Threading through clouds, often with shaky turbulence and occasional thunder, 71-year-old pilot Gary Walker burns the flares on his plane's wings, releasing chemicals as he flies.

"It can be very rough as you get close to some of those clouds," he admits.

Walker, CEO of Texas-based aviation company SOAR, carries out an increasingly popular — and in some cases, controversial — effort to chemically impregnate clouds to increase rainfall.