Here it is from FAR 25.101
25.101 General.
(a) Unless otherwise prescribed, airplanes must meet the applicable performance requirements of this subpart for ambient atmospheric conditions and still air.
(b) The performance, as affected by engine power or thrust, must be based on the following relative humidifies;
(1) For turbine engine powered airplanes, a relative humidity of -
(1) 80 percent, at and below standard temperatures; and
(11) 34 percent, at and above standard temperatures plus 50' F.
Between these two temperatures, the relative humidity must vary linearly.
(2) For reciprocating engine powered airplanes, a relative humidity of 80 percent in a standard atmosphere. Engine power coitections for vapor pressure must be made in accordance with the following table:
Specific humidity w
Altitude H (ft) Vapor pressure e (in (Ib moisture per lb Density ratio p/cy
Hg) dry air) 0.0023769
0 0.403 0.00849 0.99508
1,000 0.354 0.00773 0.96672
2,000 0.311 0.00703 0.93895
3,000 0.272 0.00638 0.91178
4,000 0.238 0.00578 0.88514
5,000 0.207 0.00523 0.85910
6,000 0.1805 0.00472 0.83361
7,000 0.1566 0.00425 0.80870
8,000 0.1356 0.00382 0.78434
9,000 0.1172 0.00343 0.76053
10,000 0.1010 0.00307 0.73722
15,000 0.0463 0.001710 0.62868
20,000 0.01978 0.000896 0.53263
25,000 0.00778 0.000436 0.44806
(c) The performance must correspond to the propulsive thrust available under the particular ambient atmospheric conditions, the particular flight condition, and the relative humidity specified in paragraph (b) of this section. The available propulsive thrust must correspond to engine power or thrust, not exceeding the approved power or thrust less -
It's a text 'cut and paste', not a scan, so my apologies if the formatting looks a bit strange.
Hope that this helps.
Regards,
Old Smokey