Here the "e" doesn't refer to "sìde" (although I think it would also be fine to say "tha i fuar" meaning "it [the weather] is cold"), it's just an abstract point of reference. The choice of gender seems to be quite arbitrary; in Gaelic the masculine form happens to be used (in Ireland we also say "tá sé fuar", and our word for "weather", "aimsir", is also feminine), but in Welsh the feminine form is used: "mae hi'n oer", literally "she is cold", even though "tywydd" = "weather" is masculine. The same "e" also appears in other phrases not referring to the weather, e.g. "Tha e lethuair an deidh a naoi" = "It's half past nine"; "Tha e an urra riut fhèin" = "It's up to you", etc. Whatever the English "it" refers to in such phrases, the Gaelic "e" refers to the same thing.

Oh Simon, it's so perfect that you mentioned the website 'learngaelic.net'! That's EXACTLY where this question came from. I was studying the first lesson on weather. I was sure at one time I had learned "Tha i fluich / blàth / fuar etc". So I was confused when I saw "Tha e fliuch." I think it's good advice not to stress over anything too much in the beginning. Thanks, Simon!

Madainn mhath a Heather, I guess for the word translation to "it is" / Tha e/Tha i there's no specific rule as it can be either e or i Even on the resource https://learngaelic.net I found both variations for the word fliuch https://learngaelic.net/lg-beginners/jslessons/index.jsp?lesson=1#screen_13 https://learngaelic.net/sol/episodes/1/6.jsp So what I do not to mess my brain up is focussing on nouns and let verbs and adjectives just roll as they come out. I'm sure gaelic speakers will stil understand you. @Gaelic Teachers If I'm totally wrong please correct me