Content

Changing seasons

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. 

Placement of Earth relative to the Sun at the start of each season.
 

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.

Four Seasons - Spring Summer Fall Winter - Flickr Creative Commons by Sarah Spaulding
Meteorological versus astronomical seasons
You may have noticed that meteorologists and climatologists define seasons differently from “regular” or astronomical spring, summer, fall, and winter.

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. 

The Gulf of Mexico's dead zone, where oxygen levels drop to dangerously low concentrations, typically forms in the spring and lasts into the summer.
The Gulf of Mexico’s dead zone
The Gulf of Mexico’s dead zone typically begins in the spring and persists into the summer. Springtime floods wash fertilizers from upriver into the gulf.

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. 

Sea turtle.
Cold snaps and stunned sea turtles
Sudden cold snaps or intense, prolonged cold can be very dangerous for sea turtles, especially during the winter.

Updated February 2019