I heard that on Lag Baomer rain is a good Siman (omen) and it helps bringing a Refua Shleima to the ill. They advise that one in need should stand on the rain with his open mouth at that time. Is that for real?

Indeed surprisingly, some Sefarim mention the above, Segulos Yisroel (3: 34), Imrei Pinchos 35, Nitei Gavriel (42: 6).