APES Video Notes for 4.7 - Solar Radiation & Seasons

Solar Radiation and Seasons

Insolation and Latitude

Insolation is the solar energy reaching a given area, measured in watts per square meter (W/m²). It is calculated by dividing the incoming solar power by the area over which it is distributed.

Insolation varies with latitude for two main reasons:

• Sun angle: Near the equator, sunlight strikes Earth more directly and is concentrated over a smaller area. At higher latitudes, sunlight arrives at a more oblique angle and spreads over a larger area, reducing its intensity.
• Atmospheric path: At higher latitudes, sunlight travels through more of the atmosphere, where more energy is absorbed or scattered before reaching the surface.

The video gives approximate annual averages of 400 W/m² near the equator and as little as 200 W/m² at higher latitudes.

Earth’s Tilt and the Seasons

Earth’s axis is tilted about 23.5° as Earth orbits the Sun. This tilt changes the angle and duration of sunlight received at different locations throughout the year, producing seasonal changes in daylight and temperature.

• June solstice: The Northern Hemisphere is tilted most toward the Sun. It receives more direct sunlight and has its longest day, marking the start of summer there. The Southern Hemisphere experiences winter.
• December solstice: The Northern Hemisphere is tilted most away from the Sun. It receives less direct sunlight and has its shortest day, marking the start of winter there. The Southern Hemisphere begins summer.
• March and September equinoxes: Neither hemisphere is tilted more toward the Sun than the other. Both receive roughly equal daylight—about 12 hours—and the hemispheres transition between seasons.

At the solstices, the most direct sunlight falls near 23.5° north or south latitude: the Tropic of Cancer or the Tropic of Capricorn, depending on the season.

A useful way to reason about the seasons is to track which hemisphere is tilted toward the Sun. A hemisphere tilted toward the Sun receives more direct insolation and has longer days; one tilted away receives less direct insolation and has shorter days.

Albedo and Surface Temperature

Albedo is the portion of incoming light reflected by a surface. It affects how much solar energy a surface absorbs, but it does not change the amount of insolation arriving at that location.

• High-albedo surfaces, such as snow and ice, reflect more light and absorb less heat. A mirror is an example of a surface with nearly 100% albedo.
• Low-albedo surfaces absorb more sunlight and warm more. After a surface absorbs sunlight, it releases some energy as infrared radiation (heat).

This helps explain the urban heat island effect. Dark materials such as asphalt and blacktop absorb substantial solar energy and release heat, making urban areas warmer than nearby rural areas with more vegetation.

Practice Task

The presenter introduces an FRQ-style task: use a diagram to identify the Northern Hemisphere’s season and explain how Earth’s axial tilt causes the seasons.

Speakers and Sources

• Presenter: Referred to in the subtitles as “Mr. Speeds,” though this may be an auto-captioning error. The video metadata identifies the channel as Jordan Dischinger-Smedes.
• Other speakers or sources: None featured in the subtitles.