Wind for the most part. The outside enviroment makes it much harder to keep fog at ground level from my personal experience so more extreme cooling I found was needed.Texmaster, why do you say it would only work well inside? I have always (the past 3 years) shot my fog machine through a tube like that filled with either dry ice, or just regular ice. It has always worked well for me. This is the first year that I will make one out of a storage bin and a dryer lint hose. Haven't built it yet so I really don't know if it will be any different.