Hello and thank you for choosing Just Answer.
Have you check the power steering fluid?
And the power steering belt?
Are you getting any type of noises or is the car pulling to any side?
Well the rain really does not get under the hood. Rain would not get on the master cylinder, the hoses, the belt. I am just gathering information right now to figure out what has happen this far. With that said, the only place where rain would actually get to is the rack. You may want go get under the car while it hot out. Spray the rack down with some wd-40 and see if steering is still stiff or if that helps.
From you description it sounds like tight steering gear adjustment. You may want to see if a shop can adjust it for you or if the rack needs work.