The rhyme:

"When windows won't open, and the salt clogs the shaker, The weather will favour the umbrella maker!"

The reason:

There is also some truth in this. Moisture in the air causes wood to swell, making doors and windows hard to open. Salt is also a very effective absorber of moisture so it clumps together with it and won't leave the shaker.

With a high level of moisture in the air, the likelihood of precipitation is increased, and there's a good chance it will indeed rain.