| by Trey Holladay, DZO |
April 12, 2005
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Summertime on the way! That glorious time we all look forward to. Long daylight hours, warm weather, great parties, dangerous thermals, less flare,...WHAT?
That's right, less flare under your canopy. We'll get to the thermals in a minute. Why less flare? I'm going to introduce you to a new term..."Density Altitude".
Density altitude is absolute altitude corrected for temperature, pressure, and humidity. Everybody got it now? No? OK.
Density altitude is the altitude your parachute acts like it's in when the conditions are not standard. For example, on a 95 degree day in Thomaston when the humidity is 90%, your parachute thinks it's flying at 3684 feet during your swoop. That translates to 1/5th less of the air molecules to use for lift. This means that your canopy is faster due to less air drag but also less effective to control.
Why is this important? Becuase you can have a 10 MPH wind and still get no lift. You must anticipate this and make your canopy control inputs sooner and more exagerated for the same results as normal. Or fall down and go boom!
Thermals are a parachute's other summer arch enemy.
The sun heats the ground during summer so much that it begins to radiate that heat off the ground. Everyone has seen this on a hot pavement, makes it look blurry from a distance. These are called thermals.
Well, the ground does not heat up or discharge that heat evenly. This is what make thermals so nasty for canopies and their pilots. While in the pattern thermals can lift you as much as 500 feet up or sometimes more. This can really mess up your timing and approach. While on final approach you can hit a thermal that pushes you 10-15 feet up, then as you move forward and past it the canopy surges forward. You should be prepared for this and always thinking a few steps ahead. What if you need more area to land because a thermal kept you in the air longer than you expected?
Thermals can also create heavy turbulence which is their real danger. As hot air rises then cools it forms a rotor that can collapse your canopy while in a turn. One might reconsider high performance manuevers in these hot conditions.
Keep these ideas in mind on the next hot day and always remember, no jump is so important it that you should sacrifice safety.
Be safe, have fun |
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