Notes for new hang glider pilots--judging the wind direction while on the ground

August 25 2005 edition
Steve Seibel
steve at aeroexperiments.org
www.aeroexperiments.org

 

Sometimes pilots are fooled by the effects of parallax when judging the wind direction at launch by looking at streamers. More than once I've seen a hang glider pilot look at a wind flag and say something like "the wind seems to be crossing from the left", and a bit later look at a different flag and say "now the wind seems to be coming straight in", when in fact both of the flags were streaming in exactly the same direction the whole time. 

 

Remember that parallel lines converge in the distance.  If you are looking north and the wind is from the north, a wind streamer located ahead of you and to your right will stream slightly to the right and appear at first glance to indicate a west component in the wind, and a wind streamer located ahead of you and to your left will stream slightly to the left and appear at first glance to indicate an east component in the wind.  Lines drawn straight through all the flags would all be parallel, and would converge in the far north. 

 

To really prove this point, do this exercise sometime when the wind is fairly constant in direction: have one pilot look at a wind flag (that is not located directly in front of him) and guess the exact point, in relation to a specific landmark on the horizon, from which the wind is blowing.  Have another pilot look at a different flag in a different location and guess the exact point, in relation to a specific landmark on the horizon, from which the wind is blowing.  Then have both pilots walk to positions where they can sight straight down the lengths of their respective flags and find the true point on the horizon from which the wind is actually blowing.  The final answer will be the nearly same for both flags (assuming that the direction of the airflow isn't being greatly affected by the contours of the hill, trees, etc) but the odds are that at least one pilot's initial guess for the wind direction will differ greatly from the true wind direction. 

 

Because of this parallax effect, it's good idea to have wind flags on both sides of the launch run.  When the wind is blowing straight in, from the perspective of a pilot at the top of the launch run, the flag on the left will appear to stream slightly to the left and the flag on the right will appear to stream slightly to the right.  If the flag on the left appears to be blowing "straight back" from the perspective of a pilot at the top of the launch run, and the flag on the right appears to stream slightly toward the right, then both flags are consistent: they are both showing that the wind is slightly crossing from the left.  And so on and so forth.  It is much harder for a pilot to accurately judge the wind direction when the wind flags are all located on one side of the centerline of the launch run, even if they are not displaced very far off to the side.   

 

Because of this parallax effect, if a hang glider pilot ties a yaw string or telltale to each of his lower front wires, he'll be able to judge the wind direction on launch much more accurately than if he only ties a yaw string or telltale to one of the lower front wires. 

 

In a case where a flag or streamer is far away and hard to see clearly, especially if the day is overcast and the flag is not casting a shadow on the ground, and especially if the observer is not familiar with the shape and length of the flag, if the flag streams toward the observers left, the only thing an observer can say with absolute certainty is that the wind is coming from somewhere to the right of a line drawn between the observer and the flag.  This leaves 180 degrees of ambiguity in the observer's estimate of the wind direction!  It's a good idea to keep this in mind when looking at wind cues such as distant smoke plumes.  Unless the plume is blowing straight back toward you, it may not be giving you quite as much information as you might think at first glance. 

 

The technique of noting the exact point on the far horizon from which the wind seems to be blowing, in relation to a specific mountain or road or other feature, is a good way to keep track of changes in wind direction over time as a pilot is walking around the launch area and attending to various tasks.  If a pilot is judging the wind direction only in relation to objects in the immediate foreground, even a small change in his position can throw his judgement completely off and make it seem like the wind direction has changed.  The technique of picking the exact point on the far horizon from which the wind seems to be blowing is even more useful when a pilot is checking the wind direction at several different launches and trying to build up a mental picture of the overall airflow in the region.

 

People seem to vary greatly in their ability to sense the direction of the wind without visual aids.  Usually, by turning my head back and forth and noticing the strength of the wind on my eyes and cheeks, and listening to the relative sound of the wind in my ears, I can accurately determine the wind direction (at least right where I'm standing!), when other people seem to need to toss up blades of grass or kick up dust.  With a helmet on I lose most of these perceptions, and have to join in with the dust-kickers.

 

At many flying sites in the US, the exact direction of at least one of the four cardinal points of the compass (north, south, east, and west) is often completely obvious, because most roads and fields are laid out on a "square" grid system.  Since parallel lines converge in the distance, the four cardinal points of the compass will be marked by converging roads or by the converging lines of the borders of fields.  Even a few roads in the foreground, or the edges of a few square fields, are sufficient to give a good reference in this manner: in your mind's eye, extend these lines to the horizon, and you'll see where the cardinal points of the compass are.  (If you are somewhere like Kansas, you won't have to do any imagining, because the grid system of roads and fields really will extend all the way to the horizon in all directions!) 

 

Pilots should always strive both to be aware of the limitations of the visual cues that the world is giving them, and to open their eyes to as many of these cues as possible!