Notes for new hang glider pilots--judging the wind direction while on the ground
August 25 2005 edition
steve at 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
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
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!