I was flying last night, and I sent up one of my medium-sized quads (12" props, 910KV motors, 3 cell battery). The sky was a bit threatening with quite a few dark clouds, but it wasn't raining. The wind was blowing - mostly steadily at about 10MPH (I have a portable anemometer), but there were a few gusts that were a bit higher. I was standing in a park that had buildings and trees all around, so I thought they might be blocking the wind a bit, so I looked to see if the treetops were waving. They did a bit, all seemed OK.
The quad I was flying is no "racer", that is, it isn't high performance. But it is a really good stable platform for a GoPro. It flies slow and steady and I have the camera gimbal well adjusted. I programmed in a route that took it about a mile away. I figured that was as far as I could claim to have "Line Of Sight". After all, Line Of Sight can theoretically mean you can still see it with binoculars (which I carry).
So, I took off and flipped the switch to AUTO. It climbed to my usual 100 meters and took off to the east. Telemetry was telling me that it was going very fast for that quad - over 30 MPH (22 MPH is normal), so I figured it had a good tailwind. It went to the furthest waypoint and started heading back. Then it stopped! At first I thought that it had crashed (I couldn't see it in the sky), but telemetry was telling me that it was still at 100 Meters and the artificial horizon on my laptop told me it was angled at 35 degrees "forward" (35 degrees is the maximum tilt angle I have programmed into that one). Just then, I felt the wind come up strongly. I realized that the quad was heading into a 22 MPH headwind and couldn't make any progress! For 45 seconds or so, the telemetry told me that it was basically hovering. I have a failsafe set such that if the battery gets too low, it will simply land. But I didn't want it have to retrieve it in someone's back yard, I wanted it to come back to me.
Finally, the wind died down a bit and the quad slowly (excruciatingly slowly) made its way back "home" and landed. It had taken only 3 minutes to get to the furthest waypoint, but 11 minutes to make it back.
Just before it landed, I got the message "WARNING! Battery at 20%".
So, if you fly, make certain that your craft can fly a lot faster than any wind you think it might encounter. And always remember that the wind usually blows harder above the treetops than it does on the ground.