There was a time when descending from an aircraft with a parachute was regarded as extremely hazardous, to be undertaken only in cases of dire necessity or as a stunt performed at flying circuses or air displays. But, in recent decades, parachuting is a widely popular sport, and one crucially dependent on the weather.
Typically, a jump takes place from an aircraft about 6,000 feet above the ground, and there are three stages to the fall. In the initial phase, lasting around 10 seconds, the parachutist experiences "throw forward" from the motion of the aeroplane. During this period the speed of descent gradually increases to terminal velocity - the speed at which the tendency to accelerate due to gravity is balanced by opposing resistance caused by motion through the air; here, terminal velocity is typically in the region of 125 m.p.h., or 180 feet per second.
Then comes free fall - the drop with the parachute closed at terminal velocity, during which horizontal displacement depends on the speed and direction of the wind. Unless the wind at this stage of the descent is greater than about 30 m.p.h., drift can be controlled by appropriate movements of the body. Free-falling through rain or hail, on the other hand, can be painful to any parts of the body that remain exposed.
The final stage begins 1,500 to 2,000 feet above the ground when the parachute opens, and the rate of descent decreases to about 12 m.p.h., or 18 feet per second. Light rain reduces the speed of descent, because a wet canopy allows less air to pass through it. Heavy rain, however, may mean a rapid descent because it is sometimes associated with a down draught. Conversely, on days when thermal convection is strong, descent may be delayed if the parachute encounters a region of rising air. Very warm air, on the other hand, is less dense and offers less resistance during the fall, and therefore allows for a very quick descent.
Wind has a very significant impact on the path earthwards during the time when the parachute is open. And at ground level, experience has shown that the incidence of injuries on landing increases rapidly when the surface wind exceeds 15 m.p.h. Consequently, this figure is often taken as being the strongest surface wind consistent with a sensible and safe parachute jump - although an experienced parachutist can make a safe landing in winds of 20 m.p.h. or even higher. Gustiness of the wind, however, makes judgment of the time and point of impact very difficult, and so increases the likelihood of injury on landing.