The phrase "to reach the ground" means something comes down or falls all the way to the Earth's surface.
Sentences with «to reach the ground»
There, they would reflect some of the sun's energy, preventing it from reaching the ground. (sciencenewsforstudents.org)
I still have the (rather good) report, which has a hand drawn graph of the spectrum of sunlight reaching the ground, with the visible band clearly marked. (realclimate.org)
Thanks to its weak gravity, if you stepped off one of its nine - mile - high cliffs, you wouldn't reach the ground until half an hour later. (bookpage.com)