Seoul is classed as having a temperate climate with four distinct seasons, but temperature differences between the hottest part of summer and the depths of winter are extreme. In summer the influence of the North Pacific high-pressure system brings hot, humid weather with temperatures soaring as high as 95°F (35°C) on occasion. In winter the city is topographically influenced by expanding Siberian high-pressure zones and prevailing west winds, and temperatures drop as low as 7°F (-13.7°C). The bitterly cold days tend to come in three-day cycles regulated by rising and falling pressure systems, however, bringing some relief. The most pleasant seasons in the city are spring and autumn, when azure blue skies and comfortable temperatures are a sure bet. The city experiences a heavy rainfall, but most of it falls in the summer monsoon period between June and September.