The ornamental design for a weathersensor, as shown and described.
FIG. 1 is a front perspective view of a weather sensor showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof, wherein the broken lines in the Figures are for illustrative purposes only and form no part of the claimed design.