Why does a heat pump freeze up in summer?

A HEAT PUMP IS AN A/C UNIT OPERATING IN THE REVERSE MODE. THE OUTDOOR UNIT BECOMES THE EVAPORATOR. THE OUTDOOR UNIT IS MOVING HEAT REMOVED FROM THE OUTSIDE AIR TO THE INDOOR AIR USING THE REFRIGERATION CYCLE. TO MAKE THE CYCLE WORK, WE HAVE TO "EVAPORATE" THE REFRIGERANT AT A TEMPERATURE BELOW THE OUTSIDE AIR TEMPERATURE. THIS TEMPERATURE IS MORE TIMES THAN NOT, BELOW FREEZING, HENCE THE FORMATION OF ICE. OPERATION IN THE "HEAT" MODE, BUILDING ICE, REQUIRES THE USE OF A DEFROST CYCLE TO REMOVE THE ICE EVERY SO OFTEN. LC