Changing Seasons

You may adapt your wardrobe and activities to the seasons, grabbing a heavy coat and skis in winter or instead, a swimsuit and snorkel in summer. However, ecosystems, plants, and animals cannot adjust their wardrobe quite so easily but they do make changes that help them survive seasonal conditions caused by the annual dance of the sun and Earth.

Birds migrate from the North Atlantic to the southern tip of South America. Whales and other marine mammals swim thousands of miles across the ocean. Seasonal changes in precipitation and temperature affect soil moisture, evaporation rates, river flows, lake levels, and snow cover. Leaves fall and plants wither as cold and dry seasons approach.  These changes in vegetation affect the type and amount of food available for humans and other organisms. Only with the recent advent of rapid transportation are fresh fruits and vegetables available in grocery stores during the winter in cold regions. Animals do not shop at grocery stores, they must find alternate food sources, move to warmer locations, or hibernate.

It may be surprising that all of these changes are the result of the Earth’s tilted axis and our planet’s yearly trip around the sun.

Seasonal vegetation changes across North America. Image shows the presence of green leaves absorbing sunlight as interpreted by satellite sensors used to calculate the Normalized Difference Vegetation Index (NDVI).
Center for Satellite Applications and Research
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As we go about our everyday activities it is not obvious that the Earth is tilted 23.5 degrees on its axis and that we orbit the sun. Nevertheless, these factors result in changes to the distribution of the sun’s energy across the surface of the Earth causing the seasons.

As the Earth orbits the sun every 365 ¼ days, the axis is always pointing in the same direction into space with the North Pole toward Polaris, the North Star. Around June 21st, the northern hemisphere is angled towards the sun, and receives the most direct radiation and the most energy. This is the start of summer in the northern hemisphere and winter in the southern hemisphere. Six months later, in December, the Earth has made half a revolution around the sun. The northern hemisphere is now angled away from the sun and receives less energy than the southern hemisphere; this is the beginning of winter in the northern hemisphere, and summer in the southern hemisphere. From north to south the results of the distribution of solar energy can be seen in the changing vegetation (see the accompanying diagram), animal behaviors, and by examining the clothes people wear.

Education Connection

What are some ways that animals adapt to seasonal changes in your region? How does this compare to other areas? How do different groups of people adjust to the seasonal changes in your region? How do the seasons impact the use of energy in your community? Investigating questions such as these may help provide relevance of seasonal changes for students. Inquiry lessons based around these types of phenomena can be used in any grade level and can help educators differentiate instruction.

Adapted from Why Do We have Seasons? and Annual Cycle and Variability

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