I've had early-stage hypothermia twice — once when I was in Smoky Mountain National Park when it was about 45 degrees. It wasn't that cold in terms of relative temperature, but it was both rainy and windy, and as I hiked along, I got colder and colder. Because of an earlier experience, I recognized the impending signs of hypothermia (intense body shivers followed by nausea and brain fog), stripped off my clothes, changed into dry ones, put plastic bags on my feet (my trusty boots had gotten soaked when I slipped in a stream), and did jumping jacks — even though I wanted to curl up on the ground for a nap. I recovered quickly and backpacked out of the park a few hours later.
Experiencing hypothermia when the air temperature is significantly above freezing is not uncommon. There are roughly 1,300 hypothermic deaths from cold each year, and more than half of them don't occur during winter or in freezing temperatures.
One of the reasons there are more deaths from hypothermia in above-freezing temperatures is that temperature alone is actually a poor indicator of how cold your body will feel out in the elements, and as a result, people find themselves cold and underdressed. That's why estimates of how cold it really feels outside are important, and wind chill is the most common way to gauge that.
How to calculate wind chill
If you want to figure out wind chill, this is a handy reference. (Chart: NWS Wind Chill Temperature index)
Wind chill can be factored a couple of ways, but all of them take into consideration wind speed and air temperature as the National Weather Service/NOAA chart above does.
It doesn't seem to make sense; the thermometer will read 45 degrees whether it's windy, or rainy or sunny — so why do we feel so much colder when the air is moving?
Unlike animals that have insulating fur, human skin is better at evaporating excess heat than it is at containing it. We lose heat quickly, because we normally radiate warmth from blood vessels beneath our skin. When the air is still, a heat envelope of sorts can form, but when the wind is blowing (or you're moving through the air, say on a bike), that heat is immediately whisked away. The faster the wind, the more quickly your body heat dissipates — and at a wind speed of 25 mph or more, the human body can no longer keep up, no matter how hard it works.
So calculating how fast the wind is moving along with the temperature means you'll get an idea of how fast you'll lose body heat. That's wind chill. (On the flip side, during heat waves, the Heat Index takes into account temperature and humidity to give you an idea of how hot it really feels.) Another method is AccuWeather's "RealFeel" estimator — it adds more information into the wind chill equation, "including the temperature, humidity, cloud cover, sun intensity, and wind."
It only takes a dip below 95 degrees in body temperature for hypothermia to begin, so keeping your eye on the wind-chill factor or "feels-like" temperature is a good idea any time of year.
If you're going to spend time in the great outdoors, you should always bring extra layers with you. The most common cause of death from hypothermia is when the weather changes quickly and people who are out outdoors enjoying a day in the wilderness are caught without enough clothing to keep them warm.