Reader James wrote in from the sailing superpower of Marblehead, Massachusetts, to ask when the “least rainiest day in the U.S.” might be. As in, the day of the year when, on average, you’re least likely to get wet.
He has good reason for asking. His birthday and his anniversary both fall on Sept. 14, and he “could never remember a rainy birthday.”
Wow, James! Congratulations for the wedding, for the birthday and — soon enough — for the greatest honor of all: A button with the face of a weird newspaper nerd on it.
It seems like the spirit of this question would be to look not at the day with the lowest average rainfall, but the day when the largest part of the U.S. isn’t rainy. For that, we turned to our buddy Ben Noll, the Post’s weather-data dynamo.
And don’t worry — the day Ben picked to marry his lovely wife, there was a hurricane. So what could go wrong here?
Ben pointed us to historical weather data — coming from satellites, weather stations, planes and ships — from the European Centre for Medium-Range Weather Forecasts, the same file piles we rummaged through to see which way the wind was blowing a while back. Those Euros and their supercomputers calculate the precipitation on just about every square of the map. (To keep things comparable here, we’ll define “dry” and “wet” based on all precipitation, even if some of it arrived in frozen form.)
The driest month in the 48 contiguous states is September, based on averages going back to 1990 — roughly our modern climate era. Specifically, the days when the largest share of the country doesn’t see a drop of rain (or snow) are Sept. 26 to Oct. 1. That may not shock your socks off. Until we remind you that, in another recent column, we found that the most common birthdays in America are Sept. 24 and 25.
In fact, trends in births and dry days seem to match throughout the calendar. Periods with a paucity of precipitation feature more procreation.
To be sure, it would be out-and-out absurd if one really caused the other. For starters, the date of birth gets determined around nine months in advance, so the weather on your due date probably has little effect on the birth month. And, while we suppose causality could theoretically run in the other direction, we’re not aware of much research demonstrating that babies can control the weather.
Instead, we reckon that the simplest explanation probably fits. Both trends are seasonal, and are both caused by underlying seasonal forces. For example, temperature can affect birth rates and it can also, of course, influence precipitation and weather patterns.
Speaking of weather patterns, it’s kinda silly to declare late September to be the driest period in the U.S. because, as you may have noticed, rain doesn’t fall evenly across these far-flung United States. In some places, “dry September” would actually be peak hurricane season. In search of more detail, we dug into the beautiful state and county precipitation averages maintained by the fine folks at the National Centers for Environmental Information.
Because of their proximity to the Atlantic Ocean and its vast expanses of moisture, most Eastern and Southern states see consistent precipitation throughout the year and therefore don’t have a rainy season to speak of. Florida is the notable exception, where clashing sea breezes spark thunderstorms on the daily from May to October.
The wettest seasons in the Pacific Northwest are fall and early winter as the North Pacific jet stream strengthens and steers storms into the region. This year, that storm pipeline was made stronger by a big marine heat wave, which contributed to destructive flooding in Washington during December.
In the Midwest, the wettest seasons tend to be spring and summer when moist air from the warming Gulf of Mexico surges northward and gets harnessed by storms, leading to violent downpours.
Between those wet regions lies the arid Plains — west of John Wesley “Lake” Powell’s hundredth meridian — and the high deserts of the Mountain West. You could be generous and say they have a rainy season too — sometime in the late spring as tornado season peaks — but even their rainiest months would seem rather dry to an Easterner, which might explain how rapidly Western tourists wilt in D.C.’s swampy summers.
This dryness is no accident: as moist air is pushed up and over the Rockies, it rains itself out on the way, leaving much drier air on the other side — a setup known as the orographic effect.
If you’re picking a dry month to get married, or to have been born, or both, then you oughta calibrate based on where you live. For much of the East Coast, that’s the fall. In the West, it’s the summer. In the plains, you’re most likely to have a dry day in winter.
Nationally, your best bet for consistently dry weather would be to visit much of California — everywhere but the North and the High Sierra, really — in mid-June. In most counties in the state, you can find at least a couple of June, July and August days that haven’t seen precipitation in at least 76 years — our data only goes back to 1951.
But in this case, we’re not sure we’d equate “no rain” with “nice weather” — that dry spell also turns the Golden State a dusty beige and kicks off California’s peak fire season.
You’ll also find plenty of dry days on the arid western half of the Great Plains, but — as strongly as we’d endorse it — we’re not confident many folks will be scheduling destination weddings in, say, Nebraska’s incomparable Sand Hills. One dark-horse option might be central Georgia, where rain has rarely fallen around April 16.
Although there’s a currently a drought, dry spells aren’t typically an issue in Florida where, for most of August, you’ve been able to find rain somewhere in the state every day since 1951. It’s also home to the county with the most consistently rainy day: We’ve apparently only had two dry Sept. 11s in Monroe County — home to the Keys and the Everglades — since we started keeping track. A few winter days in Coastal Washington are almost as consistently waterlogged.
Surrounded by warming oceans, humidity levels in the Sunshine State are rising faster than most other places, fuel for those summer deluges. And there’s absolutely nothing to block the incoming vapor: Florida’s tallest place stands at 345 feet, more than 200 feet shorter than the Washington Monument.
And going back to James The Reader, we can confirm that in the city where he grew up, Alexandria, Sept. 14 is typically rainless. So keep that in mind — or perhaps refer to the table below — if you’re looking for a birthday on which to get hitched.
Hello, there! The Department of Data continues collecting queries. Tell us what piques your curiosity: Do older home sellers get lower prices? What percentage of each city’s mail reaches its destination? Has the popularity of chess peaked? Just ask!
If your question appears in a column, we’ll send you an official Department of Data button and ID card. This week, we owe one to James in Marblehead!
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