Sucking in air is going to be dependent on where it rides in relation to the surface. In other words, if you add more water to the tank/sump to cover it, then it won't suck air. Secondly, the boyancy of the pipe is dependent on how much air is going through it. If is is totally full of water, the skimmer will be lower. As soon as air is introduced it starts a snowball effect in that it will cause the skimmer to rise and suck much more air.
As water evaporates from the system, air entering it will increase.