Snow Kiting on the back side of a steep sloping high Col in Chamonix one year, it occurred to me just how much in depth meteorological knowledge we really need, to truly appreciate the potential dangers we could be putting ourselves in during our power kite activities. As a paragliding instructor, I had to learn to understand the behaviour of wind and convey this information to students to reduce the likelihood of people flying in hazardous places. Given that we are flying airfoils and now taking them to hilly or mountain environments, some of this knowledge would benefit powerkiters too.
A good starting point is to try and imagine that the wind behaves like water, that is to say it flows over and round obstacles in its way and tries to take the easiest path available. Several effects, which I will explain in depth, such as wind gradient, venturi effect, dynamic lift and turbulent rotors, greatly affect the air in which we are flying our kites. In addition to these effects, changing weather patterns on the large scale, such as gust fronts (when giant cumulus clouds or thunderheads are approaching), as well as localised effects such as thermals or dust devils, can also have huge impact on the behaviour of air surrounding us.
Rotors & Turbulence
Many of you will have noticed that when you are flying a powerkite on a beach in an onshore wind, the wind is much smoother and cleaner than when you are flying in an off shore wind. This is quite simply because air flowing over the ocean is much less affected by ground friction and obstacles, than air which has travelled overland and been disturbed by hills, buildings, trees parked cars etc. Any object in the path of air will disturb and deflect its movement, and depending on the size and shape of the object and the strength of the wind, this will cause the laminar air flow to break up and create swirling eddies of turbulent air, called rotors (see beach huts diag 1 AND SP-KSK96.JPG ). We can feel these rotors downwind of the obstacles as mild to severe gustiness and sharp changes in wind direction. Many factors influence how far away the turbulent area of air can extend, light wind flowing over smooth obstacles – experiment with a wind sock to build up an idea of this for yourself, as obviously, it is beneficial for us to try and avoid these areas of gustiness. Many kiters have experienced these effects and the results on their kite flying and sometimes report inexplicable surges of power or complete collapses as winds switch up to 180 degrees in a heartbeat. Not so inexplicable if they have been riding a mountain board or buggy on the downwind side of buildings or trees, or on the lee side of a sharp cliff or ridge!
Wind Gradient
This is a relatively easy concept to grasp. As air moves over the surface of our planet, it is affected by friction with whatever it comes into contact with, and its movement is slowed. Consequently, air above the surface is slowed by air beneath it, but to a slightly lesser degree. So, quite simply put, the higher we go the higher the wind speed we feel. We should all notice this and can often be surprised by the difference between the air we feel on our face and how much power and wind speed we feel when we launch a kite 25m above our heads. This is wind gradient and we can, therefore, expect stronger winds at the top of a hill than we feel at the bottom of it.
Venturi Effect
Another effect of air which you may have already encountered, but not really understood is the venturi effect. Imagine running a hose pipe and then squeezing your thumb over the end of it, blocking some of the outlet area. The same amount of water is now travelling through a smaller gap so due to laws of physics, the water will accelerate. Now imagine you are riding your buggy along the beach, and an onshore breeze is blowing across the beach and then up and over a line of cliffs downwind. A gap in the cliffs would provide an easier escape route for our air, so lots of air will try and escape through this gap (rather than travelling up the cliffs), accelerating as it does so. You will also feel the effects of venturi whilst mountain boarding up a hill or snow kiting up a mountain. As you climb you will be feeling the wind strength increase due to wind gradient, and as you reach the top it will further increase due to venturi, caused by the large volume of air trying to pass over the hill or mountain, which is actually being squeezed from above by atmospheric pressure.
Dynamic Lift
Whilst hills and obstacles in the path of wind can create downwind rotors and turbulence if they are not smooth enough to allow air to flow over the top or round the sides, given a large enough surface area and a wind flowing ninety degrees on to it, they can also create dynamic lift. We have all watched sea gulls soaring effortlessly along the harbour wall or high above a windward facing cliff band. Since they are not flapping, there has to be a force keeping them up. This force is lift, and is caused by currents of air which have been forced to move beyond the horizontal axis by some hill or stucture in their path. If we have a row of terraced houses creating a barrier at the back of our beach, in a sea breeze when the air is flowing towards these houses, it is forced to rise over them in a vertical manner. This causes an area of lifting air, called a lift band, which can extend a considerable distance upwind and an even greater distance skywards. One of the principal methods of flight exploited by soaring birds, it is also one of the greater hazards for budding power kite flyers. Many kiters have been caught out attempting jumps and found themselves boosting ever higher, landing a long way from where they intended. It is easy to not have recognised the strength of the lift created by obstacles downwind, after all, air is invisible. It was the Col in Chamonix which inspired me to write this feature, since we were on a gradually steepening slope in a wind strong enough to create an incredible lift band Ozone manufacturer Robbie Whittall took air at one point and boosted high off the ground (see photo), fortunately as a former world champion paraglider pilot, he doesn’t need lessons on where the lift is weakest and easily landed again. A lesser pilot may have flown to Switzerland that day.
As kite technology improves and kite manufacturers are developing refinements in their products, kites are becoming wings and their controllers pilots. If we are going to be pilots, then better to understand the elements we are flying in. There are many books and web sites where you can educate yourself beyond the reaches of these few words, so please do so and may your winds be fair and skies blue.
Article provided thanks to Ozone.