noun
day with a high of 30°C or above; tropical day
A meteorological term for a day when the maximum temperature reaches 30°C or higher. Often used in summer weather reports. Compare with 夏日 (25°C+) and 猛暑日 (35°C+).
今日は真夏日になる見込みです。
Today is expected to be a day with a high of 30°C or above.
今年初めての真夏日を記録した。
We recorded the first tropical day of the year.
猛暑日 is for a day with a maximum temperature of 35°C or above, a more extreme heat level than 真夏日.
Compound of 真夏 (midsummer) and 日 (day), referring to a day with midsummer-like heat.