What’s Up With Soaring

Air; it’s like invisible, man.
Well it isn’t to the observant soaring pilot. Wind of course, doesn’t blow it flows. When passing an object friction causes that object to be pulled in the direction of the wind (drag). I think it’s important that a pilot understand that wind affects things it passes (and therefore those objects effect the properties of that air), rather than blowing something around it’s pulling objects along with it like a river. Once you observe wind affected objects with that in mind you will start to see the air flowing all around you. A more accurate interpretation will come once you add in all the variables that you'll learn later on.

The Beaufort scale
Irishman Sir Francis Beaufort created the scale in 1805. He didn’t scale wind to speed but rather and more importantly to its effects.

http://www.carvegp.com/misc_images/scale.gif

Dude, It's A Fluid!
Our atmosphere contains roughly 78% nitrogen, 21% oxygen, 0.93% argon, 0.04% carbon dioxide, and trace amounts of other gases, in addition to about 3% water vapor. Though air is a mixture of gases it’s fluid. Sometimes this fact is overlooked because when you think of a fluid you think liquid. In general the difference between liquids and gases as they pertain to fluid dynamics are that gases are compressible and liquids aren't (exceptions apply).

Go With The Flow
Now that we are looking at air as a fluid let's examine what makes it flow. For thermaling we must identify the things within our immediate area that will heat differently than the surrounding area. Let’s step outside into a sunny windless late summer afternoon. Now turn your back to the Sun. Do you feel the heat building on your back? Okay, now look to your shadow; the heat you feel on your back you’re robbing from the area of your shadow. It’s important to note that air by its self is not heated by solar radiation as readily as, say, pavement. In fact, air is an excellent insulator against solar radiation (i.e. dual pane glass). So, while standing in the Sun your back is heating the air around it. This is important because objects absorb, retain and radiate heat very differently. The Sun drives this differential heating, which is the source of all weather from a puff of wind to a hurricane.

Shedding Light On Density
Now, back to your shadow. Since we have robbed the heat from the shadowed area, that area is now cooler than the surrounding area. We have created our own little area of differential heating. Heating and cooling air of course changes its density, with the cooler air being denser. Maybe if you look closely to the edges of your shadow you can see a hint of air flowing out from it. Well, it is.

So What Makes It Go Up?
Hey don’t move yet! Are you still standing outside? Good! I’ll bet you’re getting a bit hot, huh? Good because I want you to imagine that you’re neck deep in cool clear water. Ah, that’s better. Oops, you just dropped the watch you were holding over your head because you were too cheap to buy a waterproof model. Now I want you to dive down to the bottom and pick it up with your teeth. I’ll bet even five feet deep you could feel the pressure increase. The same law applies to air; the air at your feet is under more pressure (air will be denser too but the water is only pressurized) than at your head. It’s a small difference but not immeasurable (my altimeter/variometer will detect it: http://en.wikipedia.org/wiki/Variometer ).

To be continued….