[elhigh]

AC systems freeze for a few reasons.

1) The refrigerant levels are low or else there is a restriction in the system. When levels are low the pressure drop on the suction side pulls the evaporation temperature of the refrigerant below the freezing point of water. If there is a restriction in the system the symptoms are similar, the compressor pulls pressures way down on the low pressure side of the circuit but cannot push more refrigerant in from the high side.

Causes: Low refrigerant levels, blockage in the plumbing or (more likely) the filter-drier canister. You can get the cheap pressure gauge/top-off can at the auto parts store or take it to a tech for a more thorough investigation.

2) Insufficient air flow. There has to be enough air flowing across the coils to blow enough heat onto them to prevent them frosting over.

Causes: air filter blockage, air duct blockage, blower running too slowly. These are the kinds of things that are easy for a shadetree mechanic to address on his own.

I live in the southeast and a LOT of what I do is HVAC repair. It's hot and muggy here 8 months out of the year, and just muggy the rest of the time. Humidity is the source of the frost but it is never the cause.