Air conditioning issues

I'm going to describe tis the best I can, so bear with me!

I had the aircon on driving home tonight and got the car nice and cold. Turned it off for about 10 minutes and turned it back on again and I had mist coming out of the vents!

Also it starts off blowing really well, then after 10 minutes it doesn't blow as well. It I turn the aircon off the fan picks up speed and the car gets nice and cold again, until it's used all the cold up.

Does it sound like the fluid needs topping up? If so why would the fan speed change when the aircon button is pressed?

Is this normal?? I have never had it on any other car but I have no idea what it could be. Any ideas?

Try to refill the gas and then check it.

I got that a couple of times, but it was very humid outside (90%+).

check if there is a problem with the thermostat or the coolant.
My uncle had the same issue before however, he hears a rumbling noise in the engine compartment after having the aircon turned on for about 10 to 15 minutes. He had to let a mechanic do the repairs because he wasnt that familiar with that particular problem either.