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Monday, June 30, 2014

What do beer and thunderstorms have in common?

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We all know that beer and flying don't mix. What you may not know is that the same thing that causes trouble for your beer also causes trouble for your flying.

If you're already a weather expert, this post may help you visualize some of the terms you already know in a different way. If you're not, have no fear: you're going to be ahead of the game because you'll understand more about some technical weather terms that you'll see on your private and/or instrument written exams than I did back when I took mine.

Those terms are "convection" and "latent heat of condensation". I made it all the way past my instrument knowledge exam without truly understanding what made convection work. I knew that it was an upward motion of a parcel of air, which was enough to bluff my way though the questions on thunderstorms, but I never could understand why a blob of air would just keep rising until it couldn't rise anymore.

The answer (and, like most things in meteorology, it's a general answer, not a 100% of the time answer) is contained in the concept of "latent heat of condensation". In this case, to keep things simple, let's look as "latent heat" as meaning "stored heat", which might make it easier to see what's happening.

In the spring of 2013, Dale Durran, a professor of atmospheric sciences at the University of Washington, studied how much of an effect the condensation that appears on the side of a beer can has on warming its contents. In this press release, he calculates that a sheen of condensation roughly the thickness of a human hair could warm the beer by 9 °F (5 °C) in only five minutes! That's a whole lot of energy in a small amount of moisture.

To see if those calculations are correct, Durran and his colleague Dargan Frierson performed some experiments, the results of which were published in this not-overly-technical and easily readable paper called Condensation, atmospheric motion, and cold beer. One of the important parts of the paper investigates how much of the heating is due to stored heat of condensation being released and how much is due to heat being transferred from the surrounding air.

They plotted the difference in heat from the surrounding air and that released in the process of condensation and came up with this:

The plot shows that the temperature rise due to latent heating increases dramatically with relative humidity. Moreover, the increase is much larger at 35 °C than at 25 °C, because of the approximately exponential dependence of the water-vapor content of saturated air on temperature. At 35 °C and a relative humidity greater than 60%, the temperature rise due to latent heating exceeds that due to heat transfer from dry air: Latent heating is the dominant factor warming your cold beer.
(Incidentally, this explains why thunderstorms are so rare in the winter: the cold, dense air in wintertime can't hold enough water vapor to store enough heat for them.)

So we've established that beer gets warmed by the release of stored energy as water condenses on the can. What does this have to do with thunderstorms?

Well, consider a cylinder of air that is a mile wide instead of the size of a beer can. Since a beer can is only about 2 1/2 inches wide, this parcel of air is going to be 25,000 times larger, and yet a mile-wide blob of air is not all that big in atmospheric terms. Think of how much stored energy is in that, yet while it's still locked up in water vapor, it's invisible!

Now let's give that cylinder of air a nudge upwards. This nudge could come from encountering a mountain range, or (quite commonly) a lift from a cold front sliding in underneath it and bullying it upward. Once it starts to rise, some of the water vapor will condense as the air cools adiabatically, and a cloud will begin to form. If there is not much moisture (i.e., water vapor) in the air, the cloud might be a small puff or a little layer.

However, given enough moisture (like on humid days), the energy that was stored in that water will be enough to heat that little blob and make it rise even more. As it rises, air from below it will be drawn up to replace it (otherwise there would be a vacuum behind it). The air from below will come up, deposit its moisture as a cloud, and heat itself up. That will make that blob rise, draw up more moist air from below it, and on and on until a towering cumulus (more formally known as cumulus congestus, but you'll usually hear pilots refer to them as "towering cumulus") forms.

This process is what convection is all about, and why pilots are always on the lookout for convective activity. If the atmospheric conditions are unstable enough, that towering cumulus can form into a full cumulonimbus: the dreaded thunderstorm. You can see a dramatic picture of the difference between the two in "Why there is no reason to fly through a thunderstorm in peacetime".

The author is an airline pilot, flight instructor, and adjunct college professor teaching aviation ground schools. He holds an ATP certificate with a DHC-8 type rating, as well as CFI, CFII, MEI, AGI, and IGI certificates, and is a FAASafety Team representative and Master-level participant in the FAA's WINGS program. He is on Facebook as Larry the Flying Guy, has a Larry the Flying Guy YouTube channel, and is on Twitter as @Lairspeed.

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