How do cacti get water in the driest desert if it doesn't rain for up to two years?

Nights are a lot cooler and the ground and the cactus cools off at night. Moisture in the air condenses on the cool cacti and the ground. It doesn't take much moisture because the plant swells up and stores the water. Notice how if you water a cactus, it gets fat and when it dries out it wrinkles, yet is alive.