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Thinking Thermally in the Fall

Thermal Technology-Ground Fog

“Ground fog” is common in many areas in the fall as cold air settles in the valleys and the moisture in it condenses. The changes in color of the fall foliage are also closely related to temperature change and the physiology of the trees.

Fall has continued to come on quickly. While we’ve enjoyed a bit of warm weather after our initial frost, the rapidly changing foliage—more colorful  every single day—is a constant reminder of the change in store. Last week, I mentioned the early morning valley fog we so often experience here in Vermont. It is pictured here sandwiched between some fall color with Camel’s Hump, our most notable mountain, in the background.

I’ve not heard from many of you with your fall Thinking Thermally observations! Hopefully you are all too busy at work, but do take time to just look around and learn from the many thermal indications at hand. I asked a few questions to prompt your thermal investigations and here are a few comments about them:

• Why do bees crawl around on a flower early in the morning, but fly from one to another later in the day? Like humans, bees too have muscles that require warming up.  So on a cold morning they tend to crawl over flowers and minimize their flying. They even target larger flowers rather than try to fly to smaller, more nectar-rich ones. The “energy balance” of bumblebees is discussed in a marvelous book called Bumblebee Economics (Heinrich). It can be a challenge to get a thermal image of a bee at work, but if you succeed, I think you’ll be impressed by what you see.

Thermal Images on Wall Heating

As we move into the fall the sun is lower in the sky and often causes south-facing walls to heat quite significantly. When viewed from inside, the wall, in this case an insulated one) can change from a normal pattern (left) to a “reversed” pattern (right). The center image shows the wall in transition and also clearly shows the affect of a large roof overhang that shades the top of the wall.

• What difference will a lower sun angle make on how your conduct building inspections over the next six months versus how you did them in the past six months? You’ll need to be much more careful about solar loading of walls, especially the south walls where the sun is more directly shining (and heating) the vertical surfaces. A wall in direct sun can “flip” in just a few hours time and remain that way for several hours after the sun is off the wall. Don’t make the same mistake I made 25 years ago (I was very “green”) of thinking a well-insulated wall was, in fact, not insulated! The key is to ask yourself “In what direction is the heat transfer occurring?”

• Think about an aluminum electrical conductor in an outdoor substation on a clear, warm day. When you view this through your imager, what “apparent” temperature difference will you see between the top and bottom of the conductor? The top will be colder and the bottom warmer. How much? Depending on conditions there could easily be a 100°F or more difference in the apparent temperature (not corrected for emissivity or background). Sound crazy? Take a look and prove it to yourself!

• You are up on a low-sloped roof in the evening, preparing to conduct a roof moisture inspection. You notice much of the roof is damp with dew, except for several rectangular-shaped areas that are dry. What do you think a subsurface moisture reading in these dry areas would indicate? When you find these conditions, you often can just put your imager down and mark the areas without condensation on them! Why? They are warmer because the trapped moisture beneath them has a higher thermal capacitance than the dry insulation.

• After a rain storm, can you predict the relative temperature of puddles of various sizes? This one is too difficult to predict from my desk but you’ll have fun—and draw some curious looks—when you try it on your own. You’ll also learn a lot about heat and temperature.

• How much energy does a Bicknell Thrush, a small bird that summers on the mountain tops in Vermont, need to store to successfully migrate this month to the Dominican Republic? This one too is tough to answer. In general birds add tremendous quantities of fat prior to migration, but they also arrive in the south with very few reserves. Their migration, of course, is ultimately powered by the sun. Like many human “snowbirds” they are chasing the sun to places where the Spring Equinox has just been celebrated and the weather will be getting warmer!

Thinking Thermally,
John Snell—The Snell Group, a
Fluke Thermal Imaging Blog content partner

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