A season is one of the four major divisions of the year, generally based on yearly periodic changes in the weather.
The four seasons
In temperate and polar regions generally four seasons are recognized: spring, summer, autumn, and winter. For spring starts out cold and ends warm, summer is warm, autumn starts warm and cools down, and winter is cold.
In some tropical and subtropical regions it is more common to speak of the rainy (or wet, or monsoon) season versus the dry season, as the amount of precipitation may vary more dramatically than the average temperature.
In other tropical areas a three-way division into hot, rainy, and cool season is used to determine the season. In some parts of the world, special "seasons" are loosely defined based upon important events such as a hurricane season, tornado season, wildfire season or a sport season.
Defining the seasons
The seasons result from the Earth's axis being tilted to its orbital plane; it deviates by an angle of approximately 23.5 degrees of an arc. Thus, at any given time during the summer or winter, one part of the planet is more directly exposed to the solar rays of the Sun. This exposure alternates as the Earth revolves in its orbit around the sun. At any given time, regardless of season, the northern and southern hemispheres experience opposite seasons and effects of sun angle on climate.
Seasonal weather fluctuations also depend on factors such as proximity to oceans or other large bodies of water, currents in those oceans, El Niño and other oceanic cycles, salt levels in ocean bodies, and prevailing winds.