T’is the Season

When the days get shorter, the air is cold,
Thermals die down: circling gets old;
When the North Pole turns dark, the jet stream south shifts,
The wind picks up: down the mountain it swifts;
When rotor clouds and lennies appear,
It means only one thing: wave season is here.

With my first wave season in Boulder upon me, I spent some time studying what I could find about the local conditions at my new soaring site in Boulder, Colorado.

  • Boulder Topography

Boulder lies directly at the foot of the Northern Front Range of the Rocky Mountains: a ~100 mile long mountain range laid out in N-S direction extending roughly from the Wyoming border in the North to Mt Evans in the South. Boulder is about in the middle and just East of the range, approx. 20 miles from Mt Arapahoe in the Indian Peaks Wilderness.

The local soaring club, the Soaring Society of Boulder, has negotiated a good-sized wave window, which allows for flights in Class A airspace above 18,000 feet, after “opening” the window with air traffic control in Denver. John Seaborn created an airspace file in Tim Newport-Peace format that you can download here to install on your flight computer (it’ll work with XCSoar). If you have an Oudie, you need a file in OpenAir format.  I could not fine one, so I created one myself using the coordinates below.  You can download it here. If you need instructions on how to install it, I found the following page from Williams Soaring to be most helpful (it obviously deals with a different wave soaring area but the installation methodology is the same.)

Arapahoe Peaks Wave Window
Cross-section of the Arapahoe Peaks Wave Area

After an initial steep ~2000 ft drop on the lee side of the Continental Divide (which triggers the wave in strong westerly winds if the stability profile of the air is favorable), the terrain slopes more gradually towards the plains. The gradient is steep enough that on fair weather days it is normally possible to reach the airport with any of the higher performance planes. That is not true in wave, however, as sink rates of 2,000 ft per minute or more can put the glider on the ground very quickly, even when flying with a tailwind. The terrain over the foothills is also basically unlandable. It is therefore paramount to stay high when flying over the hills.

Wave flights are obviously not restricted to the wave area – outside the window they just have to remain below 18,000 feet. A summary of the local procedures is here.

  • Recognizing Wave Conditions in Boulder

The air in Colorado is usually quite dry throughout the wave season.  (Summer monsoon in July and August is really the only time when it can be reasonably humid.) Nevertheless, it is fairly common for rotor clouds and/or lenticular clouds to mark the wave.

The following pictures illustrate some typical cloud formations in wave conditions at Boulder.

Rotor clouds over the foothills marking wave lift above.
A lenticular cloud and small rotor clouds underneath mark wave over Boulder.
Rotor clouds (some visible, some not), and a large lenticular cloud mark wave lift north and west of Boulder.

The following time-lapse video illustrates the formation of wave clouds over Boulder even better.

  • Flying in Wave

Flying in wave is very different from and considerably more challenging than flying in fair weather thermals (at least initially).  Locating lift (and sink) is completely different. There is (possibly severe) rotor turbulence to contend with (on tow, during flight, in the landing pattern); climb and sink rates can be extreme (and invisible); flight at high altitudes is associated with different physiological risks than one normally encounters (e.g. hypoxia, hypothermia, hyperventilation) as well as other elevated dangers (flutter risk at high airspeeds: TAS>IAS, flying above and in front of clouds), etc. Flight techniques and tactics differ, orientation can be more challenging, and there are additional rules and regulations to follow (e.g. opening and staying within the wave window, use of oxygen masks, etc.).  All those risks can be mitigated by understanding and being prepared for them. In short, it’s worthwhile to put in the time to study in advance to avoid getting caught by surprise and being confronted with an overwhelming situation and not knowing what to do.

I have added a lot of useful information in a special section about wave soaring that’s a good place to start.

Rigging and De-rigging

Nice view of Pearl Street and Boulder Canyon behind. The Continental Divide is on the horizon.

Over the last two days I received my checkout in our club’s Discus CS.  It’s a nice plane with strong performance characteristics and easy to fly.  I am particularly impressed with the quick roll rate: you can go from a 45 degree bank angle in one direction to a 45 degree bank angle in the other direction in just about three seconds – very responsive! I also like how quick the plane picks up speed and how efficiently it reconverts it into height when pulling up into a thermal.  I look forward to flying it for longer distances when the weather permits.

The foothills just West of town with the snow-covered peaks along the Divide in the background. The cumuli above the foothills looked promising but there was absolutely no lift there.

The only real downside is that the plane is always stored disassembled in its trailer. That means I always have to have someone help me rig it before a flight and derig it afterwards. I was at first a bit intimidated by the 85-point rigging checklist but once I had done it a couple of times I realized that it actually goes pretty fast. It reminded me a bit of the process involved in rigging a sailboat – like the Hobie Cat that I sailed as a kid. The automatic hook-ups of the control surfaces are a big plus so there really isn’t all that much that can go wrong. Obviously, it’s still well worth double checking everything – after all, unlike with a sailboat where mistakes are usually pretty benign, the pilot’s life is on the line when you mess up the rigging of a plane…

Circling above town. Boulder’s main commercial area is just below – the iconic Flatirons are right behind town. A cloud layer put the entire town into shade, putting an end to the little bit of instability that had kept me in the air.

The weather hasn’t been all that great for soaring. We’ve had about 10 cold days with clouds and rain,and the ground is still pretty wet (very unusual for Colorado). Temps were back around 20 degrees Celsius today but the air was stable and we stayed well short of the projected trigger temperature of 25 degrees.  There were a few weak thermals above the city of Boulder and the nearby plains but no lift at all over the foothills despite some decent looking cumuli. The cumuli had tempted me into a high tow to about 12,000 feet but 20 minutes later I was already back over town at about 8,000 feet.  There I stayed for more than an hour until I was done flying in circles that didn’t really lead anywhere.

Just before heading back to the airport after about one hour and forty minutes in the air. The runway of Boulder Municipal is just behind the little heart-shaped lake.

The air was fairly clear so I still got some nice views of the Continental Divide, glistening white in its first fresh snow cover of the season.  The trees around Boulder have also started to take on nice color.  My other satisfaction was watching other gliders take off and land while I was able to hang on. Credit to the Discus, which thermals very nicely at a relatively low speed. I’m still hoping that we’ll get a few good thermal days before it’s winter wave or nothing.