Learn Farsi: آب در صد درجه سانتی‌گراد می‌جوشد. - Water boils at one hundred degrees centigrade.

You would use ممیز (momayyez) for the decimal point. To say "100.5 degrees", you would say صد ممیز پنج درجه (sad momayyez panj darajeh). In very formal or scientific contexts, you might also hear it read as صد ممیز پنج دهم (sad momayyez panj dahom), which explicitly states "five tenths".

Why is the preposition در (dar) used to mean "at" one hundred degrees?

While در (dar) most commonly translates to "in", it is the standard preposition used in Farsi for specific points on a scale, such as temperature, altitude, or speed. English uses "at" for these points, but translating that literally would sound unnatural in Farsi.

A quick refresher: why is درجه (darajeh) singular when it translates to the plural "degrees"?

In Farsi, nouns that follow a number are always kept in their singular form. No matter how large the number is, the counted noun never takes a plural suffix like ها (-hâ). Therefore, it must be صد درجه (sad darajeh).

I have previously seen جوشاندن (jushândan) used for "to boil". Why is the verb می‌جوشد (mi-jushad) used here instead?

Farsi uses two different verbs for boiling depending on whether an action is happening to something or by someone. جوشیدن (jushidan) is intransitive, meaning the subject itself is boiling (as the water is doing here). جوشاندن (jushândan) is the causative, transitive form; you use it when someone is causing a liquid to boil (e.g., "I boil the water").