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Hot air balloons are quite obviously carried along with the wind, so how can they be navigated? Choosing the correct launch point relative to the desired landing point would seem to play a major part of this, but logic states that it is not always possible to choose the best possible launch and landing points.

Can a hot air balloon be "steered" by increasing or decreasing altitude, hence selecting just which wind it will be subject to? Does the direction of the wind vary with altitude, and if so, how much? How do balloon pilots know how the wind will vary at altitude if that is the case?

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    $\begingroup$ Welcome to aviation.SE! $\endgroup$ – Pondlife Jul 22 '18 at 0:55
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    $\begingroup$ @Pondlife, thanks. I've been lurking here for years though. $\endgroup$ – Monty Wild Jul 22 '18 at 1:07
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    $\begingroup$ Okay, given that wind direction does appear to change with altitude, how can a pilot know/predict what that change will be without using online resources? I.e. what is happening in the atmosphere to cause this change in wind direction? $\endgroup$ – Monty Wild Jul 22 '18 at 1:36
  • $\begingroup$ Not much unless the launch test balloons and observe to see what they do. Most of the time online info is better... which is why they use it. $\endgroup$ – Pugz Jul 23 '18 at 16:04
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They use the fact that the wind veers when going up, that is, changes direction clockwise, and backs, changes direction counterclockwise, when going down. This is largely (there are a lot of other factors but this is the one we are interested in) due to the tendency of air in the atmosphere to spiral away from high pressure toward low pressure (spiraling because of Coriolis effect; what makes draining water swirl - clockwise away from high pressure in the north, counterclockwise in the south).

Away from surface influence, the air spirals nearly perpendicular to the "slope" of the high pressure (think of it as a mound of air that wants to flow toward and fill in adjacent low spots but Coriolis effect makes it swirl as it does so). Closer to the surface, friction effects inhibit some of the Coriolis effect make air flow a bit more directly "downhill" toward the Low you might say, the more the lower you get.

For a high pressure area in the northern hemisphere where there is clockwise circulation as air flows away from the centre, this means the wind is more directly away from the center of the high at the surface than when higher up, where Coriolis effect is stronger and it "swirls" more. As a result the wind direction will change clockwise as you go up. And vise versa going down. In the southern hemisphere where the circulation patterns are reversed, it'll be the opposite; veering going down and backing going up.

This gives the balloonist some control over ground track by choosing a specific altitude that gives the desired track. Obviously, the amount of veering and backing being fairly small between the surface and 5000 ft, maybe 10-30 degrees, means the balloonist still has to pick a launch site that is upwind of destination. A launch site that is directly upwind based on the wind direction at, say, 2000 ft will give some left and right directional control by going higher to change track to the right, and lower to go left. The balloonist also has to account for wind backing during the descent to landing and allow for that in the "cruise" portion of the flight to get the best position for the final approach. Beyond that, it's control the altitude with the burner and hope for the best.

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    $\begingroup$ Please add some words about Coriolis force and friction close to the ground to give a credible explanation why winds change with altitude as they do. $\endgroup$ – Peter Kämpf Jul 22 '18 at 20:19
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    $\begingroup$ Well that's a whole new question isn't it. $\endgroup$ – John K Jul 22 '18 at 20:26
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    $\begingroup$ Why? That is the physical reason for the changing wind directions, and as your answer stands, it just claims how they change without presenting proof. I thought that would be a nice addition. $\endgroup$ – Peter Kämpf Jul 22 '18 at 20:28
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    $\begingroup$ Is the direction change with altitude hemisphere-dependent? If so, perhaps you should specify whether you're answering for North or South half of the world? (Your profile has no location, so I can't even guess). $\endgroup$ – Toby Speight Jul 23 '18 at 7:43
  • $\begingroup$ Ok well I gave it a shot, trying to avoid too much esoteric weather jargon. $\endgroup$ – John K Jul 23 '18 at 13:33
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That is exactly how balloons “steer” - they pick an altitude where the winds aloft are going the way the pilot wants to - assuming winds at an available altitude exist where the wind is what the pilot wants.

Many balloonists use Ryan Carlton’s wind tool found here to get a good idea of local winds: http://ryancarlton.com

NOAA has a website called RUC soundings that presents actual wind and temperature aloft data from balloon launch sites and then interpolates for locations in between: https://rucsoundings.noaa.gov/

While the pilot can plan a flight and do amazingly well to get near to where they wanted to go, at the end of the day, a balloon, as an unpowered aircraft, is at the mercy of the winds.

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Balloonists use spit and little balloons to track the variations of wind direction with altitude.

Before launch, the balloonist will release a small helium balloon and note which direction it drifts as it ascends.

After launch, the balloonist will work up a goodly gob of spit and discharge it overboard from time to time, noting which way it drifts as it descends.

Such measures are limited in accuracy and in the presence of a strong wind, the balloonist has not much choice about where to go. The ground crew in the chase truck follows the flight and knowing the flight's intended duration, will find a convenient location along the balloon's track and park there- thereby defining the balloon's "destination".

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    $\begingroup$ Shaving foam or whipped cream is used by the less saliva-oriented pilot. $\endgroup$ – Sanchises Jul 22 '18 at 6:20
  • $\begingroup$ That seems like a practical way to do it! $\endgroup$ – niels nielsen Jul 22 '18 at 6:29
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    $\begingroup$ @Sanchises Whipped cream has the disadvantage that your passengers might eat your navigation system. $\endgroup$ – David Richerby Jul 22 '18 at 10:48
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    $\begingroup$ @DavidRicherby: The counterpoint is the disadvantage to the ground crew using the spit method. $\endgroup$ – dotancohen Jul 22 '18 at 14:34
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    $\begingroup$ @dotancohen So shaving foam seems the safest. Most passengers will either be well-enough groomed that they shaved before boarding, or scruffy enough that they won't feel the need to shave during the flight. $\endgroup$ – David Richerby Jul 22 '18 at 17:00

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