I claim the ornamental design for a rain hat, as shown and described.
FIG. 1 is a front perspective view of a rain hat showing my new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a side elevation view thereof;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a perspective view thereof.
The broken lines showing a partial human figure in FIG. 5 illustrate the environment in which the design is used, and assist showing how the claimed design appears when worn. The broken-line subject matter on the rain hat itself illustrates stitching, and is part of the claimed design.