Comments on Wind Chill:

The posts over Wind Chill and whether or not it affects cameras and batteries has been both entertaining and illuminating, not only regarding the science/truths/myths/misconceptions about Wind Chill, but also about the individuals doing the posting!! Most people seem to get the concept, and much of the disagreement involves semantics and nitpicking over terminology. So let me put out a few facts about heat transfer and Wind Chill so that readers can distill what they like from the banter.

First, since credentials seem to be important to some folks in establishing credibility, I have BS and MS degrees in Physics and a PhD in Biophysics. I was the Canadian military’s expert on heat and cold stress for most of my research career, and I served on many NATO and other international panels dealing with environmental ergonomics. FWIW, one of the scientists on the Joint Action Group for Temperature Indices that redefined the Wind Chill Index for North America in November 2001 worked under my direction on that project.

The essence of Wind Chill is the increase in convective heat transfer due to the wind. In the absence of wind there is a thin boundary layer of air around every object, and it provides a small amount of thermal resistance (insulation) against heat transfer. As wind velocity increases the boundary layer gets stripped away and the result is an increase in heat transfer (loss) via convection. Wind Chill, Wind Chill Index, Wind Chill Factor, Wind Chill Equivalent Temperature, and Real Feel are all attempts to quantify this phenomenon, and some try to do it in terms that relate to the human perception of cold.

The original Wind Chill Index describes the cooling power of the wind in relation to the rate of heat loss from human skin at a mean skin temperature of 33 Celsius (that is the basis of the factor "33 - Ta" in the equation; check Wikipedia). The units are kcal/square meter/hour, or Watts/square meter. Note that you cannot derive these heat flux density units from the "units" of the components of the equation; hence, it is an empirically derived index. Values greater than 1400 exceed the body's capacity to provide enough heat to the tissues to prevent freezing/frostbite in less than 1 minute.

Wind Chill Equivalent Temperature is a calculated temperature that would provide the same sensation of cooling to bare skin that is initially at 33 Celsius with minimal (but not zero) wind speed. As such it has no direct relationship to inanimate objects that have no metabolism, or even skin that is at a different temperature. It also does not apply directly to body parts that are insulated or protected against the cold and wind.

Regarding cameras and batteries — or even human body parts, covered or not, alive or dead (but warmer than ambient) — colder temperatures and higher wind speeds will cause all of these things to cool more quickly. And as has been correctly stated, the items will not become colder than ambient temperature (unless there is evaporation of fluid from a damp surface). The cooling curve is exponential, and the instantaneous rate of cooling depends on the temperature difference between the object and the ambient temperature, the wind speed, the amount of insulation around the object, and its internal heat production. Regarding the latter point, the battery in the camera should cool more slowly if the camera is "on" due to self-heating, but the benefit of this in extending battery life might be negligible.

So while the Wind Chill Index, Wind Chill Factor or Wind Chill Equivalent Temperature do not apply quantitatively to cameras, they can be used in a comparative manner to say whether weather conditions on a given day are harsher or gentler than on another. A higher Wind Chill Factor or a lower Wind Chill Equivalent Temperature will definitely cause more rapid cooling of your equipment than if the values are “milder.”

The effect of cold on lubricants has already been mentioned (and seemingly dismissed because of modern lubricants), but two other factors to consider are mechanical shrinking of parts and the physical difficulty of using your camera due to gloves or cold bare fingers. Mechanical movements for autofocus, stopping down the diaphragm, and even image stabilization will undoubtedly be affected to some degree, but hopefully not enough to prevent anyone from getting pictures in the cold. And on the plus side, the sensor thermal noise floor should be lower in the cold.

Happy shooting!!!

I searched UHH unsuccessfully for "freeze" or "freezing" before posting this question.

I consider myself to be a street photographer and never leave home without a camera. Camera portability and image quality are most important to me. In the days of film, I had multiple bodies and lenses and even a Rolleiflex, but moved past that decades ago. My largest body is the T2i and I use that only to photograph high school reunions. Otherwise, my daily carries are either the Canon S95 and the Panasonic Lumix DMC-LX100.
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