Sandstone is sand glued together by a natural cement: clay, calcite, iron oxide or silica. In the desert Southwest that glue is often thin. Rain soaks into the rock like a sponge, softens the clay, dissolves weak cements and greases the grains. The surface can look bone-dry while the inside is still soaked, which is exactly when holds go.
How much weaker?
It depends on the rock. In lab tests on 35 sandstones, soaked samples lost anywhere from 8% to 78% of their strength, mostly depending on how much quartz and clay they held. Some hard, quartz-rich stone barely notices. Soft, porous western sandstone, like Red Rock's Aztec and Zion's Navajo, is near the bad end. A newer study from Newcastle, Australia found even short wet-dry cycles weaken solid sandstone over time.
"Mostly dry" isn't dry. Most of the weakening happens in the first sliver of moisture.
What a broken hold does to a climb
A snapped crimp doesn't grow back. The move gets harder, the grade creeps up, the beta changes for everyone, and sometimes a classic turns into choss. On sport routes a missing hold can change where you clip. On boulders it can erase the problem entirely.
How to tell if it's ready
Wait at least 24–48 hours after rain, and three to four days in a Red Rock winter. Dig an inch or two into the dirt at the base: if it's damp, the rock is too. Shady, north-facing and seeping walls stay wet longest. When in doubt, go pull on limestone or granite for the day, or check Can You Send for the call at your crag.