First it must be understood that the atmosphere is constantly in a state of thermal inversion. That is to say normally warmer(lighter) air will rise to the top of the mix. However, the atmosphere is heated from the bottom up because the suns energy warms the surface. As you go up the temperature drops at a rate of about 1.5 to 2 degrees c. per thousand feet. As the warm moist air from the low levels rises, it is cooled to saturation (100% R.H.) and then to the point where liquid particles form fog. This high altitude fog is what is commonly referred to as clouds. They start by various processes that cause "lift" such as the convergence of winds head on, thermal convection, or wind being driven up a ramp topography like a mountain.