1. thirty six point five degrees Celsius
2. thirty-six point five degrees Celsius
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Q1. Which one is right?
Q2. In which case do put a hyphen(-) between numbers?
The hyphen is needed in spelling out numbers like "thirty-five" or "twenty-two", etc. Usually, in a sentence, we use the numerals, 36.5, if the number is above ten, although that rule is flexible. We might spell out "eleven" or "twelve" and it would be acceptable. At the beginning of a sentence, we spell out the numbers no matter what they are. "Thirty-six point five degrees Fahrenheit is optimal for refrigerating milk." "Milk keeps best at 36.5 degrees Fahrenheit."
1. "Thirty six point five degrees Celsius" is the correct way to write it.
2. You would put a hyphen between numbers when you are writing them as a compound adjective modifying a noun. In the case of temperature, you typically don't use a hyphen unless it is necessary for clarity. However, if you were referring to a temperature range, such as "thirty-four to thirty-six degrees Celsius," then you would use a hyphen between the numbers because they are being used as a compound adjective to modify the word "degrees."