Just a few days after the Harvest Moon, the autumnal equinox this Sunday marks summer’s astronomical farewell for Earth’s northern hemisphere.
At 4:44 p.m. EDT on September 22, the sun will appear directly overhead at the equator before its direct rays shift into the southern hemisphere for the next six months.
While we often use the phrases “spring warmth” and “autumn chill,” the fall equinox is on average significantly warmer than its spring counterpart in March.
Astronomically, the September equinox is equivalent to March in terms of incoming solar energy, daylight, and darkness. Neither of Earth’s hemispheres is tilted toward the sun, all latitudes see roughly 12 hours of daylight and darkness, and unless you’re standing on the north or south poles, everywhere on Earth sees the sun rise due east and set due west along the horizon.
The main astronomical difference is that we’re now losing – instead of gaining – daylight at our fastest pace of the year, and the sun’s most direct rays are moving south of the equator instead of north.
So if we receive the same level of incoming solar energy as we did six months ago, why is September weather in the mid-latitudes of the northern hemisphere generally warmer than in March, as we see in the graph of average daily temperatures on both equinoxes below?
The imbalance is due to seasonal temperature lag, which leads to a delay in how average air temperatures respond to changes in incoming solar radiation. We notice this in summer and winter, when our hottest and coldest temperatures tend to occur about a month after the June and December solstices, respectively. Earth’s atmosphere takes time to warm and cool, especially since the oceans cover more than 70 percent of the planet. Water has a higher specific heat capacity than land, which means it takes longer to heat up and cool in response to changes in solar energy.
This seasonal lag time means that in September most of the northern hemisphere is much warmer than in March. If we look at the average temperatures of several U.S. cities on both equinoxes, the difference is quite pronounced. In Washington, D.C., our average 24-hour temperature on the September equinox is 21 degrees (F) higher than it was six months ago.
Note that cities closer to bodies of water see a smaller temperature difference between the two equinoxes, as summers are relatively cool and winters relatively warm compared to interior continental locations. In Seattle, the nearby Pacific reduces the average temperature spread between the equinoxes to only 13 degrees, and in Los Angeles, temperatures are only 11 degrees warmer now than in mid-March.
Tropical areas see even smaller changes in average annual temperature due to consistently high levels of insolation and high levels of atmospheric water vapor. Miami, situated just north of the tropics and along the Atlantic, is only 10 degrees warmer in September than in March.
These are, of course, average 24-hour temperatures. For a better sense of how much warmer we are now compared to March, take a look at the table of average high and low temperatures for the same cities above. In Washington, afternoon temperatures still warm into the upper 70s, while on the spring equinox, our normal daily maximum is only 57 degrees.
As the northern hemisphere responds to the decline in sunlight and incoming radiation, temperatures will continue to fall rapidly. A month from now, D.C.’s average high drops another 11 degrees and the sun will set another 45 minutes earlier. Yet even though our October shadows are about the same length as we see in February, we’re still a few months away from digging out hats, gloves, and snow shovels. After the equinox, the days will be shorter than nights, but for many, early autumn is when temperatures feel just about right.
Still have questions on your mind about the equinox? Read more about the equal day and night paradox, why sunrise and sunset don’t change at the same rate, and more technical explanations in our previous posts: