Well, it's 102 degrees out. When I leave my car idle the water gauge goes up to almost 1/2 way, then the fan comes on and it goes back to normal. It doesn't go up while driving it stays normal. Only while sitting idle it goes up.

 

Yes, because when your car is sitting still, it does not get air flow around the rad to cool the water. Also, the heat from the motor is heating the water cooling it, and in the rad. But when driving the air flow is there to keep the rad cool, and to blow the heat of the engine away from the rad.