The summer rains can take you by surprise and ruin your weekend plans or you can have a wonderful bright and dry day.
So people in Japan will say that autumn weather easily changes.
- 秋の気こうが変わりやすいです。(あきのきこうがかわりやすい です。) The autumn weather is unpredictable.
- よく変わる秋の天気 (よくかわる あきの てんき)often changing autumn weather
You can use the second phrase to compare something or someone. For instance, you can say that someone's opinion easily changes.
- あの人の意見は よく変わる秋の天気のようです。(あのひと の いけんは よくかわるのあきのてんきのようです。)
- That person's opinion changes like the autumn weather.