Less than most people imagine. In fact, you can take off with no wind at all.<\/p>\n
The range we take off in goes from 0 to 20 km\/h<\/strong>, always with the wind blowing straight at us. That figure explains almost everything worth knowing about the subject.<\/p>\n If there is no wind, the air still needs to flow over the wing somehow. And the way to achieve that is simple, by moving forward.<\/p>\n The less wind there is, the more push is needed at take-off. The more headwind, the less effort it takes. In good conditions a few steps are enough.<\/p>\n This answers the most common worry of anyone flying for the first time. No, there is no need to sprint down the slope. On most take-offs you walk, or walk briskly, alongside your pilot.<\/p>\n Above those 20 km\/h, nobody flies.<\/p>\n It may sound categorical, but it is exactly what should happen. If the day is not right, the date changes. Never the other way round.<\/p>\n That is why, when the day is being arranged, the first thing checked is the forecast. A postponed flight is always better than a forced one.<\/p>\n The wind used for flying is almost always meteorological slope wind<\/strong>. Air arrives from the sea, meets the hillside and rises.<\/p>\n Added to that, every now and then, come the thermal cycles<\/strong>. The sun heats the ground, bubbles of rising air form and for a few moments the wind picks up. Then the meteorological wind takes over again.<\/p>\n Pilots read those cycles and choose the moment to launch. It is one of those things learned over the years that cannot be seen from the outside.<\/p>\n Having the right strength is not enough, because direction matters just as much as force.<\/p>\n Take-off only happens with a headwind<\/strong>, within a window of roughly 45 degrees<\/strong>. Outside that margin nobody launches, however gentle the wind may be.<\/p>\n With crosswind or tailwind there is no flying. It is a safety matter and admits no exceptions, because the wing needs air flowing head-on to inflate and hold properly.<\/p>\n That is why flights happen at several sites facing different directions. If the wind direction does not work at one launch site on a given day, it may well work at another.<\/p>\n Here is a detail almost nobody mentions, and it explains why the Iza\u00f1a launch site is different.<\/p>\n At 2,200 metres above sea level<\/strong> the air is thinner, with fewer molecules in the same volume. And since what holds the wing up is the mass of air passing through it, not just its speed, the maths change.<\/p>\n In practice this means that up there take-off is possible with slightly more wind<\/strong>, around 25 km\/h, than at sea level. The anemometer reads higher, but the effect on the wing is equivalent.<\/p>\n That is why wind strength is never looked at on its own, because it matters at what altitude it is being measured.<\/p>\nWith little wind, you generate it on take-off<\/h2>\n
What if there is too much wind?<\/h2>\n
Where the wind comes from<\/h2>\n
The wind has to come head-on<\/h2>\n
Mount Teide is a special case, because of air density<\/h2>\n
So which days are flying days?<\/h2>\n