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Fall is in the Air (and the Sky)

As the Northern Hemisphere edges quickly into Fall—and here in Vermont those changes are undeniable despite a week of sunny days—I find it useful to consider the factors that determine the temperature of the objects we so often look at with our imagers. It is not uncommon, for example, to find one elevation of a building being several degrees cooler than another. And, I clearly remember my surprise at finding few problems in the upwind end of a large substation and almost none on the downwind end. There are many reasons for a surface to be at a given temperature and understanding them is part of the challenge of what we do.

Key Factors Affecting Temperature
Over the next couple of weeks, I’ll be writing about the key factors affecting temperature. If I’m going to avoid making dumb mistakes and have a chance of making an accurate analysis, I’ve learned that I need to pay attention to these issues. I’m not talking about the accuracy of the measurement (i.e. have we set the correct emissivity and background?), but are we truly understanding the influences on the temperatures we are seeing?

The thermal world is constantly in flux with energy being transferred in various ways over time and that  affects the temperature. Often we show up at a single moment and take one image and have one set of temperature data.  From that small bit of data, we make our analysis and draw our conclusions. No matter what I am looking at thermally, I always need to be asking myself “What is influencing this object at the moment to cause it to be the temperature I’m seeing and measuring?”

Thermal Technology-Radiational Cooling

Dew and frost on the windows of a car parked outside on a clear night are both perfect indicators of temperature. I’m always amazed by the fact that the front and back windows can be significantly cooler than the side windows due primarily to differences in radiational cooling and heating. Here we can see frost on the rear window and condensation on the side one with a temperature difference of about 12F (7C).

Radiational cooling
The clear sky, night or day, is a large “heat sink.” Whatever faces a clear sky, say the side of a building, will nearly always be losing heat faster than similar objects that don’t face that sky. You can see this very clearly this time of year on an early morning walk by observing dew or frost on the windows of cars that were parked outside. It is not unusual to find upward of a 10°F (6°C) or more difference in temperature due mainly to radiational cooling.

The next clear and crisp Fall morning you have, look out on a lawn or field where “Jack Frost” has visited and notice the variations in temperature as represented by the presence/absence of frost or its thickness. Much of that will be due to radiational cooling.

Note: Frost and dew do not “settle” on surfaces or form as a result of gravity. They form when surfaces are below the freezing or dew points and moisture is available. That can easily happen on the underside or a surface before it happens on the topside.

Radiational cooling and heating dramatically affect the differences in temperature on the elevations of buildings. Here we see the upper walls much cooler than the area under the porch because they radiate heat to the sky. The house and porch roofs, though unheated, are also showing the affect of “clear sky cooling.”

The same radiational transfers are occurring on buildings, outdoor substations, and industrial equipment in the yard—literally anything we are inspecting under the sky! If you want to better see and understand these influences, start by narrowing the span setting of your imager and look at the variations in temperature on surfaces that are otherwise similar. You’ll find a great deal of variation in the actual temperatures depending on where they are radiating to and what is radiating back at them.

The difference between the roofs and the walls of houses are fairly easy to see. If you look more closely, you can see the difference in walls facing the sky versus those facing other houses or trees. Look at where the frost or dew forms first or heaviest as confirmation of your hypotheses.  The underside of equipment mounted in the yard is typically warmer than the topside and complex or vertical shapes often remain warmer longer than broad horizontal ones. We may not be able to actually see the radiation “zooming” around but we certainly can begin to imagine what is happening using our imagers and our knowledge of heat transfer.

Radiational heating
One of the best parts of Fall is being warmed by the sun on an otherwise crisp day. We know that many surfaces, including human skin, are very efficient at absorbing radiation much of which is then converted into heat.

Seeing the thermal effects of the sun is quite straightforward. Full-spectrum solar radiation, when absorbed by a surface, results in it being heated. As you are walking by a parking lot full of cars in the sun, take a look at the wide variation in temperature based primarily on the color of the car!

Thermal Technology-Radiational Heating

A “Think Thermally” t-shirt drying on the clothesline clearly shows the difference in absorption of full-spectrum sunshine, and the consequent heating effect, between light and dark colors.

More challenging is to understand how much heating occurs? That depends in large part on the absorbency to the various wavelengths and the angle at which the sun strikes the surface. It is obvious that dark surfaces at right angles to the sun are very efficient absorbers. Again, look around the parking lot and notice the variations due to the angle of incident. Now, before your boss wonders what you are doing out in the parking lot, transfer some of that understanding to your work!

But radiational heating is not related only to direct sunlight. For instance, why is the area under a tree, even one without leaves, often free of frost on a cold, clear fall morning when the rest of the grass is covered in the stuff? The tree is radiating heat it stored from the daytime back at the ground! The same thing happens under the eaves of a house. That area remains warmer than the wall below in large part because of radiational warming.

Next week, I’ll write about other influences to the temperatures we see and measure. Between now and then, I hope you’ll spend some time thinking about and observing the effects of radiational cooling and heating. You don’t need to have your imager with you,  but that certainly makes it more interesting! When you find some good examples, I hope you’ll take a minute to post a response here.

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

1 comment to Fall is in the Air (and the Sky)

  • Stella Newport

    It has always been my belief that good writing like this takes research and talent. It’s very apparent you have done your homework. Great job!

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