Changing seasons

In many parts of the United States, you might change your wardrobe with the seasons, grabbing a heavy coat in winter, while wearing only a light t-shirt in summer. Although ecosystems, plants, and animals cannot adjust their attire quite so easily, they have evolved to make changes that help them survive seasonal conditions caused by the rotation of the Earth around the sun.

A graphic of the Earth’s orbit around the sun. Meteorological seasons: Winter starts on Dec. 1, spring on Mar. 1, summer on June 1, and fall on Sept. 1. Astronomical seasons: Winter begins on the winter solstice (Dec. 21) when the North Pole is tilted to the max extent away from the sun, spring begins on the spring equinox (Mar. 20), summer begins on the summer solstice (June 21), when the North Pole is tilted to the max extent toward the sun, and fall begins on the autumnal equinox (Sept.21).
Do you know the difference between meteorological and astronomical seasons? Astronomical seasons are based on the position of the Earth in relation to the sun, whereas meteorological seasons are based on the annual temperature cycle. The Earth’s annual trip around the sun forms the basis for the astronomical calendar in which seasons are defined by two solstices and two equinoxes. The Earth is tilted 23.5 degrees on its axis of rotation, and how the North Pole is oriented toward or away from the sun determines two of these astronomical dates. At the winter solstice, the North Pole is tilted away from the sun, whereas at the summer solstice, the North Pole is tilted toward the sun. The equinoxes occur halfway between these events when the sun’s path is aligned with the Earth’s equator. The dates shown describe the astronomical seasons for the Northern Hemisphere in 2022; the exact dates vary slightly from year to year. Meteorological seasons are broken down into groupings of three months in our civil calendar based on the annual temperature cycle. We generally think of winter as the coldest time of the year and summer as the warmest time of the year, with spring and fall being the transition seasons. In the Southern Hemisphere, the seasons are reversed where summer occurs when the South Pole is oriented towards the sun, and winter happens when the South Pole faces away. (NOAA Office of Education/Kaleigh Ballantine)
Click to see this graphic for the Southern Hemisphere

A graphic of the Earth’s orbit around the sun. Meteorological seasons: Winter starts on Jun. 1, spring on Sep. 1, summer on Dec. 1, and fall on Mar. 1. Astronomical seasons: Winter begins on the winter solstice (Jun. 21) when the South Pole is tilted to the max extent away from the sun, spring begins on the spring equinox (Sep. 22), summer begins on the summer solstice (Dec. 21), when the South Pole is tilted to the max extent toward the sun, and fall begins on the autumnal equinox (Mar. 20).
Do you know the difference between meteorological and astronomical seasons? Astronomical seasons are based on the position of the Earth in relation to the sun, whereas meteorological seasons are based on the annual temperature cycle. The Earth’s annual trip around the sun forms the basis for the astronomical calendar in which seasons are defined by two solstices and two equinoxes. The Earth is tilted 23.5 degrees on its axis of rotation, and how the North Pole is oriented toward or away from the sun determines two of these astronomical dates. At the winter solstice in the Southern Hemisphere, the South Pole is tilted away from the sun, whereas at the summer solstice, the South Pole is tilted toward the sun. The equinoxes occur halfway between these events when the sun’s path is aligned with the Earth’s equator. The dates shown describe the astronomical seasons for the Southern Hemisphere in 2022; the exact dates vary slightly from year to year. Meteorological seasons are broken down into groupings of three months in our civil calendar based on the annual temperature cycle. We generally think of winter as the coldest time of the year and summer as the warmest time of the year, with spring and fall being the transition seasons. (NOAA Office of Education/Kaleigh Ballantine)

Why do we have seasons?

The Earth is tilted 23.5 degrees on its axis, which affects the distribution of the sun’s energy across the surface of the planet. 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 22, the Northern Hemisphere is angled towards the sun, and receives the most direct radiation and the most energy. This is the start of astronomical summer in the Northern Hemisphere and winter in the Southern Hemisphere.

Six months later, in December, the Earth has completed 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 changing vegetation, animal behaviors, temperatures, and day length.

Astronomical and meteorological seasons

Meteorologists and climatologists use a slightly different time scale for meteorological seasons than for astronomical winter, spring, summer, and fall. Why do meteorological and astronomical seasons begin and end on different dates? In short, it’s because the astronomical seasons are based on the position of the Earth in relation to the sun, whereas the meteorological seasons are based on the annual temperature cycle and help scientists track climate and weather trends.

Seasonal effects

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. Animals in temperate and polar regions must find alternate food sources, move to warmer locations, or hibernate.

Beyond the big four

Though in the United States, we often think of the seasons as winter, spring, summer, and fall, but there are many different ways seasons change throughout the world, and even within the United States. Can you think of other annual climate events or ways to define seasons?

EDUCATION CONNECTION

What are some ways that animals adapt to seasonal changes in your region? How does this compare to other areas? How do people adjust to the seasonal changes in their region? How do the seasons impact the use of energy in your community? The resources in this collection help educators encourage their students connect their own observations of the seasons to concepts in Earth science and biology.