By analyzing the breakdown threshold obtained from effective magnitude or RMS (root mean square) of microwave field, it is pointed out that the assumption of threshold is not suitable for microwave air breakdown. Variations of effective electron temperature and breakdown threshold in microwave fields, which are studied theoretically and numerically by fluid-based plasma equations coupled with the Maxwell equations, are compared with those in static fields. It is found that the effective electron temperature varies greatly with the microwave field at high pressures (electron energy transfer frequency is much larger than microwave frequency) due to its high energy transfer frequency. This causes the microwave air breakdown threshold to be smaller than that obtained from the RMS of field at high pressures because the ionization frequency varies faster than electron energy loss at an effective electron temperature. On the other hand, the effective electron temperature variation with the microwave field is insignificant at low pressures (electron energy transfer frequency is much smaller than microwave frequency) due to the negligible electron energy loss in a microwave period. In this case, the microwave air breakdown threshold is approximately equal to that obtained from the RMS one of the field. The fit formula of microwave air breakdown threshold is obtained by numerical and theoretical analysis.