Convergence
- How to recognize convergence lift. Convergence is the lifting mechanism that results when and where two different air masses, traveling in opposite directions towards each other, meet. They often form in the vicinity of orographic obstacles, such as a mountain range, that provide a natural barrier for separating the two air masses. Visual clues for a convergence line may include a different look between the two air masses, e.g. clear air on one side and hazy air on the other. Soaring birds also use convergence lift. Other indicators may be a noticeable temperature change in air and/or a change in the direction of drift angle and ground speed. There may also be a noticeable difference in the height of the cloud base as shown on the following graphic that depicts a sea breeze convergence (source: Wikipedia):
The strongest lift will be found under the part of the cloud where the base is higher.
- Convergence in the mountains (in German). Excellent article written by Gavin Wills (Omarama, NZ) and originally published in English in glidingmagazine.com (now defunct). The article is a translation by Martin Strolz for streckenflug.at. Key learning points from the article are:
- Convergence zones are areas where different air masses meet. These air masses can differ from each other based on their temperature, their humidity content, their level of stability, and/or their wind direction and speed. Convergence zones can occur at a very large scale (e.g. when a front is moving in), or they can be small and local (e.g. wind flowing around a hill and meeting with itself on the lee side).
- Often thermals are triggered in convergence zones. E.g., when wind flows around a mountain and meets with itself downwind of the obstacle, the air on the ground tends to be calm and thus easily heated by the sun. The incoming wind from either side will trigger thermals in that area. These thermals tend to be “bubbly” and short-lived. Clouds that are formed by these thermals tend to come and go quickly. Gliders soaring along such a conversion often circle in different thermals.
- When prevailing winds get deflected by bigger mountain chains (such as in the Colorado Rockies) they will move along the valleys. Multiple meeting points are possible downstream, wherever those valleys meet.
- These winds may also merge with thermally-induced valley-winds, thereby producing strong thermals. If the prevailing winds are typically the same (such as westerly in Colorado), these thermals tend to occur everyday at the same places, even under blue conditions.
- Daily adiabatic winds often cause convergence zones along the spine of mountain ridges when winds from both sides of the ridge converges.
- Katabatic winds descending from the mountains in the evening can cause convergence zones in the middle of valleys.
- One of the most spectacular convergence zones in the world is the long N-S line to the East of the Rocky Mountains. It separates the more humid air of the continental eastern states from the dryer air masses to the West. (I suppose this is the line I have now experienced twice while soaring just a few miles to the West of the Boulder airport.)
- Convergence zones in the mountains may be recognizable by stepped-up cloud bases, a line of cloud-veils, or cumuli. They may also be noticeable by different (and often unusual) wind directions on the ground (e.g. by spotting different wind directions on the surface of nearby lakes); by rows of misty veils, whirled-up dust above the ground in a valley, or sometimes even by visible differences in the clarity of two air masses.
- Flugtechnik – Konvergenzen (in German). This excellent article from Torsten Hahne, published in dhv.de, appears to build upon the one above from Gavin Wills (but without directly quoting it). Torsten goes into more detail with respect to various types of convergence lift particularly useful for soaring. Key points:
- Mountain Breeze Convergence: in German often referred to as “Umkehrthermik”. After a thermal day in the evening, katabatic winds stream down the mountain slopes into the valley. Mountain breeze convergence appears in the middle of valleys where these winds meet and the air therefore has to rise.
- Valley Breeze Convergence: during thermal days, air flows up along the valleys towards the mountains to replace the air that is lifted by mountain thermals. Where two valleys meet (e.g. at a gap between mountain ranges) the air flowing up from each valley has nowhere else to go but straight up. Depending on the strength of the two valley breezes the meeting point may be either directly over the gap or to either side of it. Crossing valleys is made much easier when flying along such a convergence line.
- Convergence of a Valley Breeze with Prevailing Winds: Where a valley breeze meets a prevailing wind blowing in the opposite direction, a convergence line forms that is often marked by noticeably different cloud bases. This seems to be a key phenomenon in Boulder, where the easterly valley breeze moves up along the canyons and foothills of the front range where it meets the prevailing westerly winds flowing over the Continental Divide. The convergence line forms parallel to the Divide where those air masses meet. The location of the convergence line on a particular day depends on the strength of the valley breeze and the strength of the prevailing winds. The lift tends to be much smoother along the convergence line and much rougher and more turbulent to either side of it. The difference in cloud bases may be so great that it could be difficult to return from the lower side to the higher one. This is often a phenomenon in Southern Tyrol when humid air from the Po valley gets pulled towards the Dolomites and forms much lower cloud bases in the South. This is particularly true if the strength of the prevailing northerly winds is sufficient that the convergence line is pushed further to the South.
- Lee Convergence is the result of wind flowing around a mountain and meeting itself on the leeward side. The generation of useful lift depends on the form of the obstacle relative to the direction of the wind, on the strength of the wind, and also on the angle of the sun in the lee. If the wind is too strong one may only find sink or even turbulent rotors instead of the desired smooth convergence lift. Best conditions can be expected if smooth, constant, and rather light winds stream around a mid-size mountain where the lee is located in bright sunshine. Thermals can then easily be triggered in the convergence zone where good climb rates can be found.
- Air Mass Convergence: A good example for air mass convergence is when a sea breeze meets another airmass, or when see breezes meet from different directions (e.g. on sufficiently narrow islands like New Zealand, or the Canary Islands).
- Convergences are often mixed with other phenomena such as lee thermals, orographic winds, rotors, etc. Often, it is not easy to clearly distinguish among the various lift sources.
- Konvergenz (in German). A more technical article about convergence with good illustrations from Gerd Pfeffer.
- Flight along a convergence line (You Tube). Note the flight along the line with the higher cloud base to the left and the lower cloud base to the right. The pilot follows along the line, just to the left of the lower cloud base.
- Another nice video of a flight along a convergence line. Note the different look of the two air masses on either side of the line: the left side (when viewed in flight direction) is much dryer, the visibility is better, and the cloud base is high. The right side is more moist: visibility is worse due to haze and higher humidity, and the cloud base is lower. The pilot recognizes the convergence and follows along the higher cloud base.