As has already been mentioned the mass ascents happen six times over the period of the fiesta. In the best mass ascents over 100 balloons can take off in under half an hour from a very small patch of land, a feat which earned Bristol international balloon fiesta a place in the record books. This is all the more impressive when it is considered that launching land is at such a premium that there is only enough room to inflate less than half the balloons at a time!
The amount of mass ascents, and their quality is definately weather dependent. For it to be safe to use hot air balloons the wind conditions have to be relatively calm, and there has to be no precipitation with good visibility. Some balloons are better at coping with more abnormal conditions than others, with generally the special shapes being the ones which require the most precise weather conditions to fly. However, I cannot remember a single year when no balloons took off, and so even in the British summertime there is at least one day with decent weather generally!
With the right conditions it is possible that the balloons can continue to fly for a very long period of time and for relatively long distances. However many balloons don't attempt to fly a long distance and instead try to catch the 'hare'. This is a balloon that takes off ahead of the rest, and the pilot from the 'hounds' which can land his balloon nearest the 'hare' wins a prize. This can lead to a great concentration of balloons landing in very close proximity to one another, which is great fun for the locals particularly children as the pilots are always very friendly.
For more information,please visit http://www.bossgoo.com