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Re: Ice in hose

You can form ice in a hose (or frost, etc) on a clear night without the 
actual air temperature reaching freezing because of radiational losses.   The 
key is it has to be clear and calm night and you can't have anything overhead 
(such as branches or cloudy skies).  That's why you sometimes see frost only 
in open areas.  

Intuitively it makes sense if you think of how standing in the sun or in 
front of a fire can warm your skin even if the air temperature is low -- 
that's the classic example of radiation.  But with the freezing hose, it's 
taking place in reverse - the hose is radiating thermal energy out to space 
at a faster rate than heat is being transferred into the hose through 
conduction and convection (it's quite complex if you want to start talking 
about emissivity and the variables that actually dictate heat transfer).

If you had the hose beneath a tree, the freezing would not happen because 
(unless of course, the temperature was low enough) because the heat transfer 
through radiation would have been blocked by the trees -- not much different 
than how you cool off quickly when you are shaded by a tree, except in 

Hope this helps. 

Joe Swift

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