Airport reference temperature

Airport reference temperature is defined as the monthly mean of average daily temperature (T_a) for the hottest month of the year plus one-third the difference of this temperature (T_a) and the monthly mean of the maximum daily temperature (T_m) for the same month of the year.

Thus airport reference temperature is

T_a + \frac{T_m - T_a}{3}.

References

Airport Engineering by Norman J. Ashford and Paul H. Wright, 3d edition, page 234.