Overview

This page first discusses microbursts, from a glider pilots perspective, and then a discussion of windshear follows.  Windshear is a part of every microburst.  There is more to a microburst than the sudden downdraft!
 

Microbursts

A microburst is a strong downdraft. It is what we glider pilots would call really heavy sink. A microburst seldom lasts more than 15 minutes, is typically less than one mile wide and can be as strong as 6,000 feet per minute (FPM). To put it into perspective, lift at 1,000 FPM is actually 10 kts of wind in the vertical axis. Thus 6,000 FPM is the same as 60 kts in the vertical axis, and it is DOWNWARD! As it is nearing the ground, it begins to change to a horizontal flow 1,000 to 3,000 feet above the surface, creating a wind near the surface as high as 45 kts. And if you are unfortunate enough to be approaching the center of the microburst, the wind will change from a 45 kt headwind to a 45 kt tailwind as you pass through the center of this descending air, resulting in a wind shear of up to 90 kts! Rephrasing the situation - you are in heavy sink, experiencing a strong headwind, and the headwind changes to a strong tailwind, and during this transition from headwind to tailwind, the sink becomes even greater. It is miraculous that any one ever survives such an experience, but some do. The keys to survival are knowledge, awareness, and judgment.

Know that microbursts occur near developing thunderstorms, or strong cu nim clouds. Some visual cues to watch for include - abnormally strong lift, unusual rain patterns, virga, blowing dust on the surface, a localized heavy rain shaft, with light winds reported on the surface, high temperature and low dew point (a 30 degree Fahrenheit spread), and other unusual atmospheric phenomenon. Any one of these cues may indicate a microburst, and other cues may not be apparent. Once you become aware of the signs, use your good judgment, rather than pit you superior flying skills against the forces of mother nature. It is often better to orbit away from the field while the storm passes over the gliderport, rather than try to get back and land before it arrives. Only you can make that decision. In a recent issue of Soaring Magazine, George told us about just such an incident that he experienced in New Mexico in 1999. Reread that article now that you know more about microbursts.

In the late 1980's there was a glider with a highly experienced pilot at the controls who was on downwind at Truckee. He was pushed downward right into the pine trees from normal traffic altitude - that may have been a microburst.

In the late 1990's, Nevin wrote about his experience in the PASCO WestWind.  He was just north of Sierraville, and he was forced over 3,000 feet nearly straight down - leaving hardly enough time to do a single turn before he touched down in the middle of a field. Doug told me of a similar experience that day.

I have heard of numerous incidents at Minden of similar incidents. If you have a report along these lines, please share your story with our readers. We need to make people aware of the frequency and strength of microbursts.
 

What causes a microburst?

A microburst usually will not occur unless there is evidence of updrafts, because the microburst is a downdraft, and is associated with strong updrafts. If there is virga next to a cu nim cloud, it is occurring because the preceding convective action raised the air to the saturation level, that is, the parcel of air that was rising cooled as it was raised aloft, it could no longer hold the moisture that was in it, the moisture became visible in the form of a cloud, and the water droplets formed precipitation, and started to descend in the from of rain. If there is a wind aloft, the descending air will be adjacent to the rising air. When the air below the rain is dry, the water evaporates, and is now called virga rather than rain. When evaporation takes place, cooling takes place. The result....? The parcel of cool air, which was already descending, begins to descend even faster. Imagine fllying anywhere near when the downdraft hits! Here is a graphic showing the downdraft truning into a  strong surface wind. Thus, even if you are not directly below the microburst, you will encounter a headwind, or a tailwind as it spreads outward - depending on which way you are traveling.  That wind may be in excess of your current fllying speed.  Several microbursts in the mid-90's produced a wind force of nearly 100 knots, and the one near Washington D.C. was measured at over 200 kts.  Hopefully, there will not be an unaware glider pilot cruising in the location of a microburst.

A microburst may be dry - or wet. Those below virga are considered dry, and those in a rainshaft are considered a wet microburst. Depending on the winds aloft, a dry microburst could be in the clouds just above you and to the side of your flight path, especially if you are flying adjacent to a thunderstorm, and you will never even see it develop. Thus a microburst could seemingly appear out of the blue!
 

Windshear

Windshear is an unexpected change in the headwind/ tailwind component. The component may suddenly change because there is a change in the direction of the wind, or there could be an existing temperature inversion in the atmosphere, and the wind just above that inversion is vastly stronger or weaker that the wind just below it. Whatever the reason, a windshear is dangerous to glider pilots.

If you experience a windshear while landing at a field with other gliders, don't hesitate to share the knowledge, with a radio call, that a windshear exists. It is invisible and not detectable until you are in it, and then we are back to using our superior flying skills. After you stop the aircraft, announce on the radio - in a concise and understandable manner, what you experienced. Do not use positive or negative terminology. You do not need to offer flying lessons. Don't tell people how to handle it, just report it. State concisely what happened. Some examples-

• Abrupt loss of 20 knots at 200 feet, landing on runway 16
• Airspeed increase of 15 kts on short final
• Gradual loss of 30 knots from 400 feet to the surface

In 1996, I was just south of Woodland, and was heading south. I began to experience heavy sink, and began a run towards Yolo County Airport, less than five miles away. I was at 3,000 feet, with plenty of altitude, or so I thought. At 500 feet AGL I was still heading west, on an extended left base leg landing to the south, evaluating the hay fields between me and the runway, thinking that if I got any lower, I would land straight ahead in one of those fields. It didn't feel like a headwind, only like heavy sink. Even though I was flying at 60 kts. I was sinking like a rock. The nearest rain shafts were over Winters and Vacaville. I made a low base to final turn and rolled to a stop on the runway at mid-field. I felt lucky to have made it. I later evaluated the gps data log, and I saw that I did in fact have a wind that increased from about 10 to 30 kts of headwind on that extended base leg, and my sink rates were excessive from 3,000 right down to 300 ft. My ground speed was actually 30 while I was flying at 60 kts.

In 1997, I was flying with Key and Sebastian, returning to Williams after a nice flight in the Mendicinos. Five miles northwest of the field, I calculated I could make it to the runway at Williams. As I got lower the headwind increased. I put it into a muddy rice field on base leg just across the street from the gliderport. Curse the bad luck, I said. But luck had nothing to do with it. If I had paid closer attention to the reported surface winds at Williams, and compared them to what I had at 1500 feet, I would have been aware that I was going to be facing an increasing headwind type of windshear.

I remember flying at Air Sailing a few years ago while participating in Racing Camp. While flying on course the wind seemed like about 20 kts out of the west - pretty routine, but as I descend for landing, the wind became stronger, and there was turbulence at 1500 ft above the field. Wind in the pattern was probably 45 knots, based on the crab angle on downwind and base leg, and then at 50 feet above the surface it dropped off significantly. I landed ok because I was carrying lots of extra airspeed because I could see it was windy, but my decision to fly faster was reinforced by the report I heard on the radio that Al had broken the fuselage on his DG on landing just a few minutes earlier, due to that sudden loss of headwind.

At the PASCO Safety Seminars in mid-90's David gave an example of landing in a field surrounded by tall trees. It was adequate in size, but he asked the audience the question of where do you suppose he landed in the field, considering there was a 10 kt headwind. The answer was, that he touched down near mid-field, because as he descended below the tree line, the 10 kt headwind went to zero wind, which would have been a 10 kt loss of airspeed, but since he was flying his  airspeed precisely, he immediately lowered the nose, and maintained the airspeed, compensating for that loss of headwind. Now with a zero headwind, and again fllying at the correct approach speed, his ground speed was faster than normal, and he ended up landing at mid-field.

Increasing headwind. If you are on final glide, whether starting from 40 miles out or on short final at one quaterer mile out - and the headwind increases, your glide ratio is less, you will no longer be able to land in that same spot. You will be short of that spot. Unless...... you were carrying extra speed, and can now fly closer to your optimum glide speed, rather than too fast. .......Or, unless you had the spoilers deployed, and can now stow the spoilers, thereby increasing you glide ratio. Regarding flaps, you should normally never decrease your flap setting from the landing position to something less while close to the ground, because the aircraft will sink as you raise the flaps, and you will need to rotate the ship in order to control your airspeed. Landing flaps should not have been selected unless the landing was assured.

Decreasing Headwind. If on final glide, and the headwind decreases, your glide ratio is better. If you want to land in that same spot without doing any turns, you must increase your drag. Spoilers are the best way to increase drag, and landing flaps could be selected, but be sure the landing is assured. Slipping the aircraft is a second method. If you are far from the field, you may fly faster than your best glide speed, and your glide ratio will decrease. Flying excessively faster is not effective when at low altitude (below 1,000 ft) because the difference is glide ratios is less than significant in the short term.

Since most of us now carry GPS recorders on every flight, we now have the means to measure the severity of these incidents. If you have had similar windshear incidents, please share the details with us.

If you can find or have a graphic which depicts a windshear, please send it to me, and I will add it to this page. But don't send me any files that are over 100 kilobyte, as I don't even download them on my slow computer.  People don't want to wait for downloads while reading web pages - that's why all of my graphics are simple.