Actual vs "Feels like" temp

[ddoppke]

John, When they say the actual temp is 35 but if feels like 29 due to wind chill or whatever how does that effect the freezing of water. Does the water react like it's 29 and freezes or like it is 35 and stays in a liquid state?

 

[jd]

Wind chill only effect exposed skin that uses evaporation as a cooling mechanism. Things like water and other inanimate objects are not impacted by the wind chill effect.

-John

 

[ddoppke]

Then why do we put our temp sensors in a shield box (other than sun) if the wind does not effect the temp they would feel? If you have an exposed thermometer out in the cold windy day would it not read colder than one in a shelter? Maybe I didn't explain my question well enough because from what I get from your answer is the water would still only react to the actual temp and the colder wind blowing would have no effect toward freezing the water. That is what I am trying to understand.
Thank you and Happy New Year!

 

[jbs]

I copied the following from a different site. I don't know if this will help or not:

For an inanimate object, windchill has an effect if the object is warm. For example, say that you fill two glasses with the same amount of 100-degree water. You put one glass in your refrigerator, which is at 35 degrees, and one outside, where it is 35 degrees and the wind is blowing at 25 mph (so the windchill makes it feel like 8 degrees). The glass outside will get cold quicker than the glass in the refrigerator because of the wind. However, the glass outside will not get colder than 35 degrees -- the air is 35 degrees whether it is moving or not. That is why the thermometer reads 35 degrees even though it feels like 8 degrees.

 

[frnash]

Then why do we put our temp sensors in a shield box (other than sun) if the wind does not effect the temp they would feel? If you have an exposed thermometer out in the cold windy day would it not read colder than one in a shelter?

1. The sun is the only reason for a "shield box" (radiation shield), although protection from rainfall is appropriate as well, as the raindrops impacting upon the temperature sensor would also yield inaccuracies.
2. The temperature sensor does not "feel" colder in the same way as exposed skin that uses evaporation (of perspiration) as a cooling mechanism.
3. No, except for the effect of solar radiation, the exposed and sheltered sensors would read the same temperature values, although the sheltered sensor might lag behind in reporting temperature changes (see the discussion of convection and advection below).

I copied the following from a different site. I don't know if this will help or not:

For an inanimate object, windchill has an effect if the object is warm. For example, say that you fill two glasses with the same amount of 100-degree water. You put one glass in your refrigerator, which is at 35 degrees, and one outside, where it is 35 degrees and the wind is blowing at 25 mph (so the windchill makes it feel like 8 degrees). The glass outside will get cold quicker than the glass in the refrigerator because of the wind. However, the glass outside will not get colder than 35 degrees -- the air is 35 degrees whether it is moving or not. That is why the thermometer reads 35 degrees even though it feels like 8 degrees.

Not quite true. The only difference due to the wind will be the rate of change of temperature, and that due to the transport of heat by convection or more precisely, advection. That is not the same as "wind chill". Wind chill by definition, refers to the effect of the wind on exposed skin².

This can be readily observed in a temperature sensor in a very hot desert climate, where in spite of a radiation shield, the blazing sun will still tend to produce a warmer "micro climate" in the nearly stagnant¹ air inside the shield when compared to the true ambient temperature, all the more so in calm wind conditions. Thus the need for an aspirated (see aspirator) solar shield, to effectively draw the ambient air through the "micro climate" of the shield.

(¹ Not quite stagnant, though, as even in calm wind conditions the warmer air inside the shield would tend to rise by virtue of its lower density.)

(² The method for calculating wind chill has been controversial because experts disagree on whether it should be based on whole body cooling either while naked or while wearing appropriate clothing, or if it should be based instead on local cooling of the most exposed skin, such as the face. The internal thermal resistance is also a point of contention. It varies widely from person to person. Had the average value for the subjects been used, calculated wind chill equivalent temperatures would be a few degrees more severe.)