during late morning, and then both remain somewhat steady  for several hours during the afternoon. The height band rises  and broadens with thermal height. Sometimes the top of the  height band is limited by the base of cumulus clouds. Cloud  base may slowly increase by thousands of feet over several  hours, during which the height band also increases. Thermals  often “shut off” rapidly late in the day, so a good rule of thumb  is to stay higher late in the day.  [Figure 11-14] It is a good idea to stop and thermal when at or near the  bottom of the height band. Pushing too hard can lead to an  early off-field landing. Pushing too hard leads to loss of time  at lower altitudes because the pilot is trying to climb in weak  lift conditions. Another way to increase cross-country speed is to avoid  turning at all. A technique known as dolphin flight can be used  to cover surprising distances on thermal days with little or no  circling. The idea is to speed up in sink and slow down in lift  while only stopping to circle in the best thermals. The speed to  fly between lift areas is based on the appropriate MacCready  setting. This technique is effective when thermals are spaced  relatively close together, as occurs along a cloud street. As an example, assume two gliders are starting at the same  point and flying under a cloud street with frequent thermals  and only weak sink between thermals. Glider 1 uses the  conditions more efficiently by flying faster in the sink and  slower in lift. In a short time, glider 1 has gained distance  on glider 2. Glider 2 conserves altitude and stays close to  cloud base by flying best L/D through weak sink. To stay  under the clouds, he is forced to fly faster in areas of lift,  exactly opposite of flying fast in sink, slow in lift. At the  end of the cloud street, one good climb quickly puts glider 1  near cloud base and well ahead of glider 2.  [Figure 11-15] The best speed to fly decreases time in sink and therefore  decreases the overall amount of descent but produces the  best forward progress. Being slower in sink increases time  descending and slows forward progress, while being fast in  lift decreases time in lift and altitude gained. Altitude (feet) Time of day (military) 10 11 12 13 14 15 16 17 18 8,000 7,000 6,000 5,000 4,000 3,000 2,000 T h e r m al  st r e n g t h Height band Figure 11-14.  Thermal height and height band versus time of day. Fast flight  Slow flight Glider  Using dolphin flight, achieves a greater distance Straight high-speed glide achieves less distance 2 1 2 2 1 1 Figure 11-15.  Advantage of proper speed to fly under a cloud street. 11-13