Let's put some of the debate on this to bed....
There is only one way nominally pure water can freeze at atmospheric pressure: cooling it below 32 degrees Fahrenheit.
There is only one way the evaporator coil of an single speed ac unit goes below freezing: low side pressure too low.
Now, there are a couple of ways it can get too low, but 99.99% of the time, that is because there is insufficient freon in the system to maintain low side pressure after the compressor sucks away its draw.
Sometimes a system can operate seemingly normally for years with too low of a low side pressure, generally when there is enough warm air flowing over the coil to offset the below temp freon inside the coil and maintain a surface temperature of above freezing.
If airflow or dirty filters are causing the freeze up, your system is on the bubble, and is heading for a freon recharge.
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