Solar radiation and Earth's seasons

Key points

• Nearly all of Earth’s energy comes from incoming solar radiation, or insolation. The insolation reaching any one spot on Earth's surface varies according to latitude and season.
• Earth is a sphere. This means that the sun’s rays hit the different latitudes of Earth at different angles. The angle at which the sun’s rays hit the Earth determines the intensity of the solar radiation at that location.
  • At latitudes near the equator ($0\mathrm{°}$‍), the Earth’s surface is almost directly perpendicular to the angle of the sun’s rays. In these regions, solar radiation is intense because the sun’s energy is concentrated over a small surface area. As a result, equatorial latitudes generally experience hot temperatures throughout the year.
  • At mid latitudes ($23\mathrm{°}$‍ to $66\mathrm{°}$‍), the sun’s rays hit the Earth at a slant. This means that incoming solar radiation is spread over a larger surface area, and so is less intense than at equatorial latitudes. Earth’s mid latitudes generally experience seasonal warm and cool temperatures during the year.
  • At polar latitudes ($66\mathrm{°}$‍ to $90\mathrm{°}$‍), the sun’s rays hit the Earth at even more of a slant. So, at polar latitudes, incoming solar radiation is spread over an even larger surface area, and is even less intense than at mid latitudes. In addition, the sun's rays become scattered and diffuse as they travel through the Earth’s atmosphere, and this effect increases with latitude. The polar latitudes generally experience cold temperatures throughout the year.
• The Earth's axis of rotation is tilted $23.5\mathrm{°}$‍ from the plane of its orbit. As the Earth rotates the sun, this tilt causes seasons, which are divisions of the year that vary in temperature, weather, and the number of daylight hours.
• The duration and intensity of insolation at different locations on Earth varies seasonally. A location receives its most intense radiation during summer, and its least intense radiation during winter.
  • Summer occurs when a hemisphere is tilted towards the sun. This tilt causes the hemisphere to get more direct sunlight for more hours a day, and temperatures tend to be warmer. A location on Earth’s surface receives the most solar radiation on its summer solstice, which is the summer day with the longest period of daylight.
  • Winter occurs when a hemisphere is tilted away from the sun. This tilt causes the hemisphere to get less direct sunlight for fewer hours a day, and temperatures tend to be cooler. A location on Earth’s surface receives the least solar radiation on its winter solstice, which is the winter day with the shortest period of daylight.