Learn Hardcore Japanese: asita mo kion ga takai desyou kara, eakon wo tukeru to omoimasu. - The temperature will probably be high tomorrow too, so I think I will turn on the air conditioner.

気温が高い – “the air temperature is high”
- More objective / neutral.
- Refers to the measured temperature (what a thermometer would say).
暑い – “hot” (feels hot)
- More subjective, how people (or animals) feel.

In a sentence talking about temperature in a somewhat neutral, weather-like way, 気温が高い is natural:

明日も気温が高いでしょう。
“The temperature will probably be high tomorrow as well.”

If you focus more on how it feels to people, you might say:

明日も暑いでしょう。
“It will probably be hot tomorrow as well.”

Both are possible, but 気温が高い sounds a bit more “weather report / objective”.

What exactly does でしょう mean here?

でしょう after an adjective or verb often expresses:

guess / probability (like “probably / I suppose / I expect”)
in a relatively polite way.

In 気温が高いでしょう, it means:

“The temperature will probably be high.”
The speaker is not stating an absolute fact but giving a reasoned prediction.

Compared with other forms:

気温が高いです。 – “The temperature is high.” (plain statement)
気温が高いでしょう。 – “The temperature will probably be high.” (prediction / assumption)

So でしょう adds uncertainty + politeness.

What is the role of から at the end of 明日も気温が高いでしょうから?

から is a conjunction meaning “because / since”.

Here, 明日も気温が高いでしょうから is a reason clause:

明日も気温が高いでしょうから、エアコンをつけると思います。
“Since it will probably be hot again tomorrow, I think I’ll turn on the air conditioner.”

Structure:

[Reason] から, [Result].

Japanese often puts から at the end of the reason clause:

明日も気温が高いでしょう (it’ll probably be hot tomorrow) から
(because)
→ because it’ll probably be hot tomorrow…

Why is 高い (present) used to talk about the future? Shouldn’t it be something like 高くなる?

Japanese non-past form (dictionary form / -い form for adjectives) covers both:

present and
future.

So:

気温が高い can mean “the temperature is high” or “the temperature will be high”, depending on context.

Here, 明日 (“tomorrow”) clearly indicates future, so 高い naturally gets a future reading.

You could say:

明日、気温が高くなるでしょう。
“Tomorrow, the temperature will become high.”

But that focuses more on the change (“it will rise / become high”).
気温が高いでしょう just states the future state (“it will be high”).

Who or what is the subject in 気温が高いでしょう? There’s no “it”.

In Japanese, subjects are often omitted if they are obvious from context.

The grammatical subject here is:

気温 = “(the) temperature”

So the sentence structure is:

明日も [気温が高いでしょう] から、エアコンをつけると思います。

Literally:

“Because [the temperature] will probably be high tomorrow as well, I think [I] will turn on the air conditioner.”

English needs an “it”: “it will probably be hot”, but Japanese does not; 気温 itself is the subject.

Why is it エアコンをつける and not エアコンがつく?

This is about transitive vs. intransitive verbs:

つける (transitive) – “to turn something on / attach something”
- Needs an object marked by を.
- エアコンをつける = “turn on the air conditioner.”
つく (intransitive) – “(something) goes on / is turned on”
- Describes what happens to the thing itself.
- エアコンがつく = “the air conditioner comes on.”

In the original sentence, the speaker is talking about their own action:

エアコンをつけると思います。
“I think I’ll turn on the air conditioner.”

So they use the transitive verb つける with を.

Why is つける in dictionary form before と思います instead of つけます?

With と思います (“I think (that) …”), the part before と is treated like a quoted sentence.

Japanese normally uses the plain (dictionary) form for the content inside a quote, even in polite speech.

So:

エアコンをつけると思います。
Literally: “I think, ‘I will turn on the air conditioner.’”

Compare:

エアコンをつけます。 – plain polite statement: “I will turn on the air conditioner.”
エアコンをつけると思います。 – “I think I will turn on the air conditioner.”

Inside [ 〜と思います ], verbs and adjectives almost always appear in plain form:

行くと思います。 (not 行きますと思います)
高いと思います。 (not 高いですと思います)

What is the difference between でしょう and と思います in this sentence? Aren’t both “I think”?

They are related but not the same:

でしょう – expresses the speaker’s supposition / probability about something, often somewhat impersonal:
- 気温が高いでしょう。
  “The temperature will probably be high.”
と思います – explicitly says “I think …”, pointing more clearly to the speaker’s personal thought:
- エアコンをつけると思います。
  “I think I’ll turn on the air conditioner.”

In the full sentence:

明日も気温が高いでしょうから
The weather / temperature is being predicted (probability).
エアコンをつけると思います。
The speaker’s own action is presented as what they expect/plan.

You could also phrase it as:

明日も気温が高いと思うので、エアコンをつけます。
“Since I think it’ll be hot again tomorrow, I’ll turn on the AC.”

There, と思う is applied to the temperature instead of the action.

Why is the particle が used with 気温 instead of は?

が and は both can mark something like a “subject,” but they have different nuances.

Here:

気温が高い – “(the) temperature is high”, presenting 気温 as the thing whose state is being described in that clause.

Using は:

気温は高いでしょう。
This can sound more like setting 気温 as a topic and then commenting on it:
“As for the temperature, it will probably be high.”

In a weather-like, neutral description, 気温が高いでしょう sounds very natural.
気温は高いでしょう is not wrong, but が here fits the pattern of describing the state of something that is not already a big discourse topic.

Could I split this into two sentences like: 明日も気温が高いでしょう。だから、エアコンをつけると思います。?

Yes, that is perfectly natural.

明日も気温が高いでしょう。だから、エアコンをつけると思います。
“It will probably be hot again tomorrow. Therefore, I think I’ll turn on the air conditioner.”

Differences:

Original:
明日も気温が高いでしょうから、エアコンをつけると思います。
– One sentence, with から linking cause → effect more smoothly.
Alternative: two sentences with だから:
– Slightly more emphatic / step-by-step: first give the fact, then explicitly say “therefore”.

Both are common and correct; it’s largely a style choice.

What exactly does エアコン mean? Is it only “air conditioner”?

エアコン is a Japanese loanword from “air conditioner.”

Usually it refers to the room air conditioning unit (both cooling and, in Japan, often also heating).
It does not normally refer to a simple fan. A fan is:
- 扇風機（せんぷうき） for a standing/table fan.
In cars, エアコン can also mean the car’s AC / climate control system.

So in this sentence, エアコンをつける is “turn on the air conditioner.”

What is the politeness level of this sentence? Is it casual or polite?

The sentence is in polite form, thanks to:

でしょう (polite form of だろう / だ)
思います (polite form of 思う)

So overall it’s appropriate for:

talking to people you aren’t very close to,
or in neutral / everyday polite conversation.

A casual version might be:

明日も気温が高いだろうから、エアコンをつけると思う。

More formal/polite could be, for example:

明日も気温が高いでしょうから、エアコンをつけようと思います。
(using つけようと思います, “I’m thinking of turning it on / I intend to.”)

Could I say 明日も気温が高いと思いますから、エアコンをつけます。 instead? How does it differ?

Yes, that sentence is also natural:

明日も気温が高いと思いますから、エアコンをつけます。
“Since I think it will be hot again tomorrow, I will turn on the air conditioner.”

Key differences in nuance:

明日も気温が高いでしょうから…
- Emphasizes an impersonal-looking prediction about the temperature.
- Feels like a general statement: “It will probably be hot…”
明日も気温が高いと思いますから…
- Emphasizes your personal opinion / belief about the temperature.
- Feels a bit more like “because I think it will be hot…”
エアコンをつけると思います vs エアコンをつけます:
- つけると思います = “I think I will turn it on” (slightly tentative).
- つけます = “I will turn it on” (more decisive).

So your alternative is a bit more like:

“Because I think it will be hot, I will (definitely) turn on the AC.”
while the original is more:
“Since it’ll probably be hot, I think I’ll turn on the AC.”