With dozens of hot air balloons taking part in a cross- Channel world record attempt today, we look at the impact weather can have on balloonists.
Here at the Met Office we provide forecasts for a range of different aviation customers, all of whom are affected by the weather in different ways. The weather that affects a large commercial airline is very different to the weather affecting hot air balloonists, who have their own precise requirements. Ideal conditions for flying include dry weather, with good visibility, no low cloud, light winds and not too much turbulence. So let us have a look in more detail at how different weather parameters affect hot air balloons…
The wind speed is critical in deciding whether a balloon flight can take place. Surface wind speeds of more than 10 knots mean that inflating the balloon in the first place is extremely difficult. The wind speed above the ground (at 1000ft or 2000ft) must also not be too strong because this could lead to a high speed landing in which the balloon is swept into obstacles or is tipped over and dragged along the ground.
Extremely light winds can also be problematic, as being able to steer a balloon relies on the change in wind speed and direction with height. There have been occasions when balloons have become stuck over urban areas and forced to land in peoples gardens or in the middle of a street or have hit buildings or trees.
Wind direction is important in planning the start and end points for a flight, for making sure that the balloon doesn’t end up over water, in controlled airspace near an airport, or where there is nowhere suitable to land. Near to the coasts on a fine day in spring or summer sea breezes can develop which whilst the wind at the surface is coming onshore, it can prove hazardous to balloonists if they are caught up in the offshore part of the circulation and they are swept out over the sea.
Visibility and cloud
Hot air balloon pilots must be able to see the ground at all times in order to navigate when they are flying, so this means that cloud bases below about 2000ft will stop flying as will the visibility decreasing below about 3000m during misty, hazy or foggy conditions.
Balloonists do not like to fly in wet weather for a number of reasons. Rain falling onto a balloon in flight evaporates from the surface of the balloon causing the air inside to cool down. This means a pilot will need to use the burners more (using more fuel) to maintain the same amount of lift, and controlling the balloon becomes more difficult. In addition to this, no ground crew wants to pack away a soggy balloon.
Heavy showers and thunderstorms are a big hazard to balloonists because as well as rain they generate strong and gusty winds, strong updraughts which could lift a balloon thousands of feet in a few minutes, and could also contain hail and lightning.
Have you ever wondered why hot air balloon flights take place soon after dawn, on in the couple of hours before dusk? This is because thermal activity is less at these times which allows for a smoother and safer flight. Thermals are rising columns or blobs of warm air that develop through the day, caused by the sun heating the ground, and which become more intense as the air temperature rises. They make controlling a hot air balloon difficult, and mean that a balloon cannot fly even though all other conditions may be perfect.
We would like to wish all the balloonists taking part in today’s world record attempt from Dover to Calais every success!
Did you know?
The Met Office provides area and site specific ballooning forecasts across the UK on its General Aviation Website, which is free to use. More information on how to sign up can be found here: http://www.metoffice.gov.uk/aviation/ga
Karen Shorey- Aviation Product Manager