From reading other comments on similar posts, "tha i X" is used when using an adjective to describe the weather e.g wet or warm. When describing the weather with a noun (frost, thunder, lightning) we need to use "tha X ann".
Why isn't it "Tha i reòthadh"? In other words, when is it "Tha i (weather)" and when is it "Tha (weather) ann"?
Because reòthadh is a noun ("frost"), not an adjective ("frosty"). "There is frost", not "It is frost."
same question as Conrad below?