It's the best event in Bristol's calendar and attracts guests from across the country - but if the weather isn't right, then the Bristol Balloon Fiesta's main attractions simply cannot take to the skies.



How does the Fiesta decide whether it's safe to fly or not?

Ahead of the Fiesta, the festival's flight director Clive Bailey and flying committee closely monitor detailed forecasts which are designed specifically for hot air balloon flights. They are particularly interested in surface wind speeds, wind speeds at height, low level spot winds, the rainfall radar and visibility.


The optimum conditions needed for a safe hot air balloon flight are light winds, good visibility and no rain. However, this can be harder to predict than it sounds due to a number of complex conditions.

Mr Bailey said: "The main weather factor for hot air ballooning is the wind. Hot air balloons rely on the wind being light for the first few hours after sunrise and to decrease before sunset, which is why the mass ascents are always planned for 6am and 6pm daily - this is when the air is at its most stable.

"Ideally for a safe and enjoyable flight balloons require the surface wind to be less than 10 knots and ideally less than 20 knots at 2,000 feet."

Another important factor in deciding whether it's safe to fly or not is visibility - but this is the hardest element to predict. A forecast for a misty morning is often based on expecting rain to fall overnight, which either may not happen at all or more rain than is predicted can fall.

Mr Bailey added: "If there has recently been rain in the air, then balloonists need to look out for mist, fog and low cloud, which tends to be more of an issue in the early mornings than later in the early evening.

"Visibility is very difficult to predict so the most up-to-date information is rarely available until just a few hours before a scheduled morning flight. A forecast for a misty morning can be based on the expectation of overnight rain, which may never come, or its effect can vary greatly depending on the time and amount of rainfall."

Why can flights be cancelled at short notice?

The Bristol Balloon Fiesta is returning to Ashton Court in August.

It's no secret that British weather is notoriously unpredictable - and this makes predicting whether a flight will take place or not incredibly hard work.

As the forecast is prone to changing so frequently, a decision on whether a mass ascent will happen or not is rarely confirmed more than 30 minutes in advance - simply because knowing exactly what will happen is often impossible any further in advance.

Mr Bailey said: "For ideal ballooning weather you are looking for light winds, good visibility and no rain however with so many factors to take into consideration this can be a lot more complicated than it sounds.

"As with the rest of the UK, in advance of the event the team are reliant on weather forecasts and with the incredibly changeable weather we have seen during the past few weeks this can make it very difficult to tell if the balloons will fly in advance."

The Bristol International Balloon Fiesta nightglow.

However, it's often possible to rearrange flights at very short notice due to the changeability of the weather - which is why ascents will sometimes be delayed for a short time. This is because a brief 'weather window' may appear during a spell of bad weather where conditions are briefly favourable.

Mr Bailey added: "Forecasts can change quite dramatically in just twelve hours and sometimes if balloon teams wait as little as one hour to delay a flight, a weather window can appear.

"This is why a final decision on a mass ascent is rarely made more than 30 minutes in advance."