"There is lightning."
Translation:Tha dealanaich ann.
Dealanaich looks like the plural or genitive of dealanach (which it is), so I guess that originally dealanach was 'a bolt of lighting' and dealanaich 'a number of bolts' or used in some expression that translates as 'a bolt of lightning'. Mark give dealanach but says used in the form an dealanaich in Lewis. This suggests that they started using the plural as singular in Lewis. Which seems more common will depend on where you come from. Dealanach is more common on the BBC (in a search) including being introduced to children in an episode of Peppa Pig. Most hits for dealanaich turned out to genitives.