As far as I know, a big obstacle to attaining laminar flow in aircraft today is the so-called "bug layer", presumably a lower layer of the atmosphere where bugs are encountered. So, my question is simply this: how high up do insects stop becoming a serious issue for laminar flow?
In 1961 study a plane flew 116,684 miles sampling the air, catching whatever was up there, and managed to capture a single termite at 19,000 feet. This is the highest ever observed.
You just never get above the "bug layer" in a smaller aircraft (Cessna 172 limit is 15,000 feet). Likely this is not a proper naming.
I don't have any scientific data, but in my flying experience, including some time back in the 70s in a Breezy, where, as on a motorcycle, you have to be careful to keep your mouth closed, you don't encounter that many bugs above 1000 ft agl (they are there, but few and far between relatively). Descending in the Breezy in a warm summer late afternoon with a low sun angle, you could really start to make out the little back-lit specks in the air up-sun as you descended below about 500 ft agl. It did almost feel like you were descending into a layer of something.
In my current airplane, bug spatters on the windshield mostly appear below 1000 ft, although I get the odd one higher. The vast majority of the bug spatters on the wings, nose and windshield happen on takeoff and landing.
In the Mooney M20, there is a ram air induction system that supplies unfiltered air to the engine. The POH only refers to "clear air" on when to use ram air, but if you go on Mooney forums where the question comes up, there seems to be a consensus not to use ram air below 1000 ft due to bugs, and preferably above 5000 ft.
So it's not really a "layer". If you plotted bug density, you would likely find a parabolic curve with the bulk of the curve below 1000 ft, tapering off above that to effectively zero somewhere above 10000 ft.
Yesterday in Oregon I was collecting bugs on my leading edges like a crop duster - at 3500’ AGL. Low pressure, springtime over agricultural fields and vineyards.
As already answered, most places it is rare to get many bugs over 1000’. Mylar balloons are almost as common at altitude in California ;-)
Laminar flow isn't necessarily the panacea of design or function; indeed, many aircraft use devices to interrupt laminar flow or energize it.
"Bug layer" is a made-up term, perhaps more applicable to the boundary layer than an imaginary atmospheric one.
I've hit bugs in freefall around 18,000 and have certainly seen birds much higher. I've hit birds and some large bugs with some frequency above 10,000.'
Icing is perhaps more common and a regular occurrence at higher altitudes.
Some aircraft are much more affected by boundary disturbances, than others. In the Piaggio P180, even flight through fair weather cumulous clouds at low flight altitudes has a noticeable effect. With the autopilot engaged, it's nearly imperceptible, but when hand flying, requires movement of the control column back and forth, and the change is quite apparent. Likewise, bugs or other interruptions of the boundary layer, including rain or any ice, makes a significant difference in control, speed, and climb performance.