There are two basic types of air flow, laminar and turbulent. Despite the fancy names, the ideas are very simple. Picture the smoke rising off the tip of a cigarette. Near the hot end the smoke flows smoothly upward. At a certain point, it starts curling and swirling. That’s turbulent flow.
Why should that little bit of fluid physics matter to skydivers? Because turbulent flow is one of the major contributors to certain high risk circumstances. Turbulence can turn your body in air unexpectedly and it can cause your canopy to misbehave.
That can lead to fatalities. More than one skydiver has had his main collapse because of turbulence when he was too low for the reserve to inflate fully before impact. Even when safely on the ground turbulent air flow can knock you off your feet before you detach the harness. Laminar flow that’s strong enough can too, but you are then being pushed in one direction. Turbulence tends to twist you, making you more unstable and more likely to fall.
In fact, turbulence is more likely closer to the ground in most cases. Because of boulders and other surface features, the air gets stirred up like water moving over rocks. But unlike water, turbulent air is invisible.
The possibility of turbulence is one reason skydivers always check the weather carefully before going up. Some conditions are more likely to result in significant turbulence than others. ‘Significant’ is an important qualifier because there is always a little bit of turbulent flow around.
Wind speed is one major factor. As the wind blows faster it’s much more likely to become turbulent, other things being equal. That’s why jet plane wings always experience turbulent flow in flight. As the air flows past the wing, it gets stirred up at the trailing edge. A skydiver’s body acts somewhat the same (though only approximately).
Other factors exist, too.
Temperature matters. Hot air is more likely to be turbulent than cool air, but what really matters is the temperature difference. When hot air meets cooler air, the chances for turbulence are increased. Pressure differences between one air mass and another contribute to the possibility of significant turbulence.
As a skydiver approaches the ground the problem of turbulence is increased. One reason is the obvious reduced time and distance for reacting to a problem. But the conditions themselves change as well. Any buildings near the landing point, for example, can generate turbulence like the rocks in the river.
Turbulence around buildings is a familiar phenomenon to anyone who has lived in a big city. But tall buildings also create turbulence at intervals above them, sometimes as much as ten times their height.
The size of this so-called ‘turbulent zone’ varies depending on lots of factors. But the mere fact that it can occur at heights of ten times the building means that a 20 foot building can cause turbulence 200 feet up. That’s far too low to deploy a reserve if your main canopy collapses. Yet, a 200 foot fall to any ground hard enough to support a building is extremely likely to be fatal.
Skydiving is a very safe activity, relatively speaking. The odds of a fatality are about 1 in 100,000. Knowing something about how, when and where turbulence is created can help you maximize your odds of a safe landing.