Thank you for that very good question regarding the Aprilaire humidifier operation.
The answer to your question depends on the type of humidity control that was installed with the humidifier. In the event that the humidity control was the type that used an outdoor temperature sensor to regulate the humidity, then the humidifier would not add humidity in the summer months. this type of control would disable the humidifier when the outdoor temperature would go above 60 degrees. If the humidity control does not use the outdoor temperature sensor, then you may be able to humidify during the summer months. Just be aware that the air conditioning system will remove some humidity during its normal operation; which would take longer to bring the humidity level up.