44 Seasons and Climate-Low

Seasons and Climate FOCUS Question How do seasons vary in different climate zones on Earth?

Seeing the Seasons Imagine getting on an airplane in Panama City, Panama. The air is hot and steamy. After hours on the plane, you land in Anchorage, Alaska. Here, you see your breath in the frozen air, and snow flies all around you. Now imagine you are in Boston, Massachusetts, in July. It feels like Panama—hot and humid. But if you were in Boston in January, it would feel like chilly Alaska. Planet Earth has a huge variety of climates—the long-term weather conditions in an area. There are wet tropical forests and frozen Antarctic deserts. Some places have seasons that change dramatically throughout the year. Why does this happen?

The Sun and the Seasons Earth has three major climate zones—large areas where the temperature and precipitation are similar. The coldest areas called the polar zones, are near the North and South Poles. The tropics lie near the equator. These zones are warm year-round. In between are the temperate zones, which can be both cold and hot. Climate zones are caused in part by Earth's round shape. At the poles, the Sun shines at a low angle over the curved surface. The rays scatter as they pass through more of the atmosphere. The light energy spreads out over a larger area of the surface, providing far less heat. This keeps the polar zones very cold. In the tropics, the Sun shines down on Earth's surface at a high angle. The light energy is much more focused, creating intense heat. This keeps the tropics warm year-round.

You might think the temperate zones, between the hot tropics and cold poles, would be pleasantly mild year- round. But the temperate zones are more complicated. Earth is not vertical, or straight up and down, as it orbits the Sun. Its axis—the imaginary line from pole to pole— tilts to one side. As Earth orbits the Sun, each pole leans toward the Sun for half the year and away from the Sun for the other half. This causes Earth's seasons, as explained in the diagram below.

Cold and Colder: Polar Zones Near Earth's North and South Poles, it is below freezing nearly year-round. Many areas are covered in permanent snow and ice. During summer in the Northern Hemisphere, the northern polar zone gets twenty-four hours of sunlight per day. The southern polar zone gets twenty-four hours of darkness. During summer in the Southern Hemisphere, the opposite is true. With all those hours of daylight, it seems as though the poles would have hot summers. But the Sun is always low to the horizon, and its rays spread out, never delivering much heat. The poles do experience seasons. During the brief, cool summer, some ice and snow melt. Plants sprout and animals have their young. As the long winter approaches, many animals store food, grow thick coats, or hibernate to survive.

Hot and Cold: Temperate Zones The main feature of the climate in the temperate zones is dramatically different seasons. For example, Minneapolis, Minnesota, lies in the middle of a temperate zone. During summer, when the Northern Hemisphere is tilted toward the Sun, Minneapolis gets focused sunlight from a high angle in the sky. There are also more hours of daylight than darkness. This combination of focused sunlight and long days makes for hot summers. The record hottest day in Minneapolis is 42°C (108°F)! During winter, the Northern Hemisphere is tilted away from the Sun, and Minneapolis gets only low-angled sunlight spread over a larger area. It also experiences short days and long nights. Winters can be brutally cold, with a record low of -40°C (-40°F)! These dramatic seasonal swings do not happen near the poles or in the tropics.