you look at the spelling of whether, there is not "ea".
that is means it not the part of the pair.
trick: just think 3 of these words are the part of the love triangle. weather and whether are the boys and sea is a girl.
weather and sea has "ea" in common so they are in love and go together as part of atmosphere.
whether do not have "ea" in the spelling so it alternative for sea when weather is not available.
hope this will help you to remember which spelling to use!! 