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The Role of Water

by Peter Gray

The article on thermal myths briefly discusses a notion that has floated around (sorry) about water being a good thing for making thermals, because water vapor is lighter than air. Adapted from the article:

For example, a letter published in the April 2000 Hang Gliding magazine claims that a benefit of plowed versus flat fields is that the “furrows allow moisture to rise…and then vaporize.” ... “the heated water vapor will rise, not so much because it is warmer but because water has a low molecular weight and is less dense than the rest of the atmosphere.”
In fact, water vapor has less than 2/3 the density of air. To equal the buoyancy of water vapor, we would need to heat an equal volume of air from 45°C to 210° C! Sounds good, huh?
Yes, but evaporating a quantity of water requires 300 times more energy than raising its temperature by one degree Fahrenheit. Creating 113 kg of air buoyancy consumes about 8.2 million calories. Using the same energy to evaporate water produces only 9.2 kg of lift, which makes water vapor less than 1/12 as effective! Also, the higher heat capacity of water vapor means that more energy is needed to raise its temperature (and volume), so it is about 13% less effective than air for producing lift after it evaporates. Yes, humid air is somewhat more buoyant than dry air at the same temperature, but it only reaches the same temperature at a tremendous energy cost—energy that could have gone into far more efficient dry-air lift production.

Most pilots who blunder into areas that were doused by rain the day before soon learn that wet ground, even under full direct sun, is not a good bet for thermals, yet a good deal of argument about water persists. In a Dec. 23, 2001 post to the HangGlide@yahoogroups.com discussion group, Angus Pinkerton (Angus.Pinkerton@ntlworld.com) wrote [my numbering added]:

Well, I hate to rain on such a noisy parade, but here are some facts about the contribution of water vapour and temperature to the buoyancy of thermals.

1) In a paper originally published in the September 1995 issue of the Monthly Weather Review, Nilton Renno and Earle Williams described the results of their measurements of the temperature and water vapor contributions to thermal buoyancy using a remote piloted vehicle and a tethered balloon.

In tropical conditions (Florida) they found that moisture contributes about
50% of the total thermal buoyancy.

2) In desert conditions (New Mexico) their measurements indicate that moisture contributes about 15% to morning thermal buoyancy and insignificantly or even negatively to afternoon thermal buoyancy. (In other words, afternoon thermals can be drier than the average surrounding air mass)

In all cases, the thermal temperature excess is relatively small, about 0.4° C in the tropics and 1.5° C in the desert.

3) I speculate that this variation in relative moisture may explain why the better thermals are sometimes marked by wisps of cloud below generally flat cloud bases and at other times by noticeably convex cloud bases.

From my point-by-point reply:

1) I don’t doubt that, but it does not imply that having lots of water around is a good thing for total lift production. The distinction is subtle, but quite important to XC soaring strategy. Another way to state the above observation would be: “In tropical conditions, thermals are noticeably more humid than the surrounding air.” This is no surprise, because in places like Florida, there is so much water around that the air is typically close to saturation, regardless of temperature. When the sun heats the ground and the air above it, that air can absorb more water vapor than the surrounding air, thus it cannot help but have some buoyancy contribution from water vapor (we could work some examples if we knew the temperature differences involved). So far so good…

BUT . . . again . . . this does not mean that evaporating water is a good way to create lift, compared to heating air with the same amount of energy. In fact, it is about 12 times worse, under typical atmospheric conditions. In minor addition, water vapor is slightly (about 10%) worse than dry air for conversion of solar energy to buoyancy because of its higher heat capacity. Florida is a great place to fly, not because of the water, but because it is so close to the equator.

CAVEAT: This discussion assumes that more lift production is always better. As Davis Straub (http://www.davisstraub.com) and others have alluded, this isn’t always the case. In Florida, after several years of drought, thermals have been punchier and less pleasant to fly in (i.e., stronger than usual). Thermals can be so powerful that they deter flying, but generally if we’re talking about how to find thermals at low altitude (the only place where thinking about “triggers” might be relevant), the last thing we’re worried about is too much lift. We’re scratching, sniffing, hoping to get up one more time.

2) This isn’t surprising either, and the observation might be reworded: “Morning thermals in the desert are damper than their surroundings, while in the afternoon they are equally dry or slightly drier.” This is because dew falls at night in the desert due to much lower temperatures than during the day. A little morning sun drops the relative humidity and evaporates the dew into incipient thermals (retarding their formation, by the way). Once all the dew is gone, everything is more or less equally dry, except that very high temperatures near the ground, esp. in bare, rocky areas that produce the most thermals, make the air there even drier. As a side note, lack of water is one big reason that deserts see much higher peak temperatures (and thus higher lapse rates, climb rates, and cloudbases) than wet country at the same latitude. All that water is a gigantic energy absorber.

3) I would extend this to say “Variations in relative humidity and temperature must account for the hanging wisps, convexity, concavity, etc..” On several occasions in New Mexico I’ve thermaled quite a distance up inside a cumulus, in a kind of silo of clear air (of course I wouldn’t do that now that I’m older and wiser…). Why does this happen? Because the clouds had formed over the valley after a mid-day thunderstorm cycle, then drifted over the high and dry Sandia Mountains, where drier and/or hotter thermals rose into their bellies. Note that the non-flat bottoms of typical cumulus clouds are that way only on rather close inspection, which means that the humidity and temperature variations are proportionally small. From a mile or two away, such clouds usually look like they’re sitting on glass plates.