What would cause frost to form on the evaporator coil?

Frost formation on the evaporator coil is primarily caused by a lack of airflow across the coil. When there is insufficient airflow, the coil cannot effectively transfer heat from the refrigerant to the air being circulated by the system. This can lead to the temperature of the evaporator coil dropping below freezing, causing moisture in the air to condense and then freeze, resulting in frost on the coil.

Additionally, a lack of airflow could be due to various factors such as a dirty air filter, blocked ductwork, or a malfunctioning blower motor, all of which restrict proper air circulation.

The other options indicate conditions that do not typically lead to frost formation. Excess refrigerant can cause other operational issues like flooding of the evaporator but wouldn't directly cause frost if airflow is proper, while low humidity would reduce the likelihood of moisture condensing. High ambient temperatures usually mean increased evaporator coil temperatures, which would make frost formation unlikely as the coil would be unable to reach freezing temperatures under normal operating conditions.