Why isn't this early October warmth a "Second Summer"?

The month of October has gotten off to a very warm start in the Tri-State. Through the first 5 days of the month, the average temperature is 70.4°, a whopping 7.6° above the 30-year normal. On the warmest day, October 4th, we reached a high temperature of 83° in Central Park, equal to the average high for a day in mid-August. It was like a second summer in the Tri-State... or was it?

In my youth, the term "Indian Summer" was commonly used to describe a late-season warmup. While the antiquated term is no longer recognized by the American Meteorological Society, its scientific definition and criteria are the same as for the term "Second Summer". The AMS Glossary defines Second Summer in the following way:

"A period of abnormally warm weather that occurs in mid- to late autumn and after the first frost.
The comparable period in Europe is termed the Old Wives' summer and, poetically, may be referred to as halcyon days. In England, dependent upon dates of occurrence, such a period may be called St. Martin's summer, St. Luke's summer, and formerly All-hallown summer."

Unlike other meteorological terms, like heat wave, Second Summer does not have a minimum number of days ascribed to it, nor does it have a threshold temperature or temperature departure from average. The only firm stat in the definition is a key phrase: "...after the first frost". And therein lies the reason this early-October warmup is not a Second Summer. We haven't even come close to our first frost of the season in the city. In fact, the lowest temperature we've had so far this season was a 50° low temperature on September 27th. The map below shows the average first fall frost in the Northeast and Mid-Atlantic:

Average first frost map, using the current 30-year average

from the Northeast Regional Climate Center

The Tri-State area stands out as one of the latest spots on the map to experience a killing frost. The average first frost in NYC doesn't happen until November 20th, and the earliest first frost happened at the Central Park observation site all the way back on October 15th, 1876.

Even though they haven't been bitten by frost, a lot of the Tri-State's summer plant life is starting to wither. Our vegetable gardens have largely stopped producing anything new to eat. Despite the August-like temperatures we had this week, the October sun angle is preventing new growth from taking off. Just compare the image below of the sun angle's path through Central Park in October (first image) to its angle through the Park in June (second image):

NYC is solidly in the "midlatitudes", almost exactly halfway from the Equator to the North Pole, and this leads to a big variation in seasonal daylight. On the Summer Solstice, there's just over 15 hours between sunrise and sunset, but on October 6th, we're down to 11 hours and 33 minutes of daylight. With most veggies needing more than 6 hours of full, direct sun to flourish, the October sun is just not enough. It's no coincidence that the most recent full moon we had in September is referred to as the Harvest Moon!

One final thought for the gardeners reading this post: The amount of sunlight we have today is the same as it is on March 7th (in a non-Leap Year). This is why it's super helpful to start your plants inside, before the last frost of the Spring, which typically happens in April in the Tri-State. There's usually enough sunlight in your garden several weeks before the temperatures are high enough for plants to grow. Once the danger of frost has passed, you can permanently move everything outside and let them take advantage of all that sun!