An investigation is made of the threshold for microwave breakdown in air in situations where the microwave power is strongly modulated or stochastically varying in time as, e.g. in communication systems based on multi-carrier operation where interference between the carriers may cause occasional high power peaks in the microwave power. Thresholds are established for the scenario of coherent and co-phased carriers as well as for breakdown in an electric field with a stochastically varying amplitude.