def METHOD_NAME(T, flag=1):
    # calculate saturation vapor pressure(Es)
    #  T is temperature (K)
    #------------------------------------
  if flag == 1:
    a      = 7.5
    b      = 237.3
  elif flag == -1:
    a      = 9.5
    b      = 265.5
    #---
  tdeg   = T - 273.16
  es = 6.11 * 10**(a * tdeg / (b + tdeg)) * 100.0
  return es