How can it

[entwined]

frost when the temperature is 40°?

 

If frost is frozen water and water freezes at 32°, how can we have frost on the roof when it never got below 40°?

 

I figured one of you guys would have the skinny on that one.....

[YT2095]

I think you`ll find water freezes at Zero, not 32.

[insane_alien]

your thermometer is obviously broken or you relied on the average taken from a weather report, just because it is 40F at a monitoring station does not mean it is 40F where you are at that moment.

 

also, use centigrade more people understand it and it is only really th USA that uses it now. it is a rather antiquated system.

[ecoli]

or you're assuming the ground temperature is the same as on your roof.

 

Also, could pressure differences play a role? (unlikely)

[insane_alien]

if the pressure varies by several hundred bar in his area then yes, pressure could play a role

[swansont]

I think you`ll find water freezes at Zero, not 32.

 

Oh, you anti-Fahrenheitean!

 

 

————

You can get frost when the temperature is above freezing. I'll bet that the sky was clear. The rate of radiative heat transfer depends on the difference between T⁴ of the object and T⁴ of the surroundings (the "reservoir;" basically they each radiate, and you look at power out vs power in. No net heat transfer when the temperatures are equal because the radiation balances out)

 

Well, the nighttime sky is pretty cold: 2.7 K, so you can radiate quite efficiently when the sky is clear. When there is cloud cover, or something else nearby taking up a large amount of surface area that the objct can "see," the difference in radiated power drops dramatically, and you don't see frost above freezing.

[entwined]

Oh, you anti-Fahrenheitean!

 

 

————

You can get frost when the temperature is above freezing. I'll bet that the sky was clear. The rate of radiative heat transfer depends on the difference between T⁴ of the object and T⁴ of the surroundings (the "reservoir;" basically they each radiate, and you look at power out vs power in. No net heat transfer when the temperatures are equal because the radiation balances out)

 

Well, the nighttime sky is pretty cold: 2.7 K, so you can radiate quite efficiently when the sky is clear. When there is cloud cover, or something else nearby taking up a large amount of surface area that the objct can "see," the difference in radiated power drops dramatically, and you don't see frost above freezing.

 

I suspected that it was something like that.

 

We have had 3 frosts so far where I live and only one of those frosts involved temperatures as low as 31°. The other two, it was 40° and 36°.

 

My thought was that evaporation had something to do with it, but what you discribe makes sense......I guess.

 

I have confidence that my thermometer is not off by anything like 8°, but it is mounter under the roof portion of my back deck and the frost collects on the roof and the windshields of cars and so forth.

 

What still confuses me is how the water droplets are able to lose heat, but the surrounding air is not. Perhaps water being a better conductor?

 

And yes, on all occasions it was under a clear sky.

 

And lastly, my apologies to those who are Celsius users, but converting Farenheite to Celcius is not all that tough a calculation.

[insane_alien]

And lastly, my apologies to those who are Celsius users, but converting Farenheite to Celcius is not all that tough a calculation.

 

but this board's official stance(and most of the world,bar a country with 4% of the population) is SI or metric. i personally preffer SI.

[entwined]

but this board's official stance(and most of the world,bar a country with 4% of the population) is SI or metric. i personally preffer SI.

 

Well OK, and I would prefer that prefer is spelled with 1 f instead of two, but I had no problem understanding what you were trying to say, so I wouldn't have mentioned it......tolerance anyone?

[Klaynos]

My issue with the use of Fahrenheit, is that he didn't mention what unit set he was using...

 

But I'm all for always using SI...

[swansont]

What still confuses me is how the water droplets are able to lose heat, but the surrounding air is not. Perhaps water being a better conductor?

 

Air is moving around, so it will spend time in areas where it doesn't "see" the sky, and where it can exchange energy with other objects.

[John Cuthber]

If all you guys want to use SI units that's fine, but if that's the case then not only will water be frozen at 40 but the air will be liquid.

[insane_alien]

If all you guys want to use SI units that's fine, but if that's the case then not only will water be frozen at 40 but the air will be liquid.

 

nah mate, the air would be frozen too.

[entwined]

My issue with the use of Fahrenheit, is that he didn't mention what unit set he was using...

 

But I'm all for always using SI...

 

Sorry about that, but I assumed that mentioning than water freezes at 32 would have been the tip-off.

 

If I read a thread here and someone mentioned water freezing at 0°, I think I would know that he was talking Celcius.....

 

But, no matter, the point is that the water is freezing when the air is above the freezing point of water. I think that Swansont probably answered the question....thanks to all.