We study THz emission by two-color femtosecond filaments in air using pump wavelengths λ0 from 0.8 to 10.6 µm. Comprehensive three-dimensional numerical simulations show that tens-of-centimeter long filaments created by a 10.6-µm pump pulse can produce mJ energy yields and GV/m field strengths, while the laser-to-THz conversion efficiency exceeds the percent level. Changing temperature and humidity conditions reveals a surprisingly high stability of the THz spectra against different weather conditions. We also examine the role of many-body Coulomb effects on ionization. This additional plasma source promotes longer filaments that act as efficient THz emitters.