This satellite measures whether there were clouds or not, because think about it: If there are clouds, then you might have some rain, but if there are no clouds, then it's actually impossible for it to rain.
この衛星は 雲の有無を測定します 考えてみてください 雲が出ていれば 雨が降るかもしれませんが 雲がなければ 雨が降るのは単純に不可能なのです