Street sweepers typically don't sweep during heavy rains. They may still sweep if it's only light or sporadic rain with no flowing water in the gutters. But the reason why might surprise you. Street sweepers clean the street with one of two mechanisms... either a broom or a vacuum. Similar to how you can clean your house with a broom or a vacuum, a street sweeper works the same way... just at a MUCH larger scale. The challenge when it it comes to rain for street sweepers is that vacuum machines will vacuum up a lot of the rain water while they're sweeping. This is totally fine for the sweeper, however, the street sweeper has limited capacity in its storage hopper. If the sweeper vacuums up too much water, the driver will need to take the sweeper to dump at a designated area. The water takes up space that should be there for the street sweepings. Because their dump areas may be a long way away from where the sweep is taking place, the sweeper would waste a lot of time driving back and forth. Broom street sweepers have a different problem when it comes to the rain. Broom sweepers don't pick up nearly as much water as a vacuum sweeper, however, the rain makes debris stick to the ground. Broom sweepers are very effective at sweeping up trash and debris but it becomes much harder in the rain. Similar to trying to sweep your patio when its raining. You can likely sweep up the big materials but the small stuff is likely to stick to the ground. So there's why street sweepers typically don't sweep in the rain. They're able to do it but it's just not as effective.