The Six Seasons
Deep Winter (approx. Jan - Feb): The bonechilling times. There is likely to be at least one painfully cold* period (temperatures around 10-20 F lasting about a week); hard winters may have several of these or one that lasts for longer. Winters vary wildly in how much snow they come up with; actual snow is nice (pretty, means it's probably at least 20 degrees), the wintery mix of slush-rain-sleet-ice-snow one gets around 30-35 F is very unpleasant (heavy to shovel, soaking-cold, makes travel irritating). Dressing appropriately for the weather is a must. Throwbacks can be startlingly warm (50-60s); there may also be Very Cold Days Indeed (sub-0).
Early Spring (approx. Mar - Apr): The air is still chilly - even downright cold at times, perhaps with snow or freezing rain- but the 10-20 F hard cold snaps that mark Deep Winter are rare throwbacks. If you expect basking-in-shorts-and-a-T-shirt weather, you'll be disappointed (aside from occasional throwforwards, or if you're well-built for cold), but signs of spring are beginning to show. 30 degree days give way to 40, 50, and occasionally 60 degrees; heating bills drop, and going outdoors in your spare time stops sounding like such a bad idea.
Late Spring (approx. May - Jun): One of the nicer times of year (generally 60-70 F, +/- 10); get out there and enjoy it! There's still enough of a temperature range that you ought to either check the weather before dressing for the day or wear layers so you can adjust appropriately. Precipitation is the usual roulette wheel. Break out the sunscreen at the start of this season (or earlier, if you're fair-skinned), and if you're going to need an air conditioner come Summer, buy one now, before major heat waves hit and they all sell out.
Summer (approx Jul - Aug): Usually in the high 70s to high 80s, with occasional clusters of one to three 90F+ days. It might rain a touch less than other times of year, or might not; regardless, rain tends to come with cooler temperatures, and summer thunderstorms are awesome. Humidity averages aren't much different from the rest of the year, but you notice it a lot more when it's 90 degrees, and there are nearly always some spells of high-humidity unrelieved by rain - air conditioning is valuable as much for its drying as its cooling. Overall, though, it's not as bad as much of of the eastern seaboard; it is possible to get along without air conditioning - though I'd highly recommend having a box fan or two, being comfortable wearing very little clothing, and not living in a penthouse apartment.
Fall (approx Sep - Oct): A fantastically nice time of year, overshadowed during one's early decades by the start of school. Overall, the weather's sliding cooler - from 70s-80s down to 40s-50s - although odds of a 3-10 day Indian Summer of Septemberish weather sometime in October to early November are reasonably good. Rain can occasionally get uncomfortably cold, but the rest of the time, it's gorgeous, and the air smells fantastic. If you have any winterizing left to do, now's the time to do it, before it gets really chilly.
Early Winter (approx Nov - Dec): Objectively, the climate is quite similar to Early Spring, with temperatures from high 50s down through high 20s and sporadic snowfall potential, but usually no major blizzards or long hard cold snaps. Subjectively, it's very different; partly because it's following summer/fall (so feels "colder than it was" rather than "warming up"), partly because it's so much darker (winter solstice vs. spring equinox), and partly because it's Deep Winter looming in the future rather than Late Spring. That being said, if you dress appropriately and don't get psyched out by Deep Winter barrelling down towards you, it can be pleasant to be out and about.
* Yes, yes, there are parts of the country which get much colder. I don't want to get into a pissing contest; "painfully cold" is how I think of it when it's 10 degrees out, so that's how I'm describing it.
When I was young, I learned about the four seasons of the year: Winter, when it's cold and it snows; Spring, when it's warmer and things grow; Summer, when it's hot and you don't have school; and Fall, when it's cooling down and the leaves change color. I learned about the solstices and equinoxes, and how the season boundaries didn't quite match up to the month boundaries. And that was all. Nothing more to know; these things don't change1.
But... over the years, I noticed that my mental model of what the weather should be like didn't match what was actually out there. It's been said that unhappiness is caused by the gap between one's expectations and reality, and a vivid illustration of that is some poor, shivering soul clad in a T-shirt in mid-April, saying, "But it's spring! It ought to be warmer than this!"
Part of the difficulty, of course, is that the climate around here is not what you'd call consistent. At any time of year, you may get a throwback or throwforward: a single day seemingly yanked straight out of the previous (or following) season: eg, 68 degrees in late February, or 25 and snowing in April, or 93 degrees in May, with Mother Nature giggling all the while at the baffled people trying to figure out why they're misdressed. (Many of whom are natives and should know better - but I digress.) The difficulty of weather forecasting2 hereabouts also doesn't help.
But really, most of it had to do with the fact that my expectations were just plain wrong. Thinking about the weather in four seasons wasn't particularly accurate.
So I pondered: practically speaking, what were the major periods of climate that I noticed? When did I shift my habits of dress and behavior, if I wanted to stay comfortable?
I came up with six seasons, which loosely map to groupings of two months3. They're described in the column to the side. All temperatures discussed are daytime numbers; the night is usually 10-20 degrees cooler. (Though there are - as always - exceptions.) If you want more data, see this page of highs and lows for Boston by month (both average and record).
Of course, this is New England, so any concrete assertions about the weather should be taken with a grain of salt. (The next time you're looking at a map of US temperature bands, look at how they tend to crunch together as they swing north near the eastern seaboard. A relatively small shift can often produce a large temperature swing.) Mark Twain's pronouncements on the topic4 are well worth reading.
Precipitation around here doesn't map to the seasons - there's no one "rainy season" or "dry season"; we get stuff falling from the sky now and again all the year round. As best as I can tell, there's no real pattern to it - there might be an hour of rain, or a half a day of rain, or a few days of rain, or a weather system that hangs around for weeks like an unpleasantly damp houseguest. Similarly for clear weather.
We don't get the full supercell thunderstorms that you see on the Great Plains, and tornadoes are rather less common than in other parts of the country, but beyond that we get just about any sort of storm you care to name: spitting drizzle, gentle shower, driving whiplike rain in high wind, torrential thundershower, the occasional hurricane which makes its way this far north, etc.
2 An important thing to understand regarding Boston-area weather forecasts is that anything more than 24 hours in advance should be considered, at best, an educated guess. One of the most common errors is timing, when the forecasters correctly guess what the general weather trends will be over the next N days, but are wrong about how quickly or slowly they will unfold. Next-day temperature forecasts being incorrect aren't generally worthy of even minor comment until the level of error hits +/- 10 degrees. (An exception: Small variations in temperature around freezing can result in significant differences in how precipitation plays out, and people will often be vocally cranky if they've prepared for snow but get freezing rain or vice versa.) Every now and then, even large-scale, supposedly safe predictions (like "this huge blizzard cannot conceivably miss us") will go completely out the window, and the poor weatherfolk will be roundly mocked.
3 Some of the boundaries don't map perfectly to month borders - eg, Deep Winter usually starts closer to "the end of December" than "the beginning of January". Also, simple year-to-year variation / normal variability means the transitions may shift around somewhat. But generally, they'll be within +/- 2 weeks of the listed time.
4 Which are not
"If you don't like the weather here, wait five minutes"; that quote
seems to have been misattributed to him (and is used by people nationwide).
He did much better than that, giving
a speech on the topic, which
starts out, "I reverently believe that the Maker who made us all makes everything in New England but the weather. I don't know who makes that, but I think it must be raw apprentices in the weather-clerk's factory who experiment and learn how, in New England, for board and clothes, and then are promoted to make weather for countries that require a good article, and will take their custom elsewhere if they don't get it.
"There is a sumptuous variety about the New England weather that compels the stranger's admiration -- and regret. The weather is always doing something there; always attending strictly to business; always getting up new designs and trying them on the people to see how they will go. ..."
The whole thing is well worth reading.