long period of time in hot summer temperatures or at high  altitudes. This makes dehydration very likely for two reasons:  the gliders clear canopy offers no protection from the sun and,  at high altitudes, there are fewer air pollutants to diffuse the  sun’s rays. The result is continual exposure to heat that the  body attempts to regulate by perspiration. If this fluid is not  replaced, fatigue progresses to dizziness, weakness, nausea,  tingling of hands and feet, abdominal cramps, and extreme  thirst.  [Figure 13-8] When a glider ascends, middle ear air pressure may exceed  the pressure of the air in the external ear canal, causing the  eardrum to bulge outward. This pressure change becomes  apparent when you experience alternate sensations of  “fullness” and “clearing.” During a descent, the reverse  happens. While the pressure of the air in the external ear  canal increases, the middle ear cavity, which equalized with  the lower pressure at altitude, is at lower pressure than the  external ear canal. The result is higher outside pressure,  causing the eardrum to bulge inward. This condition can be more difficult to relieve due to the fact  that air must be introduced into the middle ear through the  eustachian tube to equalize the pressure. The inner ear is a  partial vacuum and tends to constrict the walls of the eustachian  tube. To correct this often painful condition, which causes  temporary reduction in hearing sensitivity, pinch your nostrils,  close your mouth and lips, and blow slowly and gently in the  mouth and nose. This procedure, called the valsalva maneuver,  forces air up the eustachian tube into the middle ear.  For glider pilots, prevention is the best remedy for spatial  disorientation. If the glider you are flying is not equipped for  instrument flight, and you do not have many hours of training  in controlling the glider by reference to instruments, it is  best to avoid flight in reduced visibility or at night when the  horizon is not visible. Susceptibility to disorienting illusions  can be reduced through training, awareness, and learning to  rely totally on your flight instruments.  [Figure 13-7] Water should be taken on every flight to prevent dehydration.  The effects of dehydration on a pilot’s performance are subtle,  but can be dangerous and are especially a factor in warmer  climates. Some glider pilots wear a hat to block the sun from  distracting their ability to fly and see the instruments. Pilots  should ensure that the brim of the hat is not too large, which can  interfere with the ability to scan for other gliders and air traffic.  Heatstroke is a condition caused by any inability of the body  to control its temperature. Onset of this condition may be  recognized by the symptoms of dehydration, but also has  been known to be recognized only by complete collapse. To  prevent these symptoms, it is recommended that you carry  an ample supply of water and use it at frequent intervals on  any long flight, whether you are thirsty or not. Wearing light  colored, porous clothing and a hat provides protection from  the sun. It is also helpful to keep the cockpit well ventilated,  which aids in dispelling excess heat. When flying at higher altitudes, the inside of the glider can get  cold. Proper clothing is a must since temperatures of –30° to  –60 °C may be encountered at altitude. Proper preparation for  the cold is especially difficult since temperatures on the ground  Dehydration is the term given to a critical loss of water from  the body. The first noticeable effect of dehydration is fatigue,  which in turn makes top physical and mental performance  difficult, if not impossible. Glider pilots often fly for a  13-6 Cold Weather Dehydration Heatstroke Spatial Disorientation Physical activity Loss of fluid Breathing Sweating Reduced circulation Reduced nutrients Blood more viscous Blood circulation slows Leads to: • Nausea Headaches, • Irrationality Muscle cramps • Rise in body Temperature  • Dizziness Dehydration serious effects Figure 13-7.  Learning to rely totally on flight instruments to fly  will reduce the likelihood of experiencing a disorienting illusion  while in flight.  Figure 13-8.  Symptoms of dehydration.