New Mexico, the changes in balloon capability and decrease  in burner performance are important considerations while  planning for the flight. Second, heater performance is degraded at a rate of 4  percent per 1,000 feet of altitude. This means on a standard  reference day, a particular heater will have lost 12 percent of  its efficiency at 3,000 feet, or be performing at 88 percent of  its capability. This is due to the loss of the partial pressure of  oxygen, a necessary component of combustion.  Preflight planning requires consideration of balloon loading  and performance with respect to altitude and expected  temperatures. Balloon manufacturers have provided the  information necessary to determine these factors in the  form of a performance chart in the flight manual. Referred  to as nomographs or nomograms, performance charts are  simple to use and provide excellent planning information.  [Figure 3-13] If three of the above factors are known, a fourth may be  determined. The performance charts may be used in many  ways to determine performance of the balloon on a given day.  This process does not have to be computed at the beginning of  each flight. Many pilots develop a listing of possible weights,  temperatures, and altitudes, depending on the average flying  conditions for their home area. This is an acceptable practice  0 1 2 3 4 5 10 15 70° 90° 110° 130° 150° 170° 190° 210° 230° 250° 275° -10 10 30 50 70 90 110 1,435 1,400 1,300 1,200 1,100 1,000 900 800 700 600 500 400 Envelope Temperature Pressure Altitude Pressure Altitude (x1,000 ft) Ambient Temperature (°F) Gross Lift (lb) Data Base: Calculated Sample Calculation: Ambient Temperature 60 °F Envelope Temperature 190 °F Pressure Altitude 1,500 ft  1. Enter the chart with ambient temperature (Point ). 2. Trace right to the desired envelope temperature (Point ). 3. Trace down to the pressure altitude (Point ). 4. Trace left to read GROSS LIFT (Point = 1,120 lb) A B C D A B C D EXPECTED GROSS LIFT $_{Figure 3-13. }$  Typical performance chart for a 77,000 cubic foot balloon. 3-9