In Los Angeles we didn't have to worry about such things, but here in Missouri I've had to quickly learn about wind chill and the heat index. No one had ever been able to give me a concrete definition of either term, so I turned to the ever-reliable Wikipedia for answers.
Wind chill is the apparent temperature felt on exposed skin due to the combination of air temperature and wind speed. Except at higher temperatures, where wind chill is considered less important, the wind chill temperature (often incorrectly called the "wind chill factor") is always lower than the air temperature, because any wind increases the rate at which moisture evaporates from the skin and carries heat away from the body. The phase change of water (in sweat) from liquid to vapor requires that the molecules reach a higher energy state. That energy is acquired by absorbing heat from surrounding tissue by conduction (see heat transfer).
Because wind chill is related to evaporation, dry inanimate objects are not affected by wind chill the way humans and animals are. Wind chill won't have much effect on your car, house, or outdoor machinery.
Similarly, the heat index is a measure of how much the ambient humidity is affecting evaporation.
The Heat index (HI) is an index that combines air temperature and relative humidity to determine an apparent temperature — how hot it actually feels. The human body normally cools itself by perspiration, or sweating, in which the water in the sweat evaporates and carries heat away from the body. However, when the relative humidity is high, the evaporation rate of water is reduced. This means heat is removed from the body at a lower rate, causing it to retain more heat than it would in dry air. Measurements have been taken based on subjective descriptions of how hot subjects feel for a given temperature and humidity, allowing an index to be made which corresponds a temperature and humidity combination to a higher temperature in dry air.
Humid air slows the evaporation of sweat and thereby reduces our ability to cool ourselves. Simple enough! Both pages have common formulas for calculating these values, if you're feeling adventurous.
I wouldn't have thought wind chill to have a significant impact on my car either.. until I moved to a place that had a garage that's not connected to the residence or heated in any way.
My car starts a lot easier on cold mornings when it's in the garage than when it sits outside in the wind.
thats less a wind chill its just butt cold temperatures (and as far as inside and outside, there is some limited insulation inside helping some), when in Montan its not uncommon to see extension cord sticking out of the front of cars so people can plug in block heaters when they get home.
During the day.. when my car's parked outside.. the car starts harder after a cold day when I go home than after a cold night sitting in the uninsulated garage.
Most structures have some intrinsic insulation because of building materials and it can be just enough to make a difference between inside and outside (think of the difference of being inside an igloo vs being outside an igloo), but its not really a factor of wind chill unless your car is generating heat in its environment that is blown away quicker than it can generate it while parked.
From inside the garage, there's the 2x4's for the frame and the plywood sheets that cover it. On the outside there's vinyl siding.
Just being out of the wind is the only thing I can think of that makes a significant difference, so while it may not be "wind chill", technically, the wind is the common denominator.
The temperature outside during the day can be warmer than the garage is overnight and the car will start harder after sitting out in it all day.. which is why I doubt the garage is providing any significant insulation effect.
That wiki needs to be fixed. There's a contribution to windchill from evaporation. But the big effect is eliminating the envelope of warm air that you create-essentially, in no wind, you loss heat more slowly because the thermal gradient is lower. If a car engine's been running in no wind, it's warm near the car. In a high wind, that warmed air has been moved.
Think convection vs. radiation moving heat.
Not bad. However, at seventy hundred feet in the forest, (Last of Nov and snowing) Which is best to use for ambient temperature on my body, heat index, or wind chill, (usually blowing).
Thank you.
Gary Pitchford