How is weather/season related to the action? If Gurov/Anna's affair heats up in Yalta's hot sun, why doesn't it cool down, in Moscow winter's snow?

Author's often use weather or seasons to symbolize the dominant themes of their work.  It is often the case that a passionate relationship will fade over time, and what seemed exciting and new at first becomes commonplace.  Thats the cooling off.  Either the couple will continue to find new things to enjoy about one another, or they will separate.  However, if the separation is forced, as it was here, it is easy to forget that feeling of familiarity and to be excited again when reunited.  The weather symbolizes the two stages most literally.  Hot for the passion, cold for the lack of continuing interest beyond the excitement and lust.