Common Wind Shear Patterns
Observations for the Bay Area coastal areas
Looking North at a Shear that moved in over San Francisco.
Photo Credit to Geoff Rutledge – Full Gallery
A wind shear happens when two air masses intersect and overlap each other. The air masses can be either moving in different directions or at different velocities. Among the most violent versions of a wind shear is the micro burst that occurs preceding a Thunder Shower. These are extremely violent downward blasts of air that hit the earth and radiate outward causing very high winds and hazards to pilots of any craft. Other wind shears happen in a horizontal fashion. The term shear defines the fact that one air mass is shearing off of the other. This happens because of the directional or velocity difference. Faster moving aircraft like planes and jets, will transition through the two air masses in an instant and this can be extremely hazardous. With the speed of a paraglider, we actually sometimes fly in the zone where the two air masses meet and mix.
In some locations, when the shear is of a horizontal nature, one air mass rises above the other. Sometimes the result of such an overlap can create soarable conditions much like the dynamic lift related to ridge soaring. One year, several Hang Gliding pilots got as high as 4,500 ft after getting in a wind shear and heading north of Fort Funston. The Class B airspace above Mussel Rock begins at 1,500 ft. then jumps up to 2,100 ft beginning near the Canyon of Westlake and to the North. Since the Jets from SFO depart right above us, any pilot that gets up in a shear above Mussel Rock could become a bug on a Jet’s window should they go into the Class B airspace. Even a near miss could jeopardize our beloved flying site. If you learn anything from this article, it should be that you need to stay below 1,500 ft at Mussel Rock if you get into a shear.
This article is not being written to encourage Paragliding Pilots to try to fly in wind shears. There are many days when an approaching Wind Shear is visible and pilots can avoid the hazards by seeing such before it arrives. Knowing the symptoms that identify a shear can help you to avoid putting yourself in harms way. There are some very advanced pilots that have had great flights in wind shears, but for each successful venture into a shear there are as many stories of pilots fearing for their life and safety.
I have flown in wind shears since about 1986 in Hang Gliders and occasionally in Paragliders since the early 90s. I have learned a quite a bit from each experience but mostly have learned that predicting the good from the bad ones is very difficult. Shears are less threatening to Hang Gliders than Paragliders because of their greater flying speeds. Occasionally, shears can happen with winds that are manageable by very experienced paraglider pilots, but only with knowledge of the nature and hazards of shears should this be attempted at all.
The coastal wind shears that we see near Mussel Rock and Fort Funston can often be predicted as they approach, but not always. When visible, there are specific conditions that can be observed and watched as a shear approaches. The Northern California shears in this area are most common in the Spring and Fall, but can happen any time of year. Mostly, they involve a northwest wind or west / northwest wind overlapping a southwest or west / southwest wind. I have seen some, however that were southwest over northwest.
Pacifica in Background, Shear Cloud shows the curve of the NW wind as it overlaps the W or SW. When the whitecaps approach, even on a blue shear day, you will see a curve like this.
Photo Credit to Geoff Rutledge – Full Gallery
Some shears seem to have a solid overlap of the two air masses. When you get above the mixing zone, the air smooths out. In others, it seems that the mixing layer does not end and the air can be choppy to the point of creating collapses to the paraglider.
When the shear has cloud form (shears other than blue shears), the cloud forms behind the overlap area. The cloud is like a lenticular cloud that one sees in waves. On blue days there is not a solid cloud, but sometimes you can see tiny wisps of clouds form and then disintegrate in the mixing zone. Occasionally myself and other pilots flying in a shear have soared slightly above this cloud. Getting too far back in relation to this cloud would be very dangerous, as once you get behing the overlap, laminar and even descnding and choppy air are often there. The cloud can be used to demark the shear location and XC flights can be done. Dave Ellers set the local site distance record using such a cloud (not documented, but it occurred) for Mussel Rock by flying to Montara in a wind shear. A few Hang Gliders have made it a bit to Half Moon Bay.
View from above the Cloud Form, noting where the overlap is.
Photo Credit to Geoff Rutledge – Full Gallery
Shear Cloud – likely moving slowly back toward SE,
but sometimes they just remain stationary…
Another shear cloud above Daly City.
The little wisps at bottom left are all you see in a Blue Shear.
Imagine only the faint wisps and that would be a Blue Shear.
The following are all common observations when a wind shear is present or approaching:
- A horseshoe shaped band of whitecaps is approaching (the band of whitecaps bows toward the coast like the shape of the curved part of a horseshoe). This can be well defined and easy to see. Whitecaps can be harder to see in some lighting, especially when there is cloud cover.
- Above the coast may be coastal low clouds and fog, but the horseshoe indent moves in with the whitecaps. When this occurs, this is the easiest way to recognize and see the shear coming. On other days, the whitecaps approach, but the sky remains blue. This is commonly called a “Blue Shear” by the locals.
- The winds will often get very switchy and gusty as a shear approaches. Occasionally, it can remain soarable until just before the shear arrives, but usually the wind will die and start to gust in varying directions as it gets close. The wind direction can become extremely switchy and there can be a lot of gustiness. Sometimes the wind will almost die for a while. The only rule that is consistent with regards to wind shears is that they are not predictable.
- When a shear arrives at the coast, pilots trying to get into them work their way up through the mixing layer to get to the upper level. The air in the mixing layers is often described as “hole-E” (like Swiss Cheese) meaning the air feels like there are chunks taken out of the wind. It is like flying along and suddenly the wind dies and lift disappears, then you hit some lift like a small thermal bubble. The wind drop-offs feel like holes in the air.
- Above the mixing/chop layer, the lift can increase quickly and the velocity too. In some shears, it seems like there is no layer above the chop. In others, the air becomes smooth and strong above the chopped section where the air masses overlap.
- In some shears, the wind seems to continually increase as you move up into it. In some, the wind will level off with altitude gains. If you are ever in a shear, have your guard up at all times.
The following two days occurred in the Spring of 2005 back to back. I wrote up a description of this and posted it to the local Bay Area Paragliding Group. I believe it helps to emphasize both the hazards of flying in shears and how unpredictable they are. Day 1 was a shear that was super docile and some of the pilots did not even realize what was occurring. The next day, a stronger shear approached and the gustiness and switching winds got all of the pilots out of the air. After it set in all the way, I decided to test it to see how strong it was and if it might have turned out to be like Day 1’s version. The wind on Day 2 was standard at the bottom layer, but once I got above the chop, it got stronger and stronger, faster than any shear I have ever been in.
Good Shear – Day 1
About 10 of us enjoyed one of the best Wind Shears ever at Mussel Rock. It did not move in with the normal obvious horseshoe wind lines and it was a Blue Shear (no cloud form behind it). Instead the winds built slightly and pilots entered it without much effort. Winds were about 18 or so (you could penetrate upwind at a slow pace at trim) in perfectly and glassy smooth lift that took us to 1000 ft or so way out in front (1/4 or 1/2 mile for some) of the Westlake Cliffs. I came down giddy after 45 minutes in this great shear with a perfectly clear blue sky and sharing the air with some great friends.
Nasty Shear – Day 2
Myself, Brad Smith and Nova Dasalla all saw the shear form and come in to usable level late in the day. Pilots had been flying earlier, but the air got choppy and switchy as the shear approached. All stopped flying, it was late in the day and most left other than us. We were excited, but also a little cautious about it because the white caps were bigger. I launched at Tomcat, flew to just below Lemmings and could not bridge across, so landed at Gusto Launch (the flat pad area below Lemmings). The air had been quite chopped on the flight across, but manageable for experienced pilots.
I launched at Lemmings and jumped across to the corner of Westlake and worked the lift portions of the Shear up the South end of the Westlake Cliffs. I had a radio on and was communicating with Nova and Brad at Tomcat. I talked Nova out of flying and decided to be the air testing technician (aka Wind Dummy). I was able to climb pretty quickly to about 3/4ths of the way up the cliff and the air was still choppy. I hit the biggest of the chop at about cliff-top, but then got into the shear layer. Up until this point, the winds were pretty manageable (less than 16 or so). Once I surfaced into the main shear layer, it jumped up to 18 or so, for a few seconds I was cautiously optimistic. The wind then began to get above 20. I was climbing very quickly and veered a little toward the west/southwest to penetrate and head a little to the south. It was increasing fast, so got on the speed bar around then, straight out in front of Walker. I got pretty far out, above the breakers and water’s edge. I was able to penetrate, just slightly, but as I was ascending, I could tell that it was getting stronger. I was about 300 ft over the top of Westlake, so about 900 MSL. This was less than 2 minutes after the launch at Lemmings!
I made the decision that this one was just too strong, so engaged speed bar fully and did big ears. I was now still able to penetrate, drifting south toward Tomcat but not going down. I was still going up slightly with full speed bar and big ears, my adrenaline glands became active. I also stopped penetrating and started to go back. This all happened unbelievably fast. Within 2.5 – 3 minutes from launching, I was in this situation. I had climbed incredibly quick and the winds and lift rose steadily as I had ascended. I thought about either B lining or a spiral to try to drop below the shear level. It took me less than 2 seconds to think this out and chose the spiral. I did 3 high banked corkscrew 360s rapidly and dropped as I drifted back over the mesa. It did not seem like I was dropping much, but it was in fact pretty significant (guesstimate of about 250 ft or so). At the same time, I drifted about 70 percent of the way to the cliffs at the south corner of the Westlake cliffs ( I had started in front of Tomcat and over the water’s edge (in winds like this you drift very quick). I could not continue to spiral as I would have gone back over the houses and back of the cliff, I was about 50 – 75 yards in front of the rear SE corner of the Westlake Cliffs. I exited the spiral and got on full speed bar with big ears. When I first checked, I could tell I was not penetrating with the big ears / full speed bar combo, then I notice that I was descending slowly. Within about 15 seconds, I descended about 50 more feet and began to penetrate forward. I flew forward to Tomcat and as quickly as I could get out of my harness, literally kissed the ground.
Lessons from this story:
I tell this story to raise awareness of the dangers of Wind Shears at Mussel Rock. I have been flying here for 18 years and this was the closest I have ever been to a blowback. I survived this, but I am so glad that I was the wind dummy on this day and had the right mind set to deal with it when it became critical.
Beware of shears until you have lots of coastal experience. This was one of the strongest and rapidly accelerating ones I have encountered and will be more cautious next time. Yes, I will fly in shears again, but will pick them a little more carefully in the future. If the white caps are as big as Day 2, I will likely leave it alone. They did not look that big to me, I was guessing 15 – 20. I guessed wrong and was able to survive this. On Day 1, there were white caps, but very few. It did not look like 18 or 20 on the water, but the winds were around there up in the shear. There were some new P2s flying in the shear on Day 1, most were aware of what was going on, a few were not fully aware. All of the P2 pilots did really well, using speed bars and big ears when it got stronger, and also staying way out in front of the cliffs. The two shears on consecutive days were entirely different in nature. I had really hoped Day 2 was going to be similar to the shear on Day 1, but it was way stronger than we anticipated when I decided to fly.
When a shear is approaching, if there are advanced pilots around that want to take a crack at it, watch and learn. If you are an advanced pilot and decide to try flying in a shear, know that the air will be very choppy in front of it. Know also that the wind can increase rapidly once you enter one. Look for the most experienced pilots around when a shear is present and begin to learn what is going on by asking lots of questions.