You can limit the area a selection affects. For example, in a picture of snow-capped mountains, you can select white clouds in the sky without selecting parts of the white mountain below them by selecting the entire sky, and then using the Magic Wand tool with Intersect With Selection selected and Contiguous deselected to select only the white areas included within the existing sky selection.
Select Intersect With Selection in
the options bar, and select an area that intersects the existing
selection.
Hold down Option+Shift so that cross hairs appear next to the pointer, and select an area that intersects the existing selection.