This indicated altitude is correct, however, only when the  sea level barometric pressure is standard (29.92 "Hg), the sea  level free air temperature is standard (+15 degrees Celsius  (°C) or 59 degrees Fahrenheit (°F)), and the pressure and  temperature decrease at a standard rate with an increase  in altitude. Adjustments for nonstandard pressures are  accomplished by setting the corrected pressure into a  barometric scale located on the face of the altimeter. The  barometric pressure window is sometimes referred to as  the Kollsman window; only after the altimeter is set does it  indicate the correct altitude. The word “correct” will need  to be better explained when referring to types of altitudes,  but is commonly used in this case to denote the approximate  altitude above sea level. In other words, the indicated  altitude refers to the altitude read off of the altitude which is  uncorrected, after the barometric pressure setting is dialed  into the Kollsman window. The additional types of altitudes  are further explained later.  be lower then the indicated altitude. There is an old aviation  axiom: “GOING FROM A HIGH TO A LOW, LOOK OUT  BELOW.” Conversely, if an aircraft is flown from a low  pressure area to a high pressure area without an adjustment  of the altimeter, the actual altitude of the aircraft is higher  than the indicated altitude. Once in flight, it is important to  frequently obtain current altimeter settings en route to ensure  terrain and obstruction clearance.  Many altimeters do not have an accurate means of being  adjusted for barometric pressures in excess of 31.00  "Hg. When the altimeter cannot be set to the higher  pressure setting, the aircraft actual altitude is higher than  the altimeter indicates. When low barometric pressure  conditions occur (below 28.00), flight operations by  aircraft unable to set the actual altimeter setting are  not recommended.  It is easy to maintain a consistent height above ground if  the barometric pressure and temperature remain constant,  but this is rarely the case. The pressure and temperature can  change between takeoff and landing even on a local flight.  If these changes are not taken into consideration, flight  becomes dangerous.  If altimeters could not be adjusted for nonstandard pressure, a  hazardous situation could occur. For example, if an aircraft is  flown from a high pressure area to a low pressure area without  adjusting the altimeter, a constant altitude will be displayed,  but the actual height of the aircraft above the ground would  Adjustments to compensate for nonstandard pressure do  not compensate for nonstandard temperature. Since cold  air is denser than warm air, when operating in temperatures  that are colder than standard, the altitude is lower than the  altimeter indication.  [Figure 8-3]  It is the magnitude of this  “difference” that determines the magnitude of the error. It is  the difference due to colder temperatures that concerns the  pilot. When flying into a cooler air mass while maintaining a  constant indicated altitude, true altitude is lower. If terrain or  obstacle clearance is a factor in selecting a cruising altitude,  particularly in mountainous terrain, remember to anticipate  that a colder-than-standard temperature places the aircraft  lower than the altimeter indicates. Therefore, a higher  indicated altitude may be required to provide adequate terrain  clearance. A variation of the memory aid used for pressure  Effect of Nonstandard Pressure and Temperature  Figure 8-3.  Effects of nonstandard temperature on an altimeter.  8-4  ° ° ° 5,000 foot pressure level  4,000 foot pressure level  3,000 foot pressure level  2,000 foot pressure level  1,000 foot pressure level  Sea level  30 C  15 C  0 C 