Last week we talked about dressing for winter weather, in a ‘how-to’ post on reducing heat loss from our bodies. We’ve all experienced how difficult it is to stay warm when that “Old North Wind” is blowing cold. Wind—or forced convective heat transfer as it can also be termed—seems to “go right through” even the warmest clothing, taking with it the precious boundaries of warmer air we’ve built up in the layers of our clothes and replacing it with bulk quantities of colder air.
Newton’s Law of Cooling says, “The rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings.” In other words, the colder the air is, compared to our body, the greater the rate of loss. It is no wonder then that the “Old North Wind” in winter is so devastating to our heat balance. The solution to keeping warm on a windy day, as we learned in our Level I training course, is to either get to a warmer climate (see Michael Stuart’s thermal image of waves!) or reduce “h,” the convective heat transfer coefficient!
No offense to my engineer friends, but “h” is basically a “fudge factor” that accounts for the variables of convective heat transfer, many of which are unknown or unknowable. These, among others, include velocity, shape and orientation to flow. We all know, for example, that we can simply step out of the wind and feel warmer. By doing this, we have effectively reduced “h” and convective transfer by reducing the velocity of the moving air around us. We can also just change our position relative to the wind by giving ourselves a less obtrusive geometry (or a smaller surface area) and again, we feel warmer.
In both clothing and buildings we’ve learned that we need to install effective wind barriers if we want to minimize the power of convection and transfer of air. The structure of the materials used in both today’s clothing and buildings are engineered to not only stop the larger air molecules from moving through them, but also to allow the smaller molecules of water vapor to exit, thus reducing the potential for problems associated with build-up of condensation. Given these materials, we can now stand on top of a windy ski slope in our parka or build a house with a view of that “Old North Wind” and be much less affected by convective transfer.
The wind is just one factor in heat transfer, but at this time of year it feels—literally—like a very important one! As you are out and about this week, give some thought to how “h” affects us every single day (and night). Stay warm!
John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner