A season is a period of the year that is defined by changes in climate, ecology, and daylight hours. These changes are largely caused by the axial parallelism of the Earth’s orbital rotation.
The Earth has a tilt of 23.5 degrees, and this is the main reason why we have seasons. Use the clip below to demonstrate this with students, or get them up to model this kinesthetically.
The Sun
The seasons are marked by a cycle of increased and decreased sunlight. This is due to the fact that the Earth rotates on an axis that tilts from north to south, which causes hemispheres to receive different amounts of direct sun.
The days are longest and brightest at the summer solstice on June 21 or 22 in the Northern Hemisphere, and shortest and darkest at the winter solstice on December 21 or 22 in the Southern Hemisphere. However, the season in which you live will depend on your location, not the hemisphere that you are in.
Observable, periodic natural phenomena have been used to mark time for thousands of years. These include the moon cycle, the tides and migratory patterns of birds and other animals.
Scientists use the astronomical definition of seasons, which is defined by the positions of the Earth and sun on its orbital plane. Each year the polar ice caps and deserts experience long periods of darkness, while tropical regions have rainy and dry seasons.
Earth’s elliptical orbit also influences the amount of sunlight received by different areas. However, it’s the axial tilt that most affects which hemisphere experiences which season.
The equator, which is located directly over the Earth’s center of mass, receives almost as much sunlight as the poles, because it’s positioned closer to the sun. However, the equator is also surrounded by bodies of water, which buffer its climate from the extremes of the north and south poles.
The most pronounced seasonal changes are felt at the poles, because they’re so far away from the moderating influence of the oceans and air masses that surround them. For example, temperatures at the polar cap can get very cold and dry during the winter, while the ice sheets melt and crack under the sweltering heat of the sun in the summer. The resulting change in temperature and light affects vegetation, animal behavior, and even the length of the day. Scientists are predicting that climate disruption will cause the traditional season patterns to shift, with longer summers and shorter winters.
The Earth’s Axis
Earth rotates around its axis, which is an invisible line going right through the center of our planet from pole to pole. That rotation is what gives us the daily cycle of day and night, but it’s also what creates our seasons. Basically, the tilt of Earth’s axis is what causes one part of the world to be closer or farther from the Sun. It’s what makes one hemisphere get more of the Sun’s direct rays in summer and the other hemisphere experience less, leading to colder temperatures in winter.
For this reason, the days become longer in summer and shorter in winter. Spring and fall occur when both hemispheres are closer to the Sun and receive roughly equal amounts of sunlight. This is known as solar timing, and it’s the reason many people think the start of the summer solstice (the longest day of the year) and the winter solstice (the shortest day of the year) mark the beginning and end of summer and winter respectively.
The exact angle at which the Earth’s axis is tilted from its orbital plane is called the obliquity of the Earth’s rotation axis. It isn’t constant and the slight change over long periods of time, called axial precession, is too small to affect how we live our lives.
However, the changes in seasons caused by obliquity are still significant. They vary the amount of daylight a place gets and cause things like flowering, seed-setting and hibernation cycles in plants and animals.
The climate in a particular region also affects its seasons, as the presence or absence of water can temper temperature extremes. This is why we see different seasons in polar and tropical regions, with different plants and animals responding to their environment. For example, in the Amazon rainforest, seasonal variation is more often driven by wet and dry cycles than by temperature variations. Indigenous people living in polar and temperate regions of the world often define their seasons ecologically, according to the growing and hibernating patterns of the plants and animals they interact with.
The Water Cycle
The water cycle shuttles Earth’s most vital resource around in a life-giving loop between oceans, land and atmosphere. It’s a complex process that includes evaporation, transpiration, condensation and precipitation. It’s a critical part of the Earth system and regulates climate. However, climate change is disrupting the delicate balance that maintains the water cycle. The changes are causing problems from extreme flooding and drought to contaminated water supplies.
Heat from the sun causes water molecules to evaporate from lakes, rivers and other bodies of water. This water vapor rises in the atmosphere where it can cool and condense into clouds. The cloud droplets can then fall as rain, snow, sleet or hail. The water then travels over the landscape as runoff, seeps into the ground or is stored in glaciers and snowpack. Water is also pumped into the air by wind, creating fog.
As it travels, the water absorbs and releases energy as it shifts between liquid, vapor and solid (ice). This process also helps moderate temperatures across the planet. It’s why seasons are different in polar and temperate regions, as well as why animals and plants have cycles of dormancy and hibernation.
In the ice age, the water cycle was much slower because of the thick ice sheets that covered the world. During warmer periods, the ice sheets melted and expanded, shifting the planet’s climate. This caused the weather patterns we know and love today.
This process is a vital one for human health and the ecosystems we depend on. Water provides us with food, clean water and energy for survival. It also creates habitat for many species of plants and animals.
Many people have long recognized the importance of the water cycle and its role in the Earth’s natural rhythms and seasons. Indigenous people in polar and temperate climates across the Americas, Africa, Asia and Oceania have traditionally defined their seasons ecologically by observing the activity of plants and animals. This varied from the hibernation of polar bears in the Arctic tundra to the growing season of plants in tropical rainforests.
Barometric Pressure
For most of the planet, varying air pressure creates weather shifts with the seasons. A drop in pressure with the onset of winter results in a drier season, while an increase in pressure with the start of summer brings a wetter season. In addition, wind and temperature patterns also change with the seasons. The prevailing wind direction at a given time, as well as the amount of sun or shade, influences the overall atmosphere’s moisture content.
As the Earth rotates around its axis, the North and South poles are tilted towards or away from the Sun to give different amounts of sunlight to each region. This causes the day to lengthen in the Northern Hemisphere as summer begins and shortens in the Southern Hemisphere as winter starts. The seasonal variation in sunlight influences the growth of plants, animals’ behavior, and temperatures.
Over the last century, scientists have documented the changes in climate caused by greenhouse gases causing the lengths and onset dates of the four seasons to shift. These changes have been observed at both regional and hemispherical scales, and have been found in both temperature (Christidis et al., 2007; Park et al., 2018; Pena-Ortiz et al., 2015; Yan et al., 2011) and phenological indicators such as flowering and leaf fall (Both et al., 2015).
Changing the duration of the seasons impacts the natural world. For example, as spring comes a fortnight earlier on average and autumn two weeks later, vegetation stays alive longer and is more exposed to frosts and parasites. In turn, this increases the risk of disease and death for the plant. In addition, pollinating insects have less time to collect and transport the pollen they need for reproduction.
In addition to the lengthening and shortening of the four seasons, scientists have noted a general warming trend in the atmosphere with respect to seasonality. These changes can have many consequences for biodiversity and human society. For example, a change in the timing of the season can disrupt ecosystems and affect food supply for wildlife as they have to adjust their habitat. Recording the occurrence of early leaf fall or a new flowering pattern can help to understand the impact of global warming on nature, and support policy-making aimed at reducing future warming (e.g., Judith Garforth, Citizen Science Office for Nature’s Calendar, Woodland Trust).
