I don't claim that the connection between the pyramids and rain is well attested. If it were, there would be little point in my writing a paper on this subject. However, the connection between the pyramids and the heb sed is attested (see Hassan, 1960). The connection between the heb sed and rain is attested (see Petrie, 1925). By this association, the connection between the pyramids and rain is established.

Hassan:
[archive.org]

Petrie:
[archive.org]