Explanation

NCERT Class XI (Fundamentals of Physical Geography) states that in the middle latitudes, most diurnal (day-and-night) variations in daily weather are caused by advection alone. Advection is the horizontal transport of heat by winds: a place warms when winds arrive from warmer regions and cools when they come from colder ones. In mid-latitudes this horizontal movement of air masses — fronts, temperate cyclones, jet streams — dominates over local vertical convection or the simple day–night radiation cycle. Hence the answer is advection.