Walking outside while it is raining:[1]
One who is walking through a public area and rain falls on him and on his clothes, the Sages were not stringent on [his walking of four cubits with the drops of water on him].
The reason for this is: because this is a matter which is impossible to avoid, as at times a person is walking in a public area [with clear skies] and it suddenly begins to rain on him.
To initially walk outside when it is already raining: Therefore the Sages permitted [him to walk with the rain water on him] in all cases (and he may walk outside even in a case that he sees that it is already raining outside, as we do not prohibit matters partially.)