Low-level clouds are thought to matter more than high-level ones because they are more prevalent and because they are better at reflecting solar heat away from the Earth than they are at trapping it, blanket-like, as high clouds do.
And in a paper published two years ago in Geophysical Research Letters, Sandrine Bony and Jean-Louis Dufresne reported that an analysis of 15 climate models suggested that low-level clouds over the oceans contribute most to uncertainty about how tropical clouds affect those models.