ice, which is just frozen dew.

The warning is for people who see 37 and think they're ok regarding ice, but in fact there may be ice. The temp readout is nice, but it could give someone a false sense of security "Hey its 36 and a nice clear night, there's a nice long bridge. There can't be ice at 36 right? Let's punch it and see what it'll do!"

If it's overcast or foggy this blocks the radiation (or actually re-radiates it back down), so no ice.

It's like when you park your car outside on a clear night an there's dew on it in the morning. If water condenses on the car why not in the air to make fog?..because the car radiates down to colder than the air and condenses water if it can get down below the dew point. The ice warning is the same, but below freezing.

And, BTW, the temp sensor for the mirror is a little plastic job behind the grill. It does get a good clean sample, as long as you're moving.

Worst conditions for black ice: Clear night, no wind, dew point just below freezing, air temp just above freezing.