In addition to what has been said about wind speed,cross-winds,gusts......I notice some days have much more turbulence than others. I flew yesterday and I believe the winds were probably gusty from around 6 to 12mph. But there was some crazy turbulence. I'd be flying along straight and level,when suddenly the plane would go to knife-edge with no input from me. I've flown in these conditions before,but it certainly keeps you on your toes. And several of my landings were rough. Even dipped a wingtip into the asphalt with my low-wing at one point(doing touch-n-goes). So there are many variables and sometimes you get to the field and find the conditions more difficult than expected. Flying in gusty conditions is good practice,but can be risky if you're trying to keep your planes pristine. Generally speaking though,I find it most fun to fly when the wind is below 15mph and straight down the runway. In a cross-wind,even 10mph can be a challenge on landings.