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The Equinox

As I write this, we’ve just moved into Fall, at least here in the Northern Hemisphere. Our friends “down south” are enjoying the first bursts of Spring. In the busy world of email and text messages, rushing off to the airport and ordering a pizza for dinner, who cares about the equinox? Who even remembers that it marks the time of year when day and night are equal in length? Let’s face it, on too many days most of us no longer even seeing the sun, and our view of the stars are obliterated by light pollution or nighttime television. It’s depressing how little attention we sometimes give our surroundings!

A thermographer is, in essence, a sort of “thermal Sherlock Holmes.” We must pay attention to our thermal surroundings if we are to make sense of what we are viewing through out imagers. The concept of “thinking thermally,” is based on not even needing an imager. Look around and if you are observant, you will see countless examples of the temperature of our world. We could simply say the planet is a vast thermal engine.

Thermal image of Moonlight in Vermont

“Moonlight in Vermont” is not only a wonderful old song but also a great juxtaposition of thermal events. Note the range of this auto-adjusted image, from -71.9C (-97F) in the clear sky to 63.3C (146F) on the moon’s surface. Note also that the temperature of the moon may not be accurate due to working outside the measurement resolution of the lens/detector.

Two weeks ago, for instance, we had our first frost here in Central Vermont. This typically occurs in the mountains around the September full moon. Why the full moon? Honestly, it could be any night the sky is clear, but if the sky is clear and the full moon is up, it gets your attention! Clear sky that allows the Earth to radiate so much energy to space that we cool to the freezing point. Even with frost at higher elevations, however, we often don’t get a frost down in the valleys because they fill up with cold air, that along with the moisture in the valleys, results in clouds or ground fog forming. That fog acts just like a blanket to reduce nighttime radiational cooling. As the water vapor condenses, it too gives off a tremendous amount of heat. In the end the valleys stay warmer.

For many of us, this time of year—while we are in this seasonal thermal transition—is ripe with examples of Thinking Thermally. I’d encourage you to get out, open up your “sensors” and learn from them. One word of warning, don’t be too quick to jump to conclusions from your observations. Often we can’t even see all the variables and as a result, making a valid conclusion becomes suspect. Just enjoy some patient observing, preferably from several different points of view, until the Thermal Truth, if you are lucky, reveals itself.

Not sure where to even start? Here are a couple of suggestions:

The “color palette” you see in fall foliage is remarkable. The colors have been there all summer but were merely covered by the green chlorophyll. As cold weather sets in and the cells begin to die, the chlorophyll breaks down leaving the underlying color we so enjoy seeing.

• Why do bees crawl around on a flower early in the morning, but fly from one to another later in the day?

• 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?

• 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?

• 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?

• After a rain storm, can you predict the relative temperature of puddles of various sizes?

• 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?

By the way, despite the horrendous flood damage much of Vermont suffered from the wrath of Hurricane Irene, the annual color show of Fall Foliage looks like it will be spectacular. We’d love to have you visit and see this amazing sight, as clear a thermal signature as you will ever see! Check out the foliage report here.

Many thanks also to Scott Warga for his comments and discussion last week. He is a superb example of someone who is very thermally inquisitive! Next week I’ll explore some of these issues and any others you’d like to discuss.

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

5 comments to The Equinox

  • When I measured the Moon with my Ti25, I got different results. But with clear sky it was about 90-110 °C, depending on how good was my focus

  • Yes, I’d expect we might get varied results. At a distance of approximately 400,000km and a diameter of approximately 3500km, the target size is fairly close to the approximately 75:1 measurement ratio of the Ti25. If either of us was slightly out of focus or just a bit off center, I’d expect some variation. That variation could also be quite large given we’d be averaging a bit of deep outer space into the measurement.

    Wiki tells me the surface temperature of the moon at the equator is approximately 27C (80F). so clearly my measurement is also off. I don’t know what emissivity value is correct but would guess it would be approximately 0.93; the background temperature would probably be approximately -40C (-40F). And as you point our with your reference to “clear skies” there would be some affect, either negative or positive, to the measurement by the Earth’s atmosphere.

    Notice I’ve used the term “approximately” quite often in my response! Any Ti32 users with a telephoto want to give it a try for comparison?

  • I checked, in the day the Moon surface can be up to 120°C according to open source information (wiki). But actually I think here are both: surface temperature and reflected temperature. Even with quite high emissivity, about 6000 K behind meens something :) … But I need some time to make exact estimation.
    And atmosphere is usually below 10°C, so I’m in a doubt, that it can make the temperature reading higher :) .

  • Yes, you are correct! From a NASA website (http://lunar.gsfc.nasa.gov/moonfacts.html): “The surface temperature at the equator during the day is 134C (273F), and at night is – 153C (244F).” This would also suggest that unless the moon were full, you could easily be seeing some surfaces that were warm and others that were cool, again causing the average temperature to be less than might be anticipated.

    What I meant by the atmosphere affecting the reading was the EARTH’S atmosphere. Depending on the humidity and distance, it can affect the transmission of the sun’s energy being reflected off the moon.

    Good discussion, Alexander, and useful at showing us how complex measurements can be.

  • Here are some pics, taken in about one month between each other last year.
    http://pics.livejournal.com/alexbard/pic/002994qc/
    http://pics.livejournal.com/alexbard/pic/0029a0cf/

    One of them with clouds and the moon is not full. Another one with full moon.

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