It’s that time of year again when we usher in another change of seasons – and the autumnal equinox – which marks the beginning of astronomical fall in the Northern Hemisphere. This important threshold was reached at 8:44 a.m. on Sunday.

Of course, we often associate changing weather with the transition of the seasons, and New England has already experienced quite a swing in evening temperatures from June through August (known as meteorological summer) to September — a sharp drop to fresh but pleasant sleeping weather.

And now our days will also see a shift, from the warm, summery temperatures we’ve been enjoying for the past few weeks to more seasonal temperatures in the 60s. You’ll notice a chill in the air, which will last at least through the weekend. Part of that is due to the Earth’s position relative to the sun at this time of year. During the autumnal equinox, the Earth’s axis is aligned so that the sun is directly overhead and day and night are of equal length at all latitudes. The sun then continues its journey south after crossing the equator.

From this point on, our days become even shorter. As a result, daytime temperatures become cooler as daylight hours continue. to slip away, and shorten more rapidly in October. In September, the length of daylight in New England decreases by 1 hour and 22 minutes — or about 20 minutes per week.

We will continue to lose sunshine hours as we move into October until the winter solstice, which occurs on December 21st, when the days will begin to lengthen again. Or in other words, the autumnal equinox marks the halfway point between the longest day of summer and the shortest day of winter.

Keep in mind that the start of astronomical autumn does not mean that we change our clocks. That only happens on November 3, when daylight saving time ends and we “fall back,” setting our clocks back an hour and losing an hour of daylight.

From warm and humid to pleasant and cool

What has been most noticeable in recent weeks is the transition from night-time temperatures that are well above average in the meteorological summer months (those warm and sticky evenings) to the cold night-time temperatures that we have measured so far in September.

The reason? In September there are mainly high pressure areas with little humidity, which allows the heat to be retained at the earth’s surface, just like in summer.

The map below shows that summer nighttime minimum temperatures were 1 to 4 degrees above average in New England.

And so far in September it has been a huge swing to 1 to 5 degrees below average. Cooler air can’t hold much moisture and when you have dominant high pressure areas the surface heating from the sun can escape freely through the atmosphere.

Reporting centers near Providence and Springfield have seen temperature swings of 5 to 6 degrees above average, which will be noticeable if you bring a light jacket in the evening or early morning.

That’s a tough change in a short period of time, and it will be interesting to see what the next few weeks bring. Right now, our general setup is favoring high pressure more often than not, leading to warm afternoons and cooler temperatures at night — at least for the next few weeks. After a brief cooldown this weekend due to more clouds and a northerly flow, the end of September looks set to be warm.

Seven of the last ten autumns have had above-average temperatures, and Boston and much of New England are currently right on average.

Is drought a problem?

Our parched lawns and suffering plants and vegetation may seem different, but the Boston area has not officially entered drought territory with this 29-day dry spell that we are finally emerging from today. Rain is expected in the city and southeastern parts of Mass. today through Friday, but it will only provide temporary relief for the region. Boston is nearly 2 inches below average for the month, with most of New England below average in September, adding to concerns about drought heading into October.

According to the latest Drought Monitor report released Thursday, parts of New England are experiencing “abnormally dry” conditions, including Greater Boston, Cape Cod, south-central New Hampshire and parts of Maine. If conditions become more severe, they will fall into the “moderate drought” category.

Fall also means more hurricanes

Yes, fall is known for its shorter days and cooler weather, but it’s also known to produce some powerful hurricanes. And while this disappointing Atlantic season hasn’t quite lived up to meteorologists’ predictions of a hyperactive one, there’s still a long way to go — hurricane season ends on November 30.

NOAA’s updated outlook for August calls for 17 to 24 named storms with eight to 13 hurricanes in the Atlantic. The tally currently stands at seven named storms with four hurricanes.

Remember, it only takes one tropical system to create a memorable season. Fortunately, New England came out relatively unscathed, with the exception of two remnant storms that left some areas devastated with extensive flooding.

Ken Mahan can be reached at [email protected]. Follow him on Instagram @kenmahantheweatherman.