Visualizing “Seasonal Lag” Effect

Green Building

Visualizing “Seasonal Lag” Effect


I recently completed teaching a unit on the topic of solar geometry. Among the various facets of the subject matter, I talked to the class at length about the Earth’s orbit around the Sun (which, evidently, is a revelation to 1/4 of American population), the northern hemisphere’s 23.5° tilt toward and away from the Sun during the summer and winter, respectively, and how the distribution of radiant flux is affected by the relative tilt of the Earth’s surface at a given location (i.e., the “cosine effect”).

As we discuss the Sun’s journey across the sky in northern latitudes, it becomes clear that solar income should theoretically be most available at the summer solstice (i.e., June 21) – when the Sun has its longest course, its highest altitude, and is shining most normal (e.g., closest to perpendicular) to the Earth’s surface at a given location north of the equator.

Strangely, the air temperatures at the Earth’s surface typically lag behind June 21 by several weeks. For instance, in Indianapolis, Indiana, peak temperatures typically occur in July and August. Both months’ average temperature is higher than that of June.

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