It would be fine to say "the temperature outside is low", but the following suggestions are some good alternatives:
The first suggestion is a very common expression that suggests that the temperature is so cold that it could / is freezing things.
The second suggestion is simply another way to say that the temperature is very low, and including 'indeed' to suggest a very low temperature.
The third suggestion uses a very casual informal term 'brass monkeys', which is used to express cold weather / low temperatures.
"the temperature outside is low" でも問題ありませんが、次のように言うこともできます。
一つ目の例は非常によく使われる表現です。これは「凍えるように寒い」という意味です。
二つ目の例はシンプルに「とても寒い」という意味です。'indeed' は「寒さ」を強調します。
三つ目の例では非常にカジュアルな 'brass monkeys' という表現を使っています。これも「とても寒い」という意味です。
If you are trying to tell someone that the temperature outside is low, you can say something like "The temperature outside is low right now." or "Right now the temperature is low outside.".
「外の気温が低い」は次のように言えます。
"The temperature outside is low right now."（今、外の気温が低いです）
"Right now the temperature is low outside."（今、外の気温が低いです）
"It's cold outside." is a general way to describe the temperature outside as low and it feels cold. "Cold" is a broadly used word that is the opposite of "warm" or "hot".
"The temperature is low." is a less common way to describe the temperature, in English. We tend to rather use the less specific comment of "It's cold" however, this is also a correct term.
"It's freezing!". is a saying we often use when the temperature is very low and it feels like the air is as cold as ice.
"It's cold outside."（外は寒い。）は、外の気温が低くて、寒いことを表す一般的なフレーズです。 "Cold" は"warm" （暖かい）や "hot"（暑い）の対義語として使われます。
"The temperature is low."（気温が低い）は、気温のことを表現するそこまで一般的ではないフレーズです。 "It's cold" （寒い）のように、あまり具体的ではない表現を好む傾向にありますが、この表現でも大丈夫です。
"It's freezing!"（すごく寒い！）は、気温がとても低く、空気が氷のように冷たいことを表すよく使われるフレーズです。
As you can see, these three sentences have a lot of similarities with the main difference being between whether we use the word, "cold," or, "low," however these two words have the same meaning within this context. Secondly, notice how when we use the word temperature that is functions as the subject head of the sentence.
If you want to explain that the temperature (outside) you can use any of these sentences:-
1. It's extremely cold outside!
Example:- What is the temperture today?
It's extremely cold outside!
2. The tempersture is below zero outside
Example:- Do you think it's good weather to go shopping?
I don't think so, the temperature is below zero outside
気温が低いことを説明したいなら、例えば次のように言えます。
1. It's extremely cold outside!（外はすごく寒い）
例：
What is the temperture today?（今日の気温はどんな感じですか）
It's extremely cold outside!（外はすごく寒いよ）
2. The tempersture is below zero outside（気温が零度を下回っている）
例：
Do you think it's good weather to go shopping?（今日は買い物に良さそうな天気かな）
I don't think so, the temperature is below zero outside（いや、違うと思うよ。気温は零度以下だから）
You would like to explain that the temperature outside is low. In that case, any of the above suggested statements may be appropriate for your scenario:
"You'll need warm clothing today."
"There's a real bite in the air today"
外の気温が低いことを説明したいのですね。これらの例がぴったりです。
"You'll need warm clothing today."
（今日は温かい服が必要だよ。）
"There's a real bite in the air today"
（今日はとても寒いよ。）
When you want to express that the temperature outside is very low; then you may say it in the following ways:
-It's really cold outside.
-The temperature outside is very low
外の気温がとても低いことを伝えるときのフレーズです。
-It's really cold outside.
（外がとても寒い。）
-The temperature outside is very low
（外の気温がとても低い。）
If you wanted to explain that the temperature outside today is low, you could say any of the following to express this: "The temperature outside is low today", "It is very cold outside" or "It is a very cold day outside today".
「今日は気温が低い」は次のように言えます。
"The temperature outside is low today"（今日は気温が低い）
"It is very cold outside"（今日はとても寒い）
"It is a very cold day outside today"（今日はとても寒い）
These are different ways to say that the temperature is low outside. Below are ways that they could be used in a sentence.
I went outside today and the temperature was very low so I went to get my jacket.
Tomorrow the temperature will be low, so I can't wear my shorts.
He told me that it would be cold outside today, but we didn't have any low temperature.
「気温が低い」と伝える言い方です。
文の中では次のように使うことができます。
I went outside today and the temperature was very low so I went to get my jacket.（今日外に出たら寒かったので上着を取りに戻りました）
Tomorrow the temperature will be low, so I can't wear my shorts.（あしたは気温が低いから、短パンは履けない）
He told me that it would be cold outside today, but we didn't have any low temperature.（彼は今日寒くなると言っていたけど、全然気温は低くない）
こんにちは。質問ありがとうございます。
Temperature → 気温、温度
low → 低い
また、似たような状況で、下のような単語も使えるでしょう。
Cold → 寒い
cool、chilly → 肌寒い、ちょっと寒い
例
気温が低い日が続いている。
The temperature has been low recently.
We have been having cool days.
またの質問をお待ちしてます。
If the temperature falls below 32 degrees Fahrenheit or 0 degrees Celsius then we call these temperatures below freezing. They are below freezing because they are below the point at which water freezes. I hope that this helps. :)
I can't feel my hands, I didn't wear gloves because I didn't expect the temperature to be so low tonight
When we say "It's quite chilly outside" we mean that it is quite cold. Not quite freezing but almost.
"It's frigid out there": is a way of saying that it is bitterly cold. You don't want to be outside without a good winter coat, a pair of gloves and a hat.
"I can't feel my hands": is another way of saying that it is so cold that your hands have lost all feeling and they are numb. It is not a good idea to forget your gloves when it is so cold. Your hands could get frostbitten which is a very serious condition
"It's quite chilly outside"（外はかなり冷える）
これは、「"Freezing" ほどではないがかなり "Cold"（寒い）」という意味です。
"It's frigid out there"（外はとても寒い）
これは「ひどく寒い」という意味です。コートや手袋、帽子なしではいられないような寒さを表します。
"I can't feel my hands"（手の感覚がない）
これは「寒すぎて手の感覚がない」という意味です。寒いとき手袋をしないのは良くありません。手が "frostbite"（凍傷）になるかもしれません。"frostbite"（凍傷）は深刻な症状です。
Depending on how cold that you feel the outside temperature is, you can use the adjectives "cold" or "freezing."
"cold"
This is a general way to describe that you feel that the temperature is lower than you find comfortable. It does not say anything about the degree to which you are uncomfortable. It just tells the listener or reader that you are uncomfortable.
"freezing"
This adjective tells the reader/listener that the temperature is far below what you find comfortable. The adjective comes from the temperature in which water freezes. So you are literally saying that you think the temperature is so cold that you think water can freeze. The temperature does not actually have to be that low, but it lets the reader/listener know that you find it to be very cold.
We are experiencing freezing temperatures this winter.
Example:
It's too cold to go out today, I think I will stay warm by the fireplace.
The temperature is in the low 20s today - you can replace with any number that is suitable.
"freezing temperatures" can be used figuratively or literally.
例：
It's too cold to go out today, I think I will stay warm by the fireplace.（今日は寒くて出掛けたくない。暖炉の前にいるよ）
The temperature is in the low 20s today -（今日は気温が20度前半しかない）
※ この数字は適宜置き換えることができます。
"freezing temperatures" は文字通りの意味（氷点下の気温）でも比喩的な意味（凍えるような寒さ）でも使われます。
It is cold outside.
Outside is freezing.
Admittedly, these might be a bit more like slang, or informal speech, however, it is how most people would say it.
I hope this helps.
Have a great day.
Will
It is cold outside.（外は寒い）
Outside is freezing.（外はすごく寒い）
少しスラング的（カジュアル）かもしれませんが、たいていの人はこのような言い方をします。
参考になれば幸いです。
良い一日を。
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