The Spark manual recommends not flying in wind speeds exceeding 10 m/s (22 MPH).
While the Spark could no doubt hover in the air at wind speeds higher than that, there will be a point where the Spark will start to make negative progress when flying into a headwind. So, you'll want to be careful when flying on windy days (even more so if flying into a headwind on the way back to the home point). Keep it close (and low) if you must fly on those days and you will likely be just fine.
I recently took this photo of a 120' cell phone tower.
I think the flag is 20' x 30'.
The wind was gusting up to 20 mph and the Spark handled it well.
It was neat watching the Spark hover in the parking lot and the camera gimball do it's thing while fighting the wind and the image on my screen was steady.