Soaring birds, like pelicans or eagles, are too heavy to fly easily by flapping but can attain great height by circling within the updraught of a thermal once they have surmounted the turbulence of the lowest one or two hundred metres. Then they peel off and glide towards the next thermal for another lift. Vultures in East Africa have been observed to climb at 2-4 m/s to 3,500 m, for instance. But they are readily caught on the ground in the morning, before convection has started. Close to the surface of the sea, birds such as herring gulls either flap or glide according to the atmosphere's instability (Figure 7.8). The gulls flap when the water is cooler than the air, causing stability, but glide in circles within thermals when the sea surface is warmer.

Glider pilots and hang gliders imitate the soaring birds in using thermals. Particularly good lift is found up to tall clouds, and climb rates are typically 1-3 m/s in Australian thermals. Weak thermals may be 2-5 km apart, while strong thermals are typically 15 km away from

Figure 7.8 Effect of instability on the way in which seagulls fly.

each other. Hang gliders may be lifted 400 m or so by a thermal, but the world-record altitude for glider pilots is 14 km, at the top of the troposphere.

Inexperienced pilots may mistake wave-lift for a thermal. Wave-lift is due to air deflected upwards by hills, offering limited ascent.

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