Although the seasons are based on astronomical events, the weather changes that are experienced during the seasons do not occur with the mathematical precision usual for most astronomical events. The day of the winter solsticeabout December 22has the shortest period of daylight and the least effective solar rays of the whole year in the Northern Hemisphere. It is rarely the coldest day of the year, however; the coldest days usually occur sometime in late January or in February. It takes time for the land and the oceans to cool after summer, and until they do, they warm the air somewhat, thus tempering the winter weather.
Similarly, the warmest weather usually occurs more than a month after the summer solsticeJune 21when the land and the oceans have been warmed to the maximums.
Around the time of the equinoxes, the hemisphere that has been cold begins to warm and the hemisphere that has been warm begins to cool. The atmosphere becomes somewhat unstable and the weather begins to fluctuate with great frequency. Thus during the spring and autumn, rains, high winds, and generally changeable weather conditions are common in many areas. The characteristic conditions of spring tend to move outward from the Equator as the sun does. Spring weather, therefore, may occur in some areas before the official opening of spring and later in other areas. Autumn weather may also begin before or after the autumnal equinox.