It is not advisable to pour concrete in the rain because it is possible that the rain can wash some of the cement out of the concrete. This can make the surface weak and potentially cause problems in the future, such as surface scaling.
Surface scaling occurs when the concrete surface becomes flaky, like the outside of a croissant, for example. This effect might not be noticed immediately. It is possible that surface scaling is not noticed until there is weight upon the concrete, such as people walking or the wheels of a bike. When this happens, the scale may break away or crumble. Since rain is sometimes unpredictable, a solution to a possible rain mishap is to get a one-year warranty on the work from the contractor who worked on it.