Diurnal shift is a fancy term that describes the variation in temperature from the highs during the day to the lows at night. This is a big deal for wine regions at high altitudes because they experience the most dramatic temperature swings. Big temperature variations affect the grapes because their exposure to sunlight increases the ripening qualities in the wine (i.e. more sugar), while the sudden drop in temperature at night preserves the balance of natural acids in the grape (i.e. lots of acidity in the wine).