Relative humidity and thermal strength

Is it true that days with high relative humidty produce weaker more mellow thermals than blue days

Think about it: What is air made of? Primarily N2 (80+%), then O2, then CO2. N2 is a bound pair of Nitrogen atoms, and the molecule has a total molecular weight of 14 + 14 = 28.

Now let's look at water, good 'ol H2O: It has a molecular weight of 16 + 1 + 1 = 18. That's substantially less than N2. So the addition of water vapor to the air makes it less dense, since it is displacing N2. For gliding, that's like saying it makes the altitude 'seem' somewhat higher than it actually is (though the barometric pressure stays the same).

So, you can sort of think of the addition of water vapor changing the behavior of the air (and thermals) somewhat like a change in elevation would. I used to know the exact formula to convert humidity to equivalent feet of altitude, but I can't find or recall it right now... But keep in mind that all the safety issues of flying at altitude can apply to flying in very humid air (especially if you normally fly in extremely dry air), such as the fact that may be harder to launch, it will be easier to stall your wing, and collapses (for us PG folk) will recover more slowly.

There are other ways to look at the addition of water vapor to the atmosphere and its effect on density, other than as an equivalent altitude change. The density of the air also has thermodynamic effects, in that less dense air is less good at transporting heat away from a heat source. That means you would expect your favorite house thermal to go off less often, but for a longer time, when the humidity is high, compared to when it is low. The difference can be quite small (depending on just how much water vapor is in the air), but it is certainly detectable. But since the air is less dense, the thermal will indeed feel weaker and more mellow (even small effects add up when literally tons of air are moving about).

Blue days can have high humidity too! And the presence of cumies does NOT necessarily mean the humidity is all that high! This is where RELATIVE humidity comes into play, along with the air temperature, the altitude-temperature profile, and the CCL (Cloud Condensation Level). While it is true that absolutely dry air won't make a cloud, it is also true that NO WEATHER IS ABSOLUTELY DRY! It is rare to see the relative humidity drop below 20%. So if it gets cold enough at altitude fast enough, you will certainly see clouds even over an otherwise bone-dry desert. And an inversion can prevent cloud formation on even the wettest of days.

The key here is to look at the dewpoint: It varies with the moisture content of the air, the temperature of the air, and the pressure of the air. If you can get your brain around it, the Skew-T diagram is the best single source of information for all this data, in graphical form at least. Denis Pagen went over all this in detail in one of his books or many articles. I defer to his explanation (mainly 'cause he's better at it).

This kind of information, and all information related to micrometerology, is vital to being able to predict and prepare for 'epic' days: Here in San Diego, the humidity (among other things) absolutely affects our flying decisions: Where to fly, when to fly, how much we can safely push the envelope, and how long and far we can expect to fly.

High humidity days, during the warm season, are most favorable for thunderstorm development and thunderstorms are not known for their 'mellow' characteristics.

What can be said about blue days vs. days with convective clouds is you'll get higher on a blue day b/c you can rise above what would have been the convective condensation level, AKA cloudbase.

A fantastic and well thought out reply!! Thank you for your information, although I didn't ask the original question I found it very useful. It's a shame that this newsgroup has died such a death, that we don't get more

Thanks

[snipped] Even after reading and rereading your argument about the fx of adding water vapor to a volume of air, I still fail to see your point about thermal strength.

Density altitude is as much about temperature as it is about the dew point, both of which relate to the RH. You seem to suggest that flying when the air temperature is 65°F and the dew point is 60°F (an RH of ~ 85%) would be inherently less safe, which of course it is not. The dew point is the issue, not the humidity.

Even so, high humidity resulting from high dew points during the warm season provide the latent energy for strong, deep moist convection observed as thunderstorms, as opposed to shallow, dry convection observed as fair wx cumulus.

What affects the thermal strength is the 1) height of the thermal plume which, with everything else being equal, is controlled by the surface temperature/dew point (T-Td) spread, 2) the structure of the deep layer vertical temperature profile, i.e. whether a strong, capping inversion exists to stop the ascent of buoyant parcels, and 3) the relative humidity above the planetary boundary layer (PBL).

1) The greater the T-Td, the higher the convective cloudbase will be, and therefore the stronger the lift; 2) The deeper the mixed layer, the higher the thermal plume will rise, and therefore the stronger the lift, and; 3) The dryer the air above the PBL, the more evaporation, which is a cooling process that maintains a steep lapse rate. Cooling aloft enhances lift as much as strong heating of the surface.

Actually, its the less dense air that transports heat away from the surface during convection. If these parcels didn/t have a lower density, then they wouldn/t ascend as thermals.

The real problem here lies with the premise of the original question, which seeks to equate relative humidity (assumed @ the surface) with the nature of thermals; who/s character and strength is determined by the thermal structure of the atmospheric layer well above the surface.