Observing boundary layer evolution with the UW ultralight

With winter weather looming and my sabbatical break from teaching nearing an end, I watched carefully last week for conditions that would allow us to make at least one more flight with the Zigolo before the end of the year.  Among other things, I wanted to try to use up the remaining fuel so that the carburetor and fuel lines wouldn’t gunk up  from standing idle for up to several months.  But I also wanted to demonstrate the ability to make meaningful meteorological measurements that could not be as easily achieved with an unmanned aerial vehicle (UAV, or drone), conventional weather balloon, or conventional manned airplane.

The most important considerations in planning a flight were wind and precipitation.  With the Zigolo’s unenclosed cockpit, we certainly didn’t want to contend with rain or snow.  More importantly, the aircraft does not have the speed and mass required to handle stronger winds, especially when those winds are gusty.  And even apart from the fundamental limits on the Zigolo’s airworthiness in strong wind, I do not have enough experience flying it yet to risk more than a mild breeze.

So I considered what kind of interesting meteorology could be done on a calm day with no threat of precipitation and without the need to travel a greater distance from the airport.  My previous flight to 1,100 ft. AGL and the temperature profile we captured that morning provided the inspiration:  instead of measuring a single static profile, why not observe the evolution of the near-surface boundary layer in response to the first warming rays of the sun?  So I decided to aim for a flight at or soon after sunrise.

The forecast for Wednesday, November 29, seemed promising.  Skies would be clear, wind would be light up to several thousand feet, and we could expect a strong radiation inversion to form overnight and then dissipate soon after the sun came up. 

When Jonathan and I arrived at the Verona Air Park a little before 7AM, it was almost too perfect.  In particular, the temperature at the surface had dropped well below freezing.  Despite having multiple layers of clothing on, I wasn’t certain how adequate those layers would be when exposed to the 35+ mph relative wind.  Nevetheless, it seemed worth a try.

Technical glitches kept me from getting off the ground as quickly as I had hoped.  First, my goggles – and my glasses inside the goggles – fogged up so badly as to effectively blind me, and I ended up tossing them aside and vowing to fly without goggles.  Certainly I wasn’t worried about hitting any bugs, but I knew my eyes would probably water like mad in the cold wind.

The second technical glitch entailed the freezing up of the device I had strapped to my knee to monitor position and altitude.  Apparently the improvised clip I made for it had pressed on the factory reset button.  So it too was put aside.  Instead I would rely on Jonathan informing me of my altitude via radio, based on the telemetry data he would receive on his laptop.

As before, we strapped a Kestrel 5500 Weather Meter to the “nose” of the open-frame fuselage and set it to log temperature, pressure, humidity, and wind data at 20 second intervals.  The recorded wind, of course, would not be the actual wind but rather the airspeed of the plane

I ended up finally getting off the ground at 7:55AM, 50 minutes after official sunrise. I ascended steadily from the airport in a broad spiral; my climb rate was about 300 feet per minute.  The combination of the surface temperature of less than –5 C  (21 F) and the relative wind of 50 km/hr (31 mph) yielded a wind chill of –16 C (2 F).  I could tell that I wouldn’t last too long if it stayed that cold for any length of time.

Fortunately, although I could not view any measurements in flight, I could certainly feel the air getting a bit warmer as I ascended to a few hundred feet above ground level.  From there, the temperature declined again as I approached 1000′ AGL.  Above that altitude, I felt myself climbing through yet another pronounced temperature inversion.  By the time I reached my target altitude of 2000′, I was experiencing the warmest temperatures (relatively speaking) of any point along the ascent. No, temperature does not always decrease with altitude!

Upon reaching 2,000′ AGL, I immediately began descending again, and the air grew colder again.  I approached the airport from the east at about 500′ AGL and made a low pass over an open pond, hoping to later see a humidity signature from that pass (I didn’t!).  Directly over the runway just a few feet above the grass, I added full power and began climbing again.


Over the course of the nearly hour-long flight, I made a total of three ascents to 2,000′ and back again, each time returning to within a few feet of the grass before ascending again.  By the third cycle, I realized that I could not continue – the cold wind was filtering through my layers of clothing, I was starting to shiver, and my hands were becoming numb inside my gloves.  

I landed, taxied back to the hangar, and shivered uncontrollably with honest-to-goodness hypothermia.  Jonathan let me sit in his car with the engine running to warm up; it was 15 minutes before the shivering fully subsided.

Lesson learned:  Get better cold weather clothes!  In particular, look into the kind of windproof winter clothing and gloves used by snowmobilers and motorcyclists.  Also, several experienced ultralight pilots suggested chemical warmers inside the gloves and boots.

In the end, the temporary discomfort was worth it: we obtained three perfect profiles of temperature and dewpoint (actually six if you count ascent and descent separately)  spaced about 14 minutes apart.  The plot below shows exactly the evolution we had hoped to see:  the strong surface inversion during the first ascent was already markedly weakened by solar heating during the second ascent (a mere 13 minutes later), and by the third ascent, it was gone altogether.   Also, convection from the surface appeared to erode the base of the higher inversion, which had started out near 320 m (1,050 ft) and then lifted to 420 m (1,375 ft) in just 15 minutes.

Temperature (solid curves) and dewpoint (dashed curves) from three ascents by the UW-Madison Ultralight on 29 November 2017. Displayed altitude is relative to ground level at the Verona Air Park (W19).

Key takeaway points:
  • To measure the same profiles, a traditional radiosonde system would have required three single-use balloon systems at a cost of $250 each, including instrument and helium, for a total approaching $800.  The UW-Madison ultralight, however, measured these profiles using less than two gallons of premium gasoline treated with synthetic oil, for a total cost of less than $10.  Also, it is normally not possible to operate conventional radiosondes so close to each other in time because of radio interference; instead, one would need to allocate three separate frequencies and three separate ground receiving stations.
  • A drone could have easily measured these profiles up to an altitude of 400′ AGL, but beyond that altitude would have required a waiver from the FAA.  The waiver process takes at least 90 days, and many waiver applications are denied.  In short, the full 2,000′ profiles we see here would have been possible with only significant advance planning and a successful waiver application.
  • Conventional manned aircraft could theoretically have also observed the profiles, but their cost of operation is typically far greater, especially when factoring ferrying time, intrument integration, etc., into the total cost.  Also, it is generally less safe for large conventional aircraft to descend to within a few feet of ground level unless the plane is actually landing.
At this point, the fuel tank, carburetor, and lines have been emptied, corrosion protection has been applied to the engine, and there is no urgency to fly again before spring.  Nevertheless, I’ll continue to watch the forecast, and if the opportunity presents itself, we’ll do our best to take advantage of it.

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