Mention western soaring to regular readers of Soaring magazine, and conversation will probably shift to breathtaking climbs in lift exceeding 1000 fpm, to cloudbases exceeding legal limits. Other topics likely to be mentioned are waves, oxygen, perhaps thunderstorms, and wide open empty spaces. Sinkholes probably won't make the list. This is an introduction to Colorado's longest continuously active soaring site, a site some characterize as a sinkhole.

Inspection of a Colorado map reveals Boulder at the eastern base of the Rocky Mountains. Black Forest/Kelly Airpark lies about 80 miles SSE, with Owl Canyon 55 miles NNE. Geographically, both lie farther from the mountains than Boulder...and the difference is significant to each site's "typical conditions." Owl Canyon [1] and Black Forest [2] have well known, and deserved, reputations as excellent soaring sites; perhaps less well known, in the local-tow/lift senses neither is a "mountain site." Boulder, though the only truly mountain site of the 3, is perhaps better known for being a university town and all that implies, than as a soaring site. If "Red Zinger," "Mork and Mindy," and marriage licenses for horses ring any mental bells, then at some level you have an impression of Boulder. [3]

But as a soaring site, Boulder is a "poor relation" reputation to the "real soaring sites" in the state. No SSA sanctioned contests have been held here. Until Richard Hall, Jr., won the 15-Meter nationals a few years ago at Minden [4], truth was no 'name" pilots flew regularly from Boulder. Although the state absolute altitude record was set from Boulder in the late 1970's, [5] quite simply, even by many locals Boulder isn't generally considered a soaring mecca. Yet so far as I know, soaring activity has been continuous from the Boulder Municipal Airport since the 1950's. [6] Makes a person wonder...

SURPRISE!

Upon arriving in fall of 1976, I was somewhat nonplussed to hear from many Boulder regulars, that it was a sinkhole. The consensus seemed to be: high tows were necessary year 'round; the nearest good lift lay 15 miles east or west; thermals were weak to non-existent; I shouldn't waste my time at Boulder but should go to (the original) Black Forest or Owl Canyon; etc. It might have been only my interpretation of course, but I actually got the impression that Colorado's well-known summer soaring camps [7] were practically necessary for the Boulder soarer, in order s/he might obtain an annual exposure to good thermals!

It wasn't initially obvious why such gloom and doom was so common; physically, the airport appears to be ideally situated as a soaring site. Boulder's well-known Flatirons, a 3000 foot high, several mile long ridge with lots of sun-catching exposed rocks, are less than 5 miles from the airport; immediately behind them begin the Rockies. The Continental Divide is situated less than 25 miles due west, with the plains stretching east. Only the town itself separates the airport from the foothills. At first glance, the only thing Boulder seems to lack might be a decent west-facing ridge, for the prevailing winds-aloft here are westerly.

With site experience and hindsight, Boulder's soaring reputation proved understandable - if inaccurate - for it's truly a rare Boulder day when round thermals permitting a glider to be parked in a constant-bank turn exist. Equally rare is a randomly spaced thermal field permitting textbook MacCready-ring runs. Instead, the atmosphere above Boulder routinely presents the soaring pilot with the following contradictions: 1) soarable "stuff' is nearly always present along the Flatirons; 2) mountain thermals east of the continental divide are generally fractured, rough, and all too often, of middling lift; 3) the mountain air is consistently, frustratingly rough despite the dearth of well-behaved thermals; 4) lift above the plains near Boulder, while not typically classic in the western sense, is generally usable even if often inconsistent in strength and spacing; and 5) textbook thermals aren't typically found until east of the 14-mile distant interstate.

THE SINKHOLE EXPLAINED

One afternoon after getting too low once too often and scaring myself beyond my terrain clearance tolerance for the day, I retreated from the incessant unrewarding pounding being dished out by the mountainous atmosphere, to gaze disgustedly at the hills from the relative smoothness east of the airfield. Smoke drifting northeast from a forest fire near Gunnison (a town 130 miles southwest) was key to developing a mental model subsequent experience has shown to be usefully valid. I envisioned the Continental Divide in the Boulder region as an atmospheric breakwater to the prevailing westerlies. In the absence of solar heating, weather fronts, etc., westerly winds spill over the Divide and roll east toward the Mississippi. Lower level winds with an easterly component (quite common), climb the hills in a turbulent manner until meeting the westerlies aloft. Variations of these two states exist in any wind condition. The key is the mountains encourage turbulent mixing all the time below the level of the Divide. The sun's heating enters the picture by providing upward components to the wind-driven mixture. Often the combined upward components are so distorted and chopped up by wind that textbook thermalling flight techniques are a waste of time. Only over the plains, usually beginning within a few miles of the foothills, do the mountains' effects begin to diminish; at low and intermediate altitudes they're largely dissipated once east of the interstate.

This model matches the message of the doomsayers, for if normal soaring techniques are used around Boulder, the return is often a falling from the sky, particularly if the pilot insists on defining lift in classic western proportions. Without a reasonably accurate mental airmass model, Boulder can indeed be a sinkhole by anyone's standards. On the other hand, using flight techniques suitable for the area, I believe lack of soarable conditions around Boulder is even rarer than smooth, round thermals. While the prevailing winds - in conjunction with the mountains and Boulder's closeness to them - are the major influences on Boulder's soaring weather, other identifiable factors tend to depress convection in the vicinity of the airport. Because Boulder is literally in the immediate wind shadow of the Rockies, while Black Forest and Owl Canyon are not, the upper-level airmass over Boulder is usually mixing with the smoother plains airmass, and - generally following the terrain - is often subsiding as well. In addition, there's much irrigated farmland nearby, especially to the east and northeast. Lift over these areas is often quite weak, even by eastern standards. Moreover, during Colorado's summertime monsoon season, some of the higher mountains serve as excellent thunderstorm generators, which prevailing winds often blow right over Boulder. It isn't unusual to see storms trailing downwind from Mount Evans and Longs Peak, 30 miles SW and NW of Boulder respectively. [8]

ADDITIONAL SOARING ATTRACTIONS

About 5 miles east lurks DIA's Class B airspace. While lift is often good in the urban Denver area, this airspace monster is large enough in diameter that few gliders choose to overfly it, as it tops out at 12,000 feet msl. (Flying through or under it is strictly of theoretical interest to most glider pilots.) DIA's traffic must also be considered when soaring from Boulder because major descent and departure routes head west just north and south of town respectively. Another aspect of Boulder soaring which can be a burden to some is the airport itself. The city-owned, Unicom-equipped field is busy and somewhat cramped for space. In simultaneous use are 3 parallel 4400' runways; centerline spacing is a scant 50-70 feet. Power traffic (usually!) uses the southernmost runway, while gliders use the northern two strips; the center strip is unpaved. Rigging is done in a narrow 100'-wide strip between the northernmost runway and Independence Road just outside airport property. East patterns are left-hand, while those to the west are right-hand; for traffic separation reasons gliders and tugs always use a full crosswind pattern. The 3-leg power pattern is outside the glider pattern. Despite 2 separate midair collisions between powerplanes and gliders since the early 1970's, in my view most of the potential airspace conflicts are not with powerplanes, but with gliders. The field gets busiest on weekends when local lift quits and gliders more or less simultaneously fall from the sky. For cheap entertainment, watch everyone sort themselves out! Typically the north runway is used for takeoffs - as well as for landings if it's clear - while the middle runway is reserved for landings. Every effort is encouraged to clear both runways of gliders as rapidly as possible, but at times both will have planes on them with more in the pattern. Such cases are easily handled by landing long (or short, if safely possible); it looks worse than it sounds, but things typically work out safely in practice. Many potentially serious incidents I can recall witnessing have involved approaches to a clear runway; perhaps therein lies a lesson. In any event, Boulder's traffic density discourages arriving back at the field "needing the runway Right Now!" Rationalizing, it encourages pattern discipline, something glider pilots typically won't win any awards for from their power brethren.

THE OTHER SIDE OF THE COIN

So why do pilots continue to soar from Boulder, given it's a crowded, less than classically good soaring site? One obvious reason is simple convenience; how many soaring sites out west lie within 30 minutes of major population centers? [9] Perhaps equally compelling is the amazing diversity of lift to be found within short miles of Boulder at any time of year. Yes, winter and spring have waves, while spring, summer and fall have thermals, but such a casual overview woefully under-describes the atmospheric melting pot above and west of Boulder.

Waves are common throughout the entire year, and are typically accessible without epic tows. However, pilot preference varies widely; curmudgeons exhibit a certain reluctance to take high, long tows, preferring instead 2000' or 3000' tows, releasing at or east of the foothills. It's routinely possible to climb from local tow into the tertiary, and from there to penetrate into the primary. Boulder has a large, conveniently located, and well-used wave window. [10]

For much of the year convective lift is the norm around Boulder. [11] Often its form makes it difficult to recognize, especially for the beginner or the pilot looking for "textbook lift." Streeting is quite common with any wind aloft from the west, but may not be recognized due to lack of clouds. Also street directions can vary widely in non-obvious ways. My logbook shows many street-aided flights in all seasons, with the best and strongest ones occurring in spring and fall. While ascending Boulder's learning curve, streets initially proved the easiest means of crossing the Divide to the west; once across, you'll find more or less classic intermountain western soaring conditions...high cloudbases, strong lift, dry air, the best lift tending to follow spines of major mountain ranges.

Ridge lift is also common at Boulder, despite no classic ridges; it comes in several flavors. Located in an atmospheric backwater much of the time, in the absence of energetic ground-scrubbing westerlies, low-level winds are mostly gently from the east. If the easterly component extends higher than the Flatirons, useful ridge lift is found on the east face usable by most glider pilots of intermediate skills. It takes little sun to kick thermals loose from the ridge in this situation. With ridge-top winds from the west, a similar situation exists, except that terrain west of the ridgeline doesn't fall away much, winds tend to be stronger, and conditions rougher. Only experienced pilots are likely to use the ridge on such days.

Not well understood by most Boulder-based pilots is the convergence condition which exists whenever mid/upper-level westerlies meet the plains' airmass. Quite common, it generally creates a band of lift paralleling the divide, sufficiently long to be used for non-circling "wimp 500K" flights...though no one has yet officially done it. Interestingly, some of the best soaring occurs in late evening. If one makes a serious attempt to climb to cloudbase later in the afternoon, the reward can typically be an hour-long glide from 18,000' to a dusk landing; ground-based lift quits long before upper atmosphere convection quits. If you fail to connect to the upper-level lift, another common phenomenon is a general evening upwelling of air all along the foothills, sometimes extending several miles east. This gentle, subsidence-induced caress can often be soared until dusk. I suspect hawks use it for pure pleasure, as I've often seen them soaring in pairs, amazingly tolerant of the sailplane during these times. The source of this lift is, I imagine, cool air sliding down from the mountains displacing the warmer air over the plains. Another evening phenomenon is gentle ridge lift on the east (shady) side of the foothills, often reaching several hundred feet above the ridge. My longest ridge flights at Boulder have utilized this lift; surprisingly to some local pilots, these have exceeded 20 miles in distance. Considering the foothills in this span are discontinuous, rounded, and low, the flights have proven satisfying beyond their geographic insignificance.

Boulder airport's mountain proximity is an advantage in yet another way. For all practical purposes gliders exceeding 30:1 glide angles will outglide the average slope of the mountains; getting-home from the west isn't the issue it might be were the airport farther from the hills. Given the dearth of suitable landing sites in the mountains, this is no small consideration! Happily, should they be necessary, good landing sites, both private airstrips and cultivated fields, exist year 'round along the other approaches to the airport.

ATMOSPHERE OF ANOTHER KIND

No soaring site description is complete without mention of the people who make it what it is. Organizationally speaking, there are three identifiable soaring groups on the Boulder Airport, two soaring clubs and then the private owners.

In a very recent development, Bruce Miller, formerly the operator of the FBO Cloud Base passed away in his sleep at age 57, and Cloud Base is no more. His passing is soaring's and Boulder's genuine loss. Since life does go on, history does suggest that Boulder will one day again host a soaring FBO. [12]

Perhaps because the airport is owned by the city, which happens to have a fairly active city council "responsive to the voice of the citizens" (read: squeaking wheels, grease, etc.), the question of gliders and noisy tow planes being welcome at the airport is intermittently raised. One result was that the FBO and both clubs had of necessity worked together toward common goals. Their relationship was a healthy one; many club members obtained ab-initio instruction through The Cloud Base. Conversely, over the years club members were found working for The Cloud Base as tow pilots, instructors and introductory ride pilots.

Regrettably, the field isn't a mecca for groundlings with curiosity interest in sailplanes; shade is limited and water non-existent on the glider side, and there's little to actively "do" save watch planes in the pattern and talk to the airport bums. [13] But airport denizens are generally friendly (although fools are little suffered), perhaps because soft drink machine and porta-potty are on-site. About 40 gliders are regularly tied down at the site. Visitors (flying and otherwise) are welcome. As a minimum, newcomers to the field must obtain a verbal field checkout for safety reasons; in fact this is more a relief than a chore since any sane pilot would seek additional information about the controlled chaos occurring on the ground and in the air before wanting to fly at Boulder. In 2-plus decades/3-home-states of soaring participation, I've found each of my home soaring sites to have a different "flavor." Contradictions aside, to me Boulder's ground-bound flavor is that of individualists working together. In the air, Boulder is a soaring technician's playground...mis-monikered as a sinkhole. Check it out if you get a chance!

ABOUT THE AUTHOR

Bob Whelan has been soaring from Boulder since 1977. Too dense, too cheap, or too lazy to routinely drive to other Colorado Front Range soaring sites, over time his knowledge of the Boulder sinkhole has become an asset to his cross-country soaring abilities.

[ illustrations with original article: Aerial view of Boulder Municipal Airport looking West. Road map of Boulder area. Photo of Bob ]

Notes added by Mile High Gliding Webmaster, October 2000:

About the Author. Too shy to brag, I guess, Bob is a booster of american-built sailplanes, having owned a 1-26, Concept 70, HP-14, and Zuni. The Zuni is still his current ship and he has flown it over 1,000 hrs mostly out of Boulder. For many years the newsletter editor for the Soaring Society of Boulder, Bob is the award-winning author of "Cloud Dancing" (an introduction to soaring) and "Exploring the Monster" (a history of the Sierra Wave project), both published since this article was written (and available from SSA).

1. Owl Canyon Gliderport is at the site of the former Waverly West Soaring Ranch, near Wellington, CO, on Interstate I-25 14 mi. N of Ft. Collins. The property is owned free and clear by the glider pilot members of the Colorado Soaring Association. CSA hosts the yearly Rocky Mountain Soaring Contest, the Kolstad Labor Day BBQ, and has lent its facilities for several Civil Air Patrol National Flight Academies. CSA offers a club atmosphere with extras such as overnight bunkrooms and winch launching. (Bald Mountain is 25 mi. away, dirt roads last 4 mi. to field)

2. Kelly Airpark is a residential airport site near Elbert, CO, 30 mi SE of Denver. Homeowners include the Black Forest Soaring Society, a now small club with the legacy of Wave Flights, Inc. from the famous "Black Forest Gliderport", closed in 1986. Specifically, pilots who visited BFGP in its heyday will recognize the hangars and office buildings that were moved to Kelly. (Pike's Peak is 25 mi. away, dirt roads last 4 mi. to field)

3. The "People's Republic of Boulder" is a University town, home to the University of Colorado. Also home to the National Center for Atmospheric Research, National Institute of Standards and Technology, Joint Institute for Laboratory Astrophysics and so forth, not to mention IBM and countless small hi-tech and counter-culture enterprises. Many of the residents consider themselves environmentalists and glider operators do their darndest to keep the towplanes quiet and not scare the eagles or prairie dogs. The most civilized soaring site in Colorado, paved roads and even the city bus can take you there, just a few blocks from grocery and hardware stores, bars, etc.

4. Richard Hall Jr. made it to the USA World Championships Team and currently flies a Discus 2 out of Boulder. A few other Nationals pilots that frequent Boulder are John Seaborn, Bob Epp, Phil Ecklund, Tom Serkowski, and David Leonard.

5. 44,100' by Jim Munn in 1976. The flight was written up in Soaring. It was in a 1-34 now owned by CSA at Owl Canyon.

6. The Soaring Society of Boulder was founded by the likes of Richard Ball and Gale Abels in 1959.

7. Most clubs and operators in Colorado mount summer expeditions to sleepy mountain airports beyond the Front Range (Salida, Buena Vista, Creede, Taos...) or sometimes to safe flatland country for early X/C attempts (Dalhart TX...). The idea is gather a towplane, a club ship or two and a few private owners and spend a long weekend of adventure. Around here, this is much more popular than holding contests.

8. Three "fourteeners" are within routine view of a modest altitude above Boulder Municipal Airport, Long's Peak (30 mi NW), Mt. Evans (30 mi. SW), and Pike's Peak (60 mi. S). All are approx. 14,200' MSL high. And yes, we can very frequently see 60 mi. away.

9. Boulder Gliderport is the closest place to go soaring from Denver and DIA. From downtown, it's a 40 min. drive, whereas Owl Canyon and Kelly Airpark are over an hour, let alone Canon City (Peak Soaring), Colorado Springs (High Flights SC), Salida (B&J Flying Service).

10. The "Arapahoe Peaks High Altitude Soaring Area" allows flights above FL 180 without a transponder starting 5 mi. W of the airport.

11. Cloudbases will often go above 18,000' within a few miles of the field, climb rates over 1,000 fpm.

12. Bruce Miller was a true sage and guru at the old Wave Flights, Inc. and at Cloud Base. Now, Mile High Gliding, Inc. fills the role of a commercial soaring presence at Boulder. In 3 years of operation we have expanded our fleet and averaged 5,000 safe flights a year.

13. Mile High Gliding, Soaring Society of Boulder, and Denver Soaring Council all offer shade, places to sit, water, and refrigerators stocked with soda pop. Even though the airport management has to forbid permanent structures on the N side of the property (too close to the federally funded runway), things are getting a little more civilized all the time.