Last week I showed this thermal image of a “tea cozy” in place covering a pot of hot tea and had intended to discuss it further this week. However, Hurricane Irene, has blown her way into our lives this week so instead I thought I would take a quick diversion to discuss convection and phase change, the makings of a hurricane.
Be clear – I am not an expert on hurricanes or a particularly skilled meteorologist. However, I do understand how to “think thermally,” and that is the essence of not only a hurricane, but all of our weather. In the end, weather systems are driven by such thermal factors such as the heat of the sun, nighttime radiational cooling and thermal capacitance.
Understanding Phase Change
Irene, like all tropical storms, began over water warmed by the sun during the course of the summer months. Stated rather simplistically, at some point in July or August conditions become conducive for a relatively small atmospheric disturbance to trigger a strong upwelling of air warmed by the sea. The warm, humid air rises, carrying with it the latent energy of vaporization, basically the heat of the sun that caused the sea water to evaporate. As the air gains elevation, it cools and water vapor condenses releasing that same energy.
If you don’t understand phase change, especially as it relates to H2O changing from ice to water to water vapor, it is important to review the details and incorporate your understanding into your day-to-day working knowledge as a thermographer. So much of the thermal world of thermographers is influenced by evaporative cooling in particular.
Back to Irene and that phase change energy. How much energy is absorbed and then released by vapor condensing? Scientists estimate a tropical hurricane can release 50 to 200 exajoules (1018J) per day. I know it is hard to get your head around that kind of number, so consider it is the equivalent of 200 times the electrical generating capacity of the entire planet for a day. Remember that 1000J = 1 Btu and 1 Btu is roughly equivalent to the heat released by burning a wooden kitchen match.
Newton’s Law of Cooling
Suffice it to say that all that energy is what drives the storm. Newton’s Law of Cooling shows this driving force clearly with the component relating to temperature difference (ΔT):
Q = h • ΔT • A
Q = heat
h = coefficient of convective transfer
ΔT = temperature difference
A = area of transfer
As ΔT increases so does the transfer of energy.
Some is converted to mechanical forces (wind), while the rest continues to “pump” the convective forces. Of course, the condensed water vapor at the top of the storm comes down as rain, rain cooled by the upper atmosphere; the cooling, in turn, tends to slow the convective forces. As long as the storm is over warm water, heat rules. As it moves over cooler land, however, the supply of warm, moist air is reduced and ΔT is reduced. Thus, both the energy transport and the convective process slow.
Hurricane Irene Aftermath
As was the case along most of the East Coast, Irene was not as damaging as we feared. I’m still impressed with the advanced preparation people took, even if we did not “need” it all. The incredibly complex task of forecasting such storms will always involve a bit of luck and luck alone will not keep you out of harm’s way. In Vermont we ended up with mostly rain, nearly 7” (18 cm) in many places. With our “water collection” system of mountains, foothills and rivers that ended up causing a great deal of flooding in the valleys.
Thermographers will be busy throughout the path of the storm over the weeks to come helping to locate moisture damage in the many buildings that were affected. Even the roof on my house sprung a few new leaks in the downpour! Thermal imaging—showing the cooling effects of the latent heat of evaporation—helped me locate them and get a couple of buckets strategically placed during the brunt of the storm.
Today Irene has moved on north and, deprived of heat, is dissipating rapidly. The sun is shining and, except for the clean up, life is good. Next week we’ll get back to that teapot and talk more about conductive heat transfer.
John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner