by Erik Baard
A kid waiting to kayak at the Clearwater Festival last Solstice weekend asked me, “If this is the longest day of the year, then why isn’t it the hottest?” It’s a logical question, and I guess a common one. The incomparable Joe Rao addressed it in his New York Times astronomy blurb last week, and USA Today explored the question as well. The graphic above comes from USA Today.
In short, if Earth lacked an atmosphere, then surface temperature, apart from heat retained by rocks, would correlate with sun exposure. But our relatively stable atmosphere slowly and steadily receives energy from sunlight over the course of the spring and summer, as days lengthen. That energy, which we feel as heat, builds higher and higher until reaching its peak in July and early August.
I suppose an economist might call day length a leading indicator of summer, whereas as temperature is a lagging indicator. Swimmers and boaters (who are perpetually potential swimmers) know that this phenomenon of delay is even more pronounced with water temperatures.
Of course both air and water move around, so there’s a good bit of chaos and complexity to the fluid dynamics that make for summer weather and those late night skinny dips in open water that feel as warm as a bathtub. But the principle of heat retention is a far more powerful truth than its exceptions.
I’m also reminded of my good friend, David Grinspoon (with whom, for the record, I’ve never skinny-dipped), who joined us with comments about the Orion Nebula for the very first essay on Nature Calendar. He’s an astrobiologist (with an Earthly incarnation as a rhythm guitarist) with prestigious Mars and Venus robotic probe science assignments from both NASA and the European Space Agency, and serves as the curator of astrobiology at the Denver Museum of Science and Nature. He’s devoted much of his career to the planet Venus. He notes that if there was intelligent groundling life on Venus, it won’t see dominant temperature zones as we see them, corresponding to latitude, but instead look skyward to temperate altitudes. This is because Venus has an atmosphere that’s so dense, and therefore conductive, that heat overloads long ago seeped across all geographic areas. The surface is now isothermic, meaning that temperatures at the poles and the equator are the same. It’s gotten so hot there, due to a greenhouse effect gone into overdrive, that David postulates Venusian life would prefer a cloud habitat.
Of course, our cities perversely punish themselves, with air conditioners dumping extra heat into the dense local ecosystem through their exhaust, building electrical wires, regional transmission lines, and the power plants required to power them. Makes you wonder if there’s intelligent life on Earth.