There are a couple effects from rain: absorption ("db per km") and scattering (bigger constant in front of your r^2 term), neither are good for FSO. Fog (and since this is airborne, clouds) is the real killer. Imagine taking your nice .1 degree wide laser beam and pointing it into one of those frosted street lamp bulbs so that it now points in every direction, then add to that the attenuation from being absorbed by the droplets. Now, in FB's case, they're doing something pretty smart to mitigate the water problem. Using RF for the ground-air links avoids most of the problems with scattering and absorbtion depending on droplet size and the band you're transmitting in. Hard to beat the bandwidth of FSO, so that's a great option for above the clouds. 60,000 feet should be above most clouds - anything that tall and you'll be wanting to get out of the way in any case.
The LCRD stuff that NASA is doing is indeed impressive. I think that their pointing problem is a few orders of magnitude harder than what FB is dealing with though - if anything we'll see technology transfer in the other direction.
The LCRD stuff that NASA is doing is indeed impressive. I think that their pointing problem is a few orders of magnitude harder than what FB is dealing with though - if anything we'll see technology transfer in the other direction.