How crucial is the weather to the balloon fiesta?

The iconic festival is returning to the city for its 46th annual event

The Bristol Balloon Fiesta relies heavily on weather forecasts, but the people who run the event say they have learned to look on the bright side even if predictions are gloomy.

Some years the directors have despaired at seemingly negative forecasts, only to wake up to perfect flying weather.

"Years ago, I used to worry about it, but worrying doesn't change the forecast," said chairman of the flying committee, Pete Dalby.

Multiple elements are factored into the decision on whether to launch or not, with wind speeds being one of the key ones.

The first mass ascent is scheduled to take place at 06:00 BST on Friday

Before each mass ascent at the fiesta a canister of coloured smoke is loaded into a tube, and thousands of pairs of eyes wait to see what colour comes out the top.

In a scene reminiscent of how a new Pope is announced, the colour of the smoke lets the crowd know if flying is going ahead. Green is good and red is bad.

Behind the scenes, the flying committee closely scrutinise a ballooning forecast provided by the Met Office.

They are not just looking at wind speed on the ground, but also at 2,000 ft (609m) in the air.

"We want light winds on the surface, good visibility and nice still weather, the opposite to what glider pilots like," Mr Bailey said.

"At 2,000 ft, if we have a fast wind in the wrong direction then we could end up in the Bristol Channel," Mr Dalby said.

"You get all sorts of forecasts and we never predict ourselves any more.

"In 2015, we thought we wouldn't fly at all, and we flew every slot in the end," he added.

This year, more than 120 hot air balloons are set to take flight

In recent years, fiesta directors have noticed how our changing climate also makes predictions harder.

"We do see more extremes. We see extreme temperatures and a change in the weather that happens more rapidly than when I started," said Mr Bailey.

"We used to say that we could guarantee that we'd have south westerly or westerly winds every year.

"Now you can't say that any more."

What is this year's forecast?

Ian Fergusson, BBC West's weather forecaster, said it might be difficult to tell whether the balloons will take flight until the morning of each day.

"Confidence in the localised weather fortunes throughout Friday, Saturday and Sunday is currently not very high," he said.

"Exactly how winds at the surface and up aloft will dictate prospects for balloon flights will be determined and assessed by fiesta organisers much nearer the times of each planned mass ascent."