9 0 1 2 6 5 4 3 9 0 1 2 6 5 4 3 Atmospheric pressure at sea level 1,013 mb Atmospheric pressure at 3,500 feet Decrease in pressure has caused aneroid capsule  to expand and rotate the needle. 890 mb The altimeter’s static pressure inlet must be  exposed to air that is at local atmospheric  pressure. 1 The pressure of the air inside the altimeter’s  casing equalizes to local atmospheric  pressure via the static pressure inlet. 2 Atmospheric pressure decreases with  altitude. 3 As atmospheric pressure decreases, the aneroid capsule expands, moving the linkages and rotating the display needle(s). 4 Figure 4-17.  How the altimeter functions.  of the glider is higher than the indicated altitude.  Fortunately, this error can be corrected by setting the altimeter  properly. When flying in the local area and communication with  ATC  is not available to provide the current altimeter setting,  there  is an alternative procedure. Set the altimeter on zero  while  on the ground to directly read AGL referenced to the  landing zone. However, for cross-country flights and flights from  higher elevation airports, that procedure may not be an  option  due to the excessive Kollsman window setting  instrument required. In those instances, the altimeter can be set to  read  the field elevation and either disregard the Kollsman  window setting or applying a correction factor, or set in the  barometric altimeter setting and remember if it reads high or low.  volume than on a cold day, and consequently the pressure  levels are raised. For example, the pressure level at which  the altimeter indicates 10,000 feet is higher on a warm day  than under standard conditions. On a cold day, the reverse  is true, and the 10,000-foot level would be lower. The  adjustment made by the pilot to compensate for nonstandard  pressures does not compensate for nonstandard temperatures.  Therefore, if terrain or obstacle clearance is a factor in the  selection of a cruising altitude, particularly at higher altitudes,  remember to anticipate that colder than standard temperature  places the glider lower than the altimeter indicates. Therefore,  a higher altitude should be used to provide adequate terrain  clearance. A memory aid in applying the above is “from a  high to a low or hot to cold, look out below.”  [Figure 4-18] Variations in air temperature also affect the altimeter. On  a warm day, the expanded air is lighter in weight per  unit  True altitude Indicated altitude 4,000 3,000 3,500 Feet (MSL) 30.1 30.0 29.9 I 0 0 F E T I 4 5 6 7 9 2 0 8 3 CALIBRATED TO  20,000 FEET ALT 30.1 30.0 29.9 I 0 0 F E T I 4 5 6 7 9 2 0 8 3 CALIBRATED TO  20,000 FEET ALT 30.1 30.0 29.9 I 0 0 F E T I 4 5 6 7 9 2 0 8 3 C ALIBR ATED TO 20,000 FEET ALT 30.1 30.0 29.9 I 0 0 F E T I 4 5 6 7 9 2 0 8 3 CALIBR ATED TO 20,000 FEET ALT 30.1 30.0 29.9 I 0 0 F E T I 4 5 6 7 9 2 0 8 3 CALIBRATED TO  20,000 FEET ALT 30.1 30.0 29.9 I 0 0 F E T I 4 5 6 7 9 2 0 8 3 CALIBRATED TO  20,000 FEET ALT C O R R E C T $_{ }$A L T I M$_{E}$ T E R S E T I N G 30.00 inches Hg C O R R E C T $_{ }$A L T I M$_{E}$ T E R S E T I N G 30.00 inches Hg C O R R E C T $_{ }$A L T I M$_{E}$ T E R S E T I N G 30.00 inches Hg Standard Colder Warmer Aircraft altimeter setting Aircraft altimeter setting Aircraft altimeter setting Figure 4-18.  Nonstandard pressure and temperature.  4-8