暑 - Weather's So Hot, Sun Feels Like It's Right On Top Of Me

Readings & Meanings:

あつ（−い）

しょ

heat, summer heat, hot, HOT WEATHER

Common Usage:

書中

蒸し暑い

Examples:

暑中お見舞い申し上げます。

関西の夏は蒸し暑い

Translations:

I hope you're doing well this summer. (Traditional phrase for the beginning of a "mid-summer card".

Kansai summers are hot and humid (sticky/wet/steamy/etc.).

Important:

Again, as in the case of 暖 and 温, the difference between 暑 and 熱 comes down to weather vs. touch.

暑 is for weather and intangible hotness. Just look at the kanji: there's a 日 (sun) bearing down over a 者 (person). The other kanji doesn't even have a 日 component! Easy!

References:

http://www.jlptstudy.com/forum/viewtopic.php?=&p=1110
http://www.thejapanesepage.com/kanji/kanji/a.htm