As we approach winter (a bit later than most DZ's around the country) new challenges arise for jumpers during the landing phase of their skydive. With winter weather patterns come higher winds and turbulence. For those that completed an AFF program during the Spring or Summer this is particularly hazardous. The keys to "survival" are knowledge, preparation (planning), and good judgment. First let's talk about the hazards we face.
High winds can be separated into two distinct categories, winds aloft and ground winds.
The winds aloft are posted daily and should be at most dropzones. Here is an example:
Now, not to get crazy technical on you, but it is important for you to know how to read this and plan for your skydive. Having a plan is very important and ignored too often. Look at it this way. When you get on the highway to go somewhere, you can get there with no directions, but it's much safer to avoid crossing six lanes of traffic at the last second to make an exit. If you know the exit is coming you can be in the right lane prepared to get off instead of placing yourself and others in a dangerous situation. For skydiving, the plan is know how many seconds you should allow after the group in front of you exits, approximately where you will exit over the ground, how far you may drift in freefall, where you will open, and where you will need to fly the canopy to land on target.
When you look at the chart above, the top line refers to altitude above mean sea level (MSL). Since we jump at 14,000-15,000 MSL we have to interpolate or average the two highest altitudes. The second line refers to the direction the winds are coming FROM. Very important to memorize direction coming FROM. Last is the winds speed in knots (Think nautical miles per hour. 1 knot is 1.15 statute Miles Per Hour, what we use in cars).
The high altitudes will tell you which way the plane will be flying on jump run and the approximate ground speed. To keep it super simple if an aircraft flies jump run at 80 knots and we have 40 knots of wind at 15,000 MSL, your approximate ground speed will be 40 knots. When the aircraft moves slowly over the ground skydivers get less separation as they jump out. Sometimes in winter we can see ZERO ground speed. For this reason we must be prepared for long exit intervals.
The 6,000 and 9,000 foot winds will show you how much freefall drift to expect and where it will push you. Winds above 25 knots have a noticable effect and more so on larger groups and tandems. This means you may exit over one side of the airport and open over the other.
The 3,000 foot wind forecast will show you where you need to hold to have an effective pattern and land on target. Note that winds above 20-25 knots will push you backwards even when facing into the wind.
There are three issues to be concerned about with ground winds. They are speed, gusts, and changes in direction. High wind speed is not necessarily an issue by itself. There are DZ's that jump in windy conditions year round. Wind gusts by themselves can be handled easily if you are prepared. Here is an example of a problem caused by gusts. At 30 feet while preparing to flare, the wind speed may drop off surging your forward and downward speed. In this case you have to recognize this condition immediately and flare more quickly and often flare harder than you expected. The opposite is true as well. You may be flaring and the wind gust will exaggerate your flare and picking you up off the ground. You must again react immediately just slightly letting off on your flare then continuing as needed until you touch down. Then comes the fun part. Learning to control a canopy in high winds while standing on the ground is an art. But the most effective way is to comletely let go of one toggle and pull the other one down steering the canopy into the ground. Then while continuing to hold the toggle all the way down, run over and step on any part of the canopy. A common mistake is to try to run to the canopy while it is still in partial flight, before the nose has hit the ground. You are then just in a rodeo with the parachute.
The next concern is variable wind direction. This is the more dangerous of the conditions and will lead us into the next section of hazards. Even when the winds are not high, a 60 to 90 degree shift in wind direction can create turbulence that will collapse a canopy. This is particularly dangerous during turns at low alitutde. If you must jump in these conditions the remedy is to make more shallow turns keeping the canopy fully pressurized as much as possible. This leads us into the next section.
Turbulence can come from variable wind direction but much of what you feel is from temperature differences over the ground, buildings, and asphault or from rotors as winds are disturbed by buildings and trees. Ground surfaces absord different amounts of energy from the sun. This energy is radiated as heat. Since the air temperature is cold in winter, the difference in temperature creates unstable rising air which affects your canopy. This can save you if you get long spot. Flying over the asphault can give you enough lift to get back the a safe landing area. However, as your canopy passes over the ground close to landing, the hot air can lift your canopy sometimes hundreds of feet. This can cause you to land long even though you planned for the winds correctly. The opposite can be true as well when you cross over from asphault to ground and your canopy begins to drop altitude quickly. Knowing what your pattern will be can help to plan for these potential hazards.
Rotors are simply columns of air that come in contact with a hill, trees, or buildings swirling in a circular pattern (think mini tornado). They can swirl in any direction, rolling like an ocean wave or horizontally like a sink drain. Also when wind gets squeezed between two buidlings it will accelerate into this swirl. Plan to land as far away as practical from object that may cause this kind of turbulence. They are known to collapse even high performance, highly pressurized canopies traveling very fast.
Most of the hazards described here can be avoided or handled with awareness and planning. However, there is no substitute for knowing the limitations of your experience level.
Some factors to consider:
- How many total jumps do you have?
- Have you jumped in high winds before?
- Are you very familiar with the canopy you are jumping in different weather conditions?
- Do you have experience jumping in every season?
- Have you recently downsized or changed your canopy type?
- Are you familiar with the DZ and landing area?