Opening Winds vs. Surface Winds: The Two-Arrow Logic
Look at the windsock. Now imagine the wind 3,000 feet above it. They're rarely the same wind, and on a busy day they're not even close — different speed, different direction, sometimes by a lot. That gap is the whole reason the spotting tool shows two arrows instead of one: an opening-altitude wind and a surface wind. They answer two different questions, and confusing them is the classic way to climb out over a perfect spot and still land off.
Why the two winds disagree
Near the ground, friction drags the wind down — terrain, buildings, trees all slow it and bend its direction. Climb a few thousand feet out of that friction layer and the wind speeds up and shifts clockwise (in the northern hemisphere, it veers). So a surface report of 240° at 8 knots can easily be 270° at 20 up where your canopy opens. Same air mass, two very different vectors, stacked on top of each other.
That's not a quirk of one DZ. It's the boundary layer doing what it always does, and it means the surface windsock is a poor proxy for what your canopy is about to ride.
What each arrow is for
The opening-altitude arrow is your drift. From the moment the canopy's open, you're a sail being pushed across the ground by the wind at altitude — and you stay in that stronger, shifted upper wind for most of your descent. That arrow tells you which way, and how hard, you'll be carried while you fly down. It's the input that decides your spot: where you leave the aircraft and where you open so the drift works for you, carrying you toward the landing area instead of away from it.
The surface arrow is your landing. Down in the friction layer for the last stretch of the flight, that's the wind you set your pattern and final approach against — the one that decides your landing direction and how much ground you cover on final. It's what the windsock and tetrahedron are showing the people already on the ground.
Plan with only the surface arrow and you've solved the landing while ignoring the drift. On a light-surface, windy-aloft day that's exactly how a textbook spot turns into a long walk: the canopy gets shoved downwind far harder than the sock suggested, and you're flying back from somewhere you didn't plan to be.
Reading it on the tool
Drag the target to your intended landing area and the tool works the two winds against your canopy's glide. The glide rings show how far you can actually reach from a given point: the Safe ring holds a conservative floor (opening you a 1,500 ft cushion to set up a normal pattern), and the Max ring is the aggressive edge of your range down to an 1,100 ft floor. Inside the Safe ring you've got comfortable options. Out past Max, you're not making it back without help from the wind you may or may not have.
Set the opening arrow to your winds-aloft data, set the surface arrow to what's on the ground, and the picture you get is the honest one: where to open so the drift carries you into the Safe ring, then which way to land once you're down in the surface wind.
Where the numbers come from
The surface arrow you can read straight off the METAR (see reading METAR for skydivers) or the windsock. The opening arrow needs winds-aloft data — forecast winds for the layers you're descending through, not the surface report. Feed the tool good numbers for both and it does the geometry; feed it a surface wind for both arrows and it'll cheerfully draw you a plan that ignores half the flight.
Two winds. Two arrows. One's your drift, one's your landing — and the day they're most different is the day it matters most to keep them straight.