We’re acutely tuned to the seasons, whether sunbathing and planning beach trips during the height of summer or groaning at the prospect of shoveling our drive during a winter snowstorm. But what causes the seasonal shifts in the weather? And why do the seasons differ depending on where on the globe we live?
If you could push a button and fast-forward through an entire year, starting on the first day of summer, you could easily see how each season correlates to Earth’s position in its orbit. On June 21, known as the summer solstice, the Sun’s rays beat directly down on the Northern Hemisphere. Because Earth is tilted at 23.5 degrees, the Sun shines straight above 23.5 degrees latitude, an imaginary line around Earth called the Tropic of Cancer.
Why is the longest day of the year not also the hottest? Thermal inertia—the tendency of temperatures to remain the same—keeps the land and water from getting instantly hot. Only after several weeks of intense Sun beating down on Earth will the hemisphere start to heat up.
If you stand on this line on the first day of summer, the Sun will be directly overhead, seemingly paused for a moment before beginning to sink lower in the sky each day. This is also the longest day of the year in the entire Northern Hemisphere, and from that first day of summer until December 21 (the winter solstice), the days will get progressively shorter. With the Sun at its highest point, and the days at their longest, common sense would tell you that June should be the hottest month of the year. But due to an effect called thermal inertia, the ocean and landmasses take time to heat up. As a result, the hottest weather usually arrives about six weeks after the summer solstice.
By the time the heat kicks in, you’re well on your way to the autumnal equinox on September 22—the first day of fall—when the days and nights are the same length. The oceans in the Northern Hemisphere are at their warmest and tropical storms and hurricanes begin to form. The weather has been getting cooler in the Northern Hemisphere, and now the leaves of many trees begin to show their true colors. On the days following the fall equinox, the Sun begins to set at the North Pole, and six long cold months of darkness set in there, while at the South Pole the Sun is rising.
The brilliant reds and oranges of fall are actually right there in the leaves all year long, but because the green color of chlorophyll is so dominant, you can’t see them until the chlorophyll level decreases. Cooler nights and shorter days reduce the amount of chlorophyll and allow the leaves to begin their yearly show.
As the days get shorter and the weather cooler, cold fronts begin to plow southward and some mornings reveal frost on the lawn. As the winter holidays approach, ice and snow can make an appearance, and by the time the first day of winter arrives on December 21, the Sun is shining directly down on the Southern Hemisphere at 23.5 degrees south—the Tropic of Capricorn—and the days are at their shortest in the North.
At the beginning of winter, the Northern Hemisphere is tilted as far back from the Sun as it can get, so the light reaching the ground is dimmer and more diffuse than at any other time of the year. Again the ground and the oceans are slow to give up the heat they’ve absorbed through the summer and fall, so the coldest weather is still to come.
As heating bills skyrocket and some cities in the North are hit by blizzards and ice storms, everyone gets tired of being indoors and wishes for spring. Then seed and garden catalogs begin to arrive, just making things worse. By March 20, just about everyone north of 35 degrees north of the equator has gone stir-crazy, but just in time, the first day of spring arrives. On that date, called the vernal equinox, the days and nights are once again equal in length, and the Sun is directly above the equator.
As the Sun rises higher in the sky each day, the land is warmed and trees begin to leaf out again, hibernating animals wake up and begin to search for food, and many people put away their treadmills. The days are getting longer, and before you know it, June is here and you’re back where you started. Unfortunately, you’re also a year older.
In this century, Earth’s northern axis points toward Polaris, the North Star. It won’t always, though, because Earth’s axis wobbles very slowly, about a half a degree per century, like a top just before it stops and falls over. This motion, called precession, causes the planet’s axis to describe a giant narrow circle in the sky that takes nearly 26,000 years to complete. So, in about 11,000 years, Earth will be closer to the Sun in July and farther away in December, the opposite of today’s situation. In 26,000 years things will be back the way they are now.
Further complicating the picture is the fact that Earth’s 23.5-degree tilt changes over time too, taking about 41,000 years to run through a full cycle that varies from about 21.5 to 24.5 degrees. When the angle is smaller, there will be less seasonal variation at middle latitudes; with a larger angle, the variations will be amplified. It’s thought that this change in tilt angle is one of the main factors that causes the periodic ice ages that sweep across our planet.
Archaeologists have found man-made lunar calendars that date back thousands of years. They indicate that Ice Age hunters carved notches and bored holes into sticks, mammoth tusks, and reindeer bones to record the days between each phase of the Moon.
Because the weather was tied to survival, most cultures kept records of its changes and erected monuments that acted as giant seasonal clocks. The most famous of these is Stonehenge, north of Salisbury, England. There is still disagreement as to the ancient stone circle’s exact purpose, but because the structure is aligned with the winter and summer solstices, many feel it functioned as a predictor of the passage of the seasons for the early residents of Britain.
At Machu Picchu, Peru’s City in the Clouds, the ancient Incas set up the Intihuatana (“Hitching Post of the Sun”), a stone that precisely indicated the date of the winter solstice. The ceremony they performed there was designed to halt the Sun in its northern migration through the sky. Machu Picchu’s Temple of the Sun features a window exactly aligned with the sunrise on the summer solstice.
The Egyptians were keen students of the Sun and stars, and around 3,000 B.C. they created a calendar very similar to the one used today. Like clockwork, the Nile river would flood each year at about the same time, and Egyptian sky watchers noted that the star Sirius would rise into the sky around that time too. The period between appearances was 365.25 days, so the Egyptians based their calendar on that time period. This calendar was later copied by Julius Caesar.
In the Americas, ancient tribes had their own ways of predicting the seasons. The early residents of Wyoming built a large circle made of stones that seems to point toward the position of the Sun at the summer solstice. The structure, known as the Bighorn Medicine Wheel, lies at an altitude of nearly 10,000 feet in the mountains.
Our ancestors did the best they could to use the Sun, the Moon, and the stars as predictors of coming climatic conditions, and some succeeded amazingly well. Their early research paved the way for the more sophisticated instruments of forecasting to come. But it’s a tribute to their resourcefulness and ingenuity that many of their methods of predicting the seasons still work today.