Last week we talked about emissivity and reflection, probably the two most talked about issues among thermographers. But that ain’t all we need to talk about! There is a reason engineering heat transfer courses are both long and notoriously challenging. We won’t go that deep into it, but as most of you know, I am a big fan of practicing Thinking Thermally.
As I said several weeks ago, Spring is a great time to see many of the basic heat transfer issues engineers study outside the classroom. Heat transfer is happening all around us, and is often visible without an imaging system. As we transition from Winter to Summer, the influence of the sun makes itself known in many different ways, all of which we can learn from. The entire planet responds to the changing solar angle as Spring arrives in the northern hemisphere (and Fall gives way to Winter, South of the equator).
Here in Central Vermont, we still have a lot of snow. As it melts, it is interesting to think about why it does so at different rates and in different patterns. Here is what my blueberry bushes look like. The same pattern of melting can be found around nearly all trees, but why? The answer lies in basic radiational physics. The plants absorb full spectrum solar radiation, heat up and then re-radiate the energy as long-wave radiation (heat) back into the snow. Snow doesn’t absorb full spectrum solar nearly as well, but it does absorb long-wave energy with an emissivity of about 0.95.
Driving around Central Vermont this past week, I noticed that all the lakes are still covered with ice, except where there might be an inlet of a feeder stream—fascinating! Water, of course, has a tremendous thermal capacitance and the latent heat of fusion—the heat required to melt the ice–is also quite large. The end result? It takes a lot of heat to melt that water. When the lake is covered with snow, the sun’s energy is, in large part, lost to reflection. Some who are bored with Winter have fun predicting when the ice will melt. One of the most notable ice-covered lakes in my area is Joe’s Pond, where you can lay your bet down, along with about 12,000 others, as to when the ice will melt. Interestingly, the data shows a distinct warming trend in Joe’s Pond over the past 25 years.
Even if you don’t have snow in your area, I’d be interested to see the thermal
signs our readers have seen of Spring’s approach. Of course that same sun that melts the snow and ice also affects our work—from heating up the sides of buildings, to making substation inspection much more challenging, to providing the heat source for roof moisture inspections. Whether the sun is helping us or making our lives harder, we need to understand the thermal affects it is having on our surroundings. Failing to do so means we will “flunk” Thinking Thermally 101 and end up in the principal’s office for not getting our job done right!
So, have some fun practicing Thinking Thermally—it will help you refine your skills as a thermographer and will make a positive difference in your work.
John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner