Seasons and seasonality are the main properties of extra-tropical climate that affect ecosystems and society. For example, agriculture, tourism, energy consumption or ecosystem phenology are primarily dependent on seasonality and on the magnitude of the meteorological events associated within each season. Changes in the seasonality of variables like surface temperature during the last decades have been widely investigated but seasonal changes of the weather have received less quantification. This paper redefines the concept of seasonality based on the extra-tropical atmospheric circulation, and on the notion that it can drive the evolution of temperature. We find that summer-like atmospheric conditions have appeared earlier and ended later since 1948. Conversely, the period with winter patterns has reduced over that period. The temperatures associated with weather patterns allow to identify the sources of temperature trends.