Words to look for in a forecast: gusty, breezy, windy, blustery, small craft warnings, high wind warnings
Wind is the most critical factor in safe ballooning; it effects every phase of a flight. More balloon flights are cancelled due to wind than for any other reason. Balloons fly best in light and stable winds of 7-10 Km's per hour. Maximum safe winds are 8-10 mph. Here are the reasons wind is such an issue:
During inflation the balloon is filled with cold air using a fan. The balloon fabric is just a giant sail, and winds approaching 10 mph make it almost impossible to fill the balloon. The wind will cave the side of the balloon in and the resulting sail effect places tremendous loads on both the fabric and the basket. These forces can be 3-10 tonnes depending on the size of the balloon. The balloon will roll around, sometimes violently. It is tied off to keep even a gentle breeze from causing it to drag downwind, but we have seen a gust cause the balloon to drag the trailer and van it was tied to across the grass! Pretty impressive to watch - not much fun!
Strong winds in flight can take the balloon farther than the pilot has room to fly. Remember that the winds aloft are generally stronger than the winds at the surface. Since a balloons flight path and the distance it will travel is dictated solely by the wind's speed and direction, this can be an issue if high winds carry the balloon into areas that are unsuitable for a landing. Such areas include: metropolitan areas, large expanses of forest, restricted airspace, and large bodies of water. All of these are factors in our immediate flying area.
Lastly, there is the landing. A balloon's speed across the ground will be the speed of the wind it is flying in. High wind speeds mean that the pilot needs a larger area to land in. A balloon relies on the friction of the basket dragging along the ground to come to a stop as balloons do not have brakes. In a high wind landing you are trying to stop 3-10 tonness, all moving at the speed of the wind, without brakes - the basket will skip, drag and bounce along the ground. It will eventually layover on its side while continuing to drag along the ground. Again, impressive just not much fun.
The winds on the surface are just one of our concerns. We have to think three dimensionally and consider what the wind is doing at altitude as well. This is perhaps the most confusing. There is not even a hint of a breeze on the ground , why aren't we flying? We look at winds at the surface (the wind you can feel) and the winds at 1 to 9,0000 metres. It tells us if we might encounter issues such as wind shear, turbulence, or strong surface winds later on. Even if there are no winds to speak of at the surface, the winds aloft may drive our decision not to fly. Winds aloft of 18-20 knots or 20 Km's per hour can be sufficient to stop us flying.
Words to look for in a forecast: foggy, hazy, misty
How far can we see?
The visibility must be 2 to 5 Kilometres, depending on where we are flying. If we don't have it, we can't fly!
Rain & Storms words to look for in a forecast: thunderstorms, rain, chance of showers or storms
The decision not to fly in rain or storms seems a simple one - of course we don't! What isn't so simple is why flights are cancelled when no storm or rain actually happens in the area. We must often make our decision based on a forecast. Despite the many advances in weather prediction, forecasting remains an imprecise science. We often refer to forecasts as "horoscopes with numbers."
Storms can be significant events to any type of aircraft, but a balloon is perhaps the most weather sensitive aircraft there is. An airplane can turn and run from a storm whereas a balloon is drawn into a storm. The winds will accelerate and head toward a building storm and flow out of a decaying storm. These gust fronts can occur 75 to 100 miles away from the actual storm and create winds that are dangerous to a balloon. Once again, it's the wind! If storms are forecast or there are storms within 100 miles we will not fly.
Since hot air balloons fly by changing the temperature inside the balloon with heat, it stands to reason that outside air temperature is going to affect balloon flights, and it does! When the air in the balloon is heated, it becomes hotter and thus less dense than the surrounding outside air. This hotter air is "lighter" and the balloon will float upward. The more heat, the higher up you go. A balloon will fly when its temperature is around 75 degrees Celsius above the outside air temperature (generally). So, the colder it is outside, the less heat it takes to fly and conversely, the hotter it is outside, the more heat it will take to fly.
The smaller the balloon, the less lift capacity it will have and the hotter it must be inside the balloon for it to fly.