EXTREMELY HOT DAY can be described with two words: SIZZLER (7 letters) and SCORCHER (8 letters). A sizzler is a very hot day, usually referring to a day with temperatures above 90 degrees Fahrenheit. A scorcher is an even hotter day, usually referring to a day with temperatures above 100 degrees Fahrenheit. Therefore, EXTREMELY HOT DAY is an apt description for a day with temperatures above 100 degrees Fahrenheit.