"There is no frost."
Translation:Chan eil reòthadh ann.
It probably be understood that way, but it’s ungrammatical nonetheless, reòthadh is a noun, act-of-freezing, frost, so it cannot be a predicate of tha or chan eil on its own.
That structure would need an adjective, eg. reòta frozen, frosty or maybe reòthanach but I think tha i reòta would rather mean she, it, is frozen, and not sure if reòthanach is used as frosty with regard to weather (Colin Mark’s dictionary doesn’t even list it).
If you mean the breve accents (the ` over the letters) that mark long vowels, they come most often on the vowel of the first syllable – in Scottish Gaelic only stressed vowels can be long and the stress always goes on the first syllable.
Compound words might be exceptions – the stress might go on the second part of the compound and it might retain its long vowel.