Solar Radiation & Earth's Seasons (College Board AP® Environmental Science): Study Guide

Effect of latitude on insolation

• Latitude refers to the distance north or south of the equator, measured in degrees (°), with 0° at the equator and 90° at the poles

• Insolation refers to the amount of incoming solar radiation that reaches the Earth's surface

Angle of solar radiation

• The angle of the Sun’s rays determines the intensity of solar energy received at different latitudes

• At the equator (0° latitude):

  • The Sun’s rays strike the Earth more directly (higher angle)

  • This concentrates solar energy over a smaller surface area

  • This results in higher temperatures and greater solar intensity

• At higher latitudes (closer to the poles, 60° to 90° latitude):

  • The Sun’s rays hit at a lower angle

  • This spreads solar energy over a larger surface area

  • This leads to lower temperatures and reduced solar intensity

Decrease in insolation from equator to poles

• The highest solar radiation per unit area is received near the equator, decreasing gradually toward the poles

• This latitudinal variation contributes to:

  • Warmer climates in tropical regions

  • Colder climates in polar regions

  • The formation of distinct climate zones (tropical, temperate, polar)

  • Wind formation and global wind patterns

Atmospheric effects on insolation

• At higher latitudes, sunlight must travel through more of the atmosphere, which:

  • Scatters and absorbs more solar energy

  • Reduces the amount of radiation reaching the surface

• This effect further contributes to cooler temperatures near the pole

Seasonal variation in solar radiation

What causes seasonal variation in solar radiation?

• The amount of solar radiation received at a location on Earth varies throughout the year

• This variation is caused by Earth’s axial tilt (23.5°) as it orbits the Sun

• The number of daylight hours also changes, affecting the total energy received at different times of the year

How Earth’s tilt affects solar radiation

Longer summer days vs. shorter winter days

• During summer, a location receives more hours of daylight and higher solar intensity

• During winter, the same location receives fewer daylight hours and lower solar intensity

• Example: The Arctic Circle experiences:

  • 24-hour daylight in summer (when the tilt causes the Arctic to face towards the Sun)

  • 24-hour darkness in winter (when the tilt causes the Arctic to face away from the Sun)

Summer and winter solstices

• The summer solstice (around June 21 in the Northern Hemisphere) marks the longest day of the year

• The winter solstice (around December 21 in the Northern Hemisphere) marks the shortest day

• These dates are reversed for the Southern Hemisphere

Spring and autumn equinoxes

• The equinoxes (around March 21 and September 21) occur when day and night are equal in length worldwide

• Solar radiation is more evenly distributed across latitudes during these times

Effect on temperature and climate

• More solar radiation in summer → warmer temperatures

• Less solar radiation in winter → cooler temperatures

• This seasonal variation creates climatic patterns that affect ecosystems, agriculture, and weather