How do they know?

How do these dormant plants know when there has been rain if they are inside the greenhouse? This is such a bizarre thing. Dudleya pulverulenta looks shriveled and dried up all summer. Even though they are in a plastic greenhouse, getting regular water. As soon as we get the first rain (outside) they start growing and look succulent and lush. Totally weird! All the Ribes are popping green buds too.