In the first part of Tom's feature posted last month he explained the conditions necessary for lee waves to form in the hope it would help you make contact more easily. Now, with characteristic clarity, he takes us a step further to show some of the complexities of wave systems. For part 1 click here.

Photo A. Short wavelength bars over Northern Ireland.

Having concentrated on the main features of lee waves the last month, this article describes some of the main varieties of wave flow one may encounter. Photo A shows an unusually regular set of lee waves. They formed in a stable layer of air blowing over Northern Ireland from the Atlantic. The westerly wind was light so the wavelength was short. Fig 1. Composite wave diagram. Tom's drawings enhanced by Steve Longland. Fig 1 shows a composite diagram of a common type of wave. On the left the solid line marked temperature shows the usual decrease with height, interrupted by a steeper section where this air had its maximum stability. The pecked line marked wind shows that speed increased with height. An intermediate layer of strong stability and a wind which increases with height are signs that the atmosphere can develop lee waves. The wind speeds shown are the average velocities on days when sailplanes gained at least 5km (16 405ft) in height. In the centre is a sketch of typical clouds. The size of low cumulus clouds is limited by the stable layer and the tops are capped by lenticular clouds. On the right a line marked lift shows how the vertical velocity of the updraft varied with height. The irregular wiggles at the foot represent surges of lift in thermals. Pilots use thermals to reach the stable layer where wave flow starts. The smooth wave lift starts in the zone marked max stability. The best lift is usually close to the top of this stable layer. The upwind edge of the smooth lenticular cloud often marks the strongest lift. Photo B. Lenticular tops of a wave over Wales. Photo C. High lenticular over Scotland. Photos by Tom.   Photo B shows the lenticular cloud capping cumulus in a Welsh wave system. The curved top of the lenticular shows the shape of the wave flow. The lift nearly always decreases above the layer of maximum stability. The arrow marked 5.5kts shows the lift at 4.4km (about 14 400ft) where it had decreased to little more than half the maximum in the stable layer. Photo C shows the top of a much higher lenticular cloud over Scotland. The cloud is thinner and flatter than lenticulars lower down. This usually means the lift is less at that height. Slim and elegant lenticulars make good pictures but the lift is much better where the clouds have more depth. Changes Of Lift With Height The lift curve can vary greatly. The wave is usually flattened if it comes up to a powerful jet stream aloft; this restricts the h