The passing of a year can bring a marked change in the weather and the surrounding environment. The four seasons — winter, spring, summer, autumn — can vary significantly in characteristics, and can prompt changes in the world around them. Let's take an overview of these four separate periods.
The annual cycle
Attributes of the seasons may vary by location, but there are still broad definitions that cross most of the boundaries.
In the spring, seeds take root and vegetation begins to grow. The weather is warmer, and often wetter. Animals wake or return from warmer climates, often with newborns. Melting snow from the previous season, along with increased rainfall, can cause flooding along waterways. [Related: 6 Signs Spring Has Sprung]
In the summer, temperatures may increase to their hottest of the year. If they spike too high, heat waves or droughts may cause trouble for people, animals, and plants. Rainfall may increase in some areas. Others may receive less water, and forest fires may become more frequent. [Gallery: Stunning Summer Solstice Photos]
In the autumn, or fall, temperatures cool again. Plants may begin to grow dormant. Animals might prepare themselves for the upcoming cold weather, storing food or traveling to warmer regions. Various cultures have celebrated bountiful harvests with annual festivals. [Gallery: A Rainbow of Fall Leaves]
Winter often brings a chill. Some areas may experience snow or ice, while others see only cold rain. Animals find ways to warm themselves, and may have changed their appearance to adapt. [Gallery: Images of Stunning Snowy Landscapes]
Location, location, location
The timing and characteristics of the seasons depends upon the location on Earth. Regions near the equator experience fairly constant temperatures throughout the year, with balmy winters barely discernable from warm summers.
But for areas to the north and south, the seasons can change more significantly. People closer to the poles might experience icier, more frigid winters, while those closer to the equator might suffer hotter summers. Other factors can also affect the weather and temperature over the seasons; some areas experience dry summers as temperatures spike, while others might call summer their "wet season," experiencing higher than average rainfall. Mountainous regions might experience more snowfall than plains within the same latitude, while oceanfront property could see an increase in violent tropical storms as the weather shifts.
The time of year a region experiences a season depends on whether it is in the northern or southern hemisphere. The Southern Hemisphere experiences winter while its northern neighbors chart summer; the north sees the slow blossom of spring while the south brings in the autumn harvest.
Earth and the sun
The cycle of seasons is caused by the Earth's tilt toward the sun. The planet rotates around an (invisible) axis. At different times during the year, the northern or southern axis is closer to the sun. During these times, the hemisphere tipped toward the star experiences summer, while the hemisphere tilted away from the sun experiences winter.
At other locations in Earth's annual journey, the axis is not tilted toward the sun but instead along the planet's path, parallel to the star. During these times of the year, the hemispheres experience spring and autumn.
The astronomical definition of the seasons relate to specific points in Earth's trip around the sun. The summer and winter solstice, the longest and shortest day of the year, occur when the axis of the Earth is either closest or farthest from the sun. The summer solstice in the northern hemisphere occurs around June 21, the same day as the winter solstice in the southern hemisphere. The south's summer solstice occurs around December 21, the winter solstice for the north. In both hemispheres, the summer solstice marks the first day of astronomical summer, while the winter solstice is considered the first day of astronomical winter.
Equinoxes are another significant day during Earth's journey around the sun. On these days, the planet's axis is pointed parallel to the sun, rather than toward or away from it. Day and night during the equinoxes are supposed to be close to equal. The spring, or vernal, equinox for the northern hemisphere takes place around March 20, the same day as the south's autumnal equinox. The vernal equinox in the southern hemisphere occurs around September 20, when people in the north celebrate the autumnal equinox. The vernal equinox marks the first day of astronomical spring for a hemisphere, while the autumnal equinox ushers in the first day of fall. [INFOGRAPHIC: Earth's Solstices & Equinoxes Explained]
But changes in the weather often precede these significant points. The meteorological seasons focus on these changes, fitting the seasons to the three months that best usher them in. December to February marks meteorological winter in the Northern Hemisphere and meteorological summer in the southern. March, April, and May are lauded as spring or autumn, depending on the location, while June through August are the months of summer for the north and winter for the south. September, October, and November conclude the cycle, ushering in fall in northern regions and spring in southern.
The seasons can bring a wide variety to the year for those locations that experience them in full. The weather in each one may allow people to engage in activities that they cannot perform in others — skiing in the winter, swimming in the summer. Each season brings with it its own potential dangers, but also its own particular brand of beauty.