Смотря где. :D

Да Arhi, опять пьяные китайцы вмешались в процесс обучения. "Chilly" - никак не прохладно, а "холодно"! "Прохладно" ближе к "cool"

It is not the same thing (althought one could argue that it is not such big of a difference). Cold vs chilly

Not quite, though the word indeed uses a prefix. It is про + хладный (an obsolete version of холодный).

Some words came from Church Slavonic and entered Russian. Such vowel changes should not suprise you. The following words are essentially descendants of the same word: one has East Slavic origin, anothe Church Slavonic one:

• голова "head" / глава "chapter"
• город "city" / -град (mostly used as a component of some toponyms)
• молодой "young" / младший "younger" (e.g., in "little brother")
• порох "powder" / прах "ashes, remains (of a person)"
• голос "voice" / гласный "vowel"
• сторож "guard, watchman" / страж "guardian"