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day, lee waves are often present all day, as long as the winds aloft continue. The daytime dissipation of lee waves is not as notable for large mountains. For instance, during the 1950s Sierra Wave Project (see www.soaringmuseum.org), it was found that the wave amplitude reached a maximum in mid to late afternoon, when convective heating was a maximum. Rotor turbulence also increased dramatically at that time. 

Topography upwind of the wave-producing range can also create problems, as illustrated in Figure 9-27 . In the first case [Figure 9-27A] , referred to as destructive interference, the wavelength of the wave from the first range is out of phase with the distance between the ranges. Lee waves do not form downwind of the second range, despite winds and stability aloft being favorable. In the second case [Figure 9-27B] , referred to as constructive interference, the ranges are in phase, and the lee wave from the second range has a larger amplitude than it might otherwise. 

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Wave flight is a unique soaring experience and requires planning, equipment and Federal Aviation Administration (FAA) notification for the flight. The Soaring Society of America (SSA) Awards and Badges offers soaring pilots Lennie Awards for completing and documenting a wave flight. [Figure 9-28] 

Isolated small hills or conical mountains do not form classic lee waves. In some cases, they do form waves emanating at an angle to the wind flow similar to water waves created by the wake of a ship. A single peak may require only a mile or two in the dimension perpendicular to the wind for high- amplitude lee waves to form, though the wave lift is confined to a relatively small area in these cases. 

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## Lift Due to Convergence 

Convergence lift is most easily imagined when easterly and westerly winds meet. When two opposing air masses meet, air is pushed aloft by the two opposing winds. Air does not need to meet head on to go up, however. Wherever air piles up, it leads to convergence and rising air. 

One type of convergence line commonly found near coastal areas is the so-called sea-breeze front. Inland areas heat during the day, while the adjacent sea maintains about the same temperature. Inland heating leads to lower pressure, drawing in cooler sea air. As the cooler air moves inland, it behaves like a miniature shallow cold front, and lift forms along a convergence line. Sometimes consistent lift can be found along the sea-breeze front while at other times it acts as a focus for a line of thermals. If the inland air is quite unstable, the sea-breeze front can act as a focus for a line of thunderstorms. Additionally, since the air on the coast side of the sea-breeze front is rather cool, passage of the front can spell the end of thermal soaring for the day. 

