Twenty seconds spent analyzing the windsock is always a good investment, especially when it identifies one of those seldom-seen wind conditions that can greatly affect your flight. The characteristics in question almost always include a degree of "sharpness," meaning the changes are hard-edged and seem to happen instantaneously. They include:
Wind snaps behind the wing. It looks like a normal gusty crosswind but will periodically and suddenly change direction and snap from in front of the wing to behind it. The directional change is so sudden it amounts to a wind shear. If it occurs during the flare, the slow airplane could drop out of the air. Lots of power is the cure.
Very sharp gusts. Gusts are always quick to act, but some border on violent. The windsock clearly shows these as hard, jerky motions. If these catch you on short final, you'll have your hands full--be ready for them.
Sharp gusts from different directions. Some winds reserve their energy for changes in direction: They'll be blow one direction fairly steadily, then suddenly spit out a hard gust from a different direction.
Don't underestimate the effect of tailwinds--no matter how small--on either takeoff or landing. Not only do they cause an airplane to use more runway in both situations, but on takeoff they'll also flatten the climb angle, which may hamper your aircraft's ability to clear obstacles. In addition, a quartering tailwind makes a tailwheel airplane difficult to control. In general, try to avoid a tailwind on takeoff or landing, and if flying a tailwheel airplane, you avoid them all together--unless you're really comfortable with that particular airplane. If landing runway length is even remotely a factor, as little as two knots on the tail can take away your margin, so landing on, or near, the numbers becomes critical. The best remedy is to avoid takeoffs or landings in tailwinds of any kind.
When we think of topography and its effect on airplanes, we often think in terms of mountains, but many other types of topography can also cause problems. A runway doesn't have to be in the mountains to have a sudden dropoff at the end, if it terminates at a lake, the edge of a small ridge, or a dropoff to a road. If the runway is as little as five to 10 feet higher than the surrounding ground, wind can create a rather exciting downward curl right at the end. Similarly, if there is a parallel line of buildings fairly close to the runway, crosswinds can create eddies over them--and the spaces between them can act as venturis, which increase the velocity of the wind and become nozzles that squirt a narrow stream of faster-moving air across the runway.
In the vegetation-covered East, downbursts out of thunderstorms aren't seen as easily as in the West where the ground is often bare dirt or sand. In those conditions, it's not unusual to see an upside-down mushroom of dust where the wind is roaring down out of the thunderstorm and hitting the ground vertically before reversing. This creates the classic gust front, and it is an indication that the storm in question is a serious one. You should give it a very, very wide berth; delay your takeoff or landing.
Neither rain nor snow is going to hurt you or the airplane as long as their density is within reason, and visibility through them is only slightly compromised. Unfortunately, those factors are hard to evaluate until you're well into the precipitation. There is no definitive rule of thumb to use in judging when to continue and when to turn back, but federal aviation regulation visibility minimums apply. If, when skirting the rain or snow, you can't see well into it--don't poke your nose in. Be absolutely prepared to do a quick 180 the instant visibility starts downhill, and make the turn long before the visibility sinks down to the legal minimums.
One of the scariest sights from an aircraft windshield is to look at moisture running up the windshield and then realize it's no longer moving. It's freezing, and a translucent cataract is in the process of covering your windshield. There is a mistaken assumption that you have to be in clouds before you'll get ice--that definitely is not the case. In fact, those ragged, low ceilings common in the East during early winter are famous for putting a layer of the ugly stuff on airplanes in VFR weather. The water vapor that's hanging out of the clouds in the form of a wispy veil is often supercooled, and all it needs is a cold surface, like a wing or windshield, passing through to convince it to turn to ice. It is generally thought that visible moisture is necessary to generate ice, but the definition of visible is flexible because there is moisture right outside of those wispy veils that would probably be visible in the right lighting conditions, but under the clouds can't be seen. Stay away from ragged ceilings, and if the temperatures and ceilings are working against you, find someplace to sit it out.
This is probably caused by local topography, possibly a dropoff at the end of the runway, and may mean that the winds just off the end of the runway are setting a trap for you. They are whipping back and forth in a way that makes it impossible to even guess their direction--if they did, you could plan and compensate for them.
If it's possible, a steeper approach that will put the airplane higher and a little farther down the runway may solve the problem. At least, if you're higher it will give you more margin for error. If forced to fly a lower approach that'll put you in the turbulence, do so knowing you'll probably need a little extra speed, and be ready to add power to go around.
It could be said that altitude, temperature, runway length, and load are the four basic aviation food groups, because they determine takeoff and landing performance. Because each is such an important factor, understand their individual effects. Even more important, they aren't mutually exclusive: the different factors are intertwined and represent some very complex relationships.
Most pilot operating handbooks include charts that let you calculate the effect altitude, temperature, and load have on takeoff and landing performance, but an innate understanding and respect for these factors is at least as important as the POH itself, because no one consults the POH every time they get ready to take off or land. You have to have the fundamentals firmly in mind so that you don't inadvertently step into a situation in which one of these factors compromises your safety. Just know that higher altitudes, temperature, and loads take more runway. When they are all present at the same time, what looks like a reasonably long runway isn't.
Ground fog is an interesting and surprisingly contradictory entity. It is nothing more than visible water vapor. But it causes problems because it blinds those who penetrate it. One of the factors that make it so dangerous is that it is born of an innocuous and difficult-to-sense relationship--the outside air temperature drops to the level necessary for moisture to condense out of the air, and suddenly there's a low, thin cloud.
Fog that forms and moves as a cloud can be avoided, but sometimes local temperatures will hit the magic number and a huge area will be enveloped in fog in a matter of seconds. This can be preceded by small patches of fog developing in low, usually cooler, spots. When that happens, get suspicious and get on the ground as soon as possible.
These are just a few of the dozens of environmental factors that bear watching when we fly. The most important habit we can develop is to think beyond a cursory look at the weather and analyze what might happen, given the available weather data. We want to know what kinds of challenges it's going to present. Remember the old aviation saying that it's better to be on the ground wishing we were in the air, than in the air wishing we were on the ground. When in doubt, don't go.
Budd Davisson is an aviation writer/photographer and magazine editor who has written approximately 2,200 articles and has flown more than 300 different types of aircraft. A CFI since 1967, he teaches about 30 hours a month in his Pitts S-2A Special. Visit his Web site.
Links to additional resources about the topics discussed in this article are available at AOPA Flight Training Online.