Going out on a limb I would say its the mean maximum temperature of the hottest month. Dont know what the statistical basis or how it is calculated.
It would mainly be used for performance calculations to "adjust" to the runway length for density altitude.
Takeoff
Altitude Correction
(7% per 1,000' above sea level)
E = Elevation
L = Takeoff length @ sea level
L1 = Length corrected for altitude
L1 = (.07 * E / 1000) * L + L
Temperature Correction
(0.5% per degree above stnd temp in hottest month)
(Stnd Temp adjusted to Sea Level)
T1 = Adjusted Stnd Temp
T = Mean Max High Temperature
L2 = Length corrected for altitude & temperature
T1 = 59 - (3.566 * E / 1000)
L2 = ( .005*( T - T1)) * L1 + L1
Effective Gradient Correction (takeoff only)
(10' for each 1' difference between Hi / Lo Pt)
G = Difference between Hi / Lo point in feet
L3 = RW length corrected for alititude, temperature & gradient
L3 = G * 10 + L2
Landing
Altitude Correction
(7% per 1,000' above sea level)
E = Elevation
L = Landing length @ sea level
L1 = Length corrected for altitude
L1 = (.07 * E / 1000) * L + L
Temperature Correction
(0.5% per degree above stnd temp in hottest month)
(Stnd Temp adjusted to Sea Level)
T1 = Adjusted Stnd Temp
T = Mean Max High Temperature
L2 = Length corrected for altitude & temperature
T1 = 59 - (3.566 * E / 1000)
L2 = ( .005*( T - T1)) * L1 + L1
Wet Pavement Correction (landing length only)
(15% increase in length based on dry conditions)
L3 = Landing RW length corrected for altitude, temperature & wet cond.
L3 = 1.15 * L2