I don’t need to remind Washington area residents that it has rained a lot in the last year. 2018 was the wettest calendar year in D.C’s official weather history, which dates back to 1871, and we recently completed D.C.’s wettest 365-day period. I like rain, but this is getting old.
Fortunately, this week, we’re catching somewhat of a reprieve from rain and storms, so I started thinking about dry periods, and how long they can last.
Washington’s longest dry period, that is, one with no measurable rainfall, lasted 34 days in the fall of 2007. That seems like a lot, but the city of Arica, in Chile, has had a period of more than 14 years with no measurable rainfall. Arica isn’t some tiny spot on a map chosen for its dryness; it has nearly a quarter of a million inhabitants. But Arica’s average annual rainfall is less than one-tenth of an inch.
In contrast, D.C.’s average annual rainfall is about 39.4 inches. (All average rainfall values are mean values and include melted snow and ice where snow and ice occur.) So you might conclude that dry places get longer rainless periods than wet ones, and you’d be right, but that’s only part of the story.
Let’s look at famously rainy Seattle, which actually averages “only” 37.1 inches of rain a year, and Portland, Ore., which averages 36 inches. Seattle’s longest rainless period is much longer than that of D.C., at 56 days, and Portland’s is more than twice that of D.C., at 71 days. Why?
Before I answer, here are a few more comparisons. Albuquerque averages a paltry 9.45 inches of rain a year, and its longest rainless spell lasted 109 days. Yet San Francisco, with nearly two-and-a-half times as much rain (23.6 inches), has gone 159 days with no rain. Los Angeles, with nearly 15 inches of rain on average, has gone for more than 200 days without rain.
For those of you familiar with the weather on our Pacific Coast, the answer must be obvious: Those places have a strongly seasonal distribution of rainfall. In other words, they have seasons where very little rain falls in most years. In Portland and Seattle, that dry season is July and August; in Los Angeles, the dryness often lasts from May through September.
There is one more factor at work, and that’s the tendency of a place with relatively low rainfall that’s strongly seasonal to experience huge ranges between driest and wettest seasons. Washington’s driest year had more than half the average rainfall and its wettest year had less than twice the average rainfall. In Los Angeles, with 14.93 inches of rain on average, the wettest year had 38.18 inches, while the driest had a mere 3.21 inches. (In Los Angeles, the rain year goes from July 1 through June 30.) That is just under a tenfold difference between driest and wettest, an unthinkable amount of variation for places such as Washington or even Seattle.
So next time it rains in the Washington area — take some comfort in knowing that you’re not going to have to suffer through a year with more than two-and-a-half times the average rainfall, and you’re not going to have to endure a seemingly endless drought.
David Policansky is a retired scientist who worked in the Division on Earth and Life Studies at the National Academy of Sciences.