The first reason is that the clouds have disappeared. When rain formed in clouds falls through the sky, it takes a long time for the rain to fall to the ground. In the middle of that long time, the clouds are swept away by the wind and disappear, so that when the rain falls to the ground, the sun is already out.
The second reason is that the rain has been washed away. This is because the rain particles formed in the sky are swept away by strong winds and fall to the ground just as the sun is rising.
The third reason is that small clouds have made it rain. This is a case where there is a small rain cloud in a large part of the sky and rain is falling from it. Because it is a small cloud, the sun is naturally out around it.
Thus, these three reasons are considered for weather rain.