Summer has come to an end and autumn is already upon us
It looks like summer is over now.
Summer time is over and autumn has begun - and you want to explain this. There are a couple of suggestions above. Of course, if summer is over, the following season is autumn, so it is clear that autumn has arrived - it need not be stated - see the second example. Stating the obvious is often seen as either 'boring' or unnecessary or time-consuming so should be avoided whenever a simple deduction may be made.
Autumn has begun.
Autumn has officially begun.
Fall is here.
Summer is fading into fall.
Summer is over and now it is fall.
Summer is over and now it is autumn.
Once again, summer leaves us and fall comes to visit.
Autumn is here at last.
Autumn has begun.
（秋が始まりました）
Autumn has officially begun.
（秋が正式に始まりました）
Fall is here.
（秋がやって来ました）
Summer is fading into fall.
（夏から秋に移り変わっています）
Summer is over and now it is fall.
（夏が終わって秋がやって来ました）
Summer is over and now it is autumn.
（夏が終わって秋がやって来ました）
Once again, summer leaves us and fall comes to visit.
（再び、夏が終わり秋がやって来ます）
Autumn is here at last.
（やっと秋がやって来ました）
Fall is another way of saying Autumn and is more often than naught used in the Americas. For example; I love the Fall weather, it's not to hot and not too cold.
"Summer's ended and Fall is here!" Here you explain that Summer is finished and Fall has started.
"Summer's left and Autumn is here!" You can used left for a season, it personifies it (gives it a human characteristic).
"Fall" は "Autumn"（秋）の別の言い方で、アメリカでよく使われます。
例えば：
I love the Fall weather, it's not to hot and not too cold.（秋の天気が暑すぎず寒すぎずで大好きです。）
"Summer's ended and Fall is here!"（夏が終わって秋がやって来ました）
- ここでは、夏が終わって秋が始まったと伝えています。
"Summer's left and Autumn is here!"（夏が去って秋がやって来ました）
- 季節についても "left"（去った）を使うことができます。擬人化です。
"seasons are changing"
This can be used to describe any season that is finishing and another starting, 'season' is used to describe summer,winter,autumn and spring.
"Summer is over and autumn is now beginning"
'summer' referenced to the season is 'over'(finished), 'beginning is referencing something that has begun, the star of.
"Seasons are changing"（季節が変わっている）
「季節が変わっている」という意味です。これはどの季節にも使えます。
'season' は夏や冬、秋や春のこと。
"Summer is over and autumn is now beginning"（夏が終わって、秋が始まっている）
'over' は「終わっている」の意味です。'beginning' は、それが始まっていることを表します。
In American English, we almost always use the word "fall" to refer to the season that follows summer. To say that something is "finally here" means that you have been waiting for it and you are probably excited about it. I hope that this helps! :)
The leaves are turning orange, must mean summer's over.
There's a chill in the air, looks like autumn is on it's way
"The leaves are turning orange" - When autumn occurs the leaves turn orange or brown, therefore it's a sign that summer is ending.
"There's a chill in the air, looks like autumn is on it's way" - Again, having colder temperature is a sign that summer is ending. And we can say that "Autumn is on it's way" as if it's a person who will be visiting. The seasons can all be referred to as if they are people, each with their own unique personalities e.g. Spring's bringing life with her.
"The leaves are turning orange"（木の葉が色づいてきた）
- 春が来ると木の葉の色がオレンジや茶色に変わります。なので、これは夏が終わるサインです。
"There's a chill in the air, looks like autumn is on its way"（空気が冷たい。秋が近いですね）
- 同様に、気温が下がることも夏が終わるサインです。"Autumn is on its way"（もうすぐ秋が来る）と、あたかもそれが人であるかのように表現することができます。季節はどれも擬人化して表すことができます、そしてそれぞれの季節にはその性格があります、例えば：
Spring's bringing life with her.
（春は命を運んでくる）
When you want to explain that summertime is over and autumn has begun, then you may say this in the following ways:
-Summer is over and autumn has begun.
-Summer is over and it is now autumn.
-It's officially autumn.
夏が終わって秋が始まったことは、次のように説明できます。
-Summer is over and autumn has begun.（夏が終わって秋が始まりました）
-Summer is over and it is now autumn.（夏が終わって秋が始まりました）
-It's officially autumn.（正式に秋が始まりました）
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