Five myths about sum­mer weather

(Spoiler alert: D.C. is not built on a swamp)

The Washington Post Sunday - - FRONT PAGE - By An­gela Fritz and Jason Sa­menow An­gela Fritz is an at­mo­spheric sci­en­tist and The Washington Post’s deputy weather editor. Jason Sa­menow is The Post’s weather editor.

It’s hot, it’s hu­mid, it’s sum­mer: full of swampy, muggy days that leave you drip­ping af­ter a quick trip out­doors and af­ter­noon thun­der­storms that seem to do lit­tle to quell the heat. All that hot weather brings a lot of mis­con­cep­tions, too. De­spite what you’ve heard, for in­stance, there’s no such thing as heat light­ning. It’s ac­tu­ally a real live thun­der­storm spark­ing elec­trons far off in the dis­tance. While we can’t make any prom­ises about cooler weather in the im­me­di­ate forecast, we can dis­pel some of the myths about the sea­son — and we guar­an­tee that some­time later this year, the heat will be gone. (And it’ll be time to deal with some myths about win­ter.)

1 D.C. is mis­er­ably hu­mid in the sum­mer be­cause it was built on a swamp. This one is a dou­ble myth. Not only was the Dis­trict not built on a swamp, but even if it had been, that wouldn’t nec­es­sar­ily af­fect our weather hun­dreds of years later. The idea that D.C. used to be swamp­land is a cliche, es­pe­cially among politi­cians who like to bash the city. But ac­cord­ing to lo­cal his­to­rian Don Hawkins, only about 2 per­cent of the land in Pierre L’En­fant’s orig­i­nal plan would meet the def­i­ni­tion.

In­stead, the Dis­trict’s ab­hor­rent sum­mer hu­mid­ity stems from the di­rec­tion of its pre­vail­ing winds, which come from the south. This air­flow draws wa­ter logged air north­ward from both the Gulf of Mexico and the At­lantic Ocean — some­times right up the Po­tomac, fun­nel­ing in even more mois­ture. It’s not the land the Dis­trict was built on that’s the prob­lem; rather, it’s the air stream­ing to­ward it.

2 D.C.’s hottest weather comes in Au­gust.

In June and July, we fre­quently de­liver de­press­ing fore­casts about pun­ish­ing heat waves. A com­mon re­ac­tion is: “It’s not even Au­gust yet!” But by the time Au­gust rolls around, the hottest days of sum­mer have usu­ally come and gone.

July is D.C’s hottest month, and by a siz­able mar­gin. The av­er­age high tem­per­a­ture in July (88.4 de­grees) is about two de­grees warmer than the av­er­age high in Au­gust (86.5 de­grees). D.C.’s av­er­age tem­per­a­ture peaks on July 15 and then be­gins a slow de­cline in Au­gust.

Cold fronts, sapped of energy at sum­mer’s height, have dif­fi­culty pen­e­trat­ing south of D.C. in July. But by Au­gust, aided by shorter days and a low­er­ing sun an­gle, they of­ten have a bit more oomph, es­pe­cially as La­bor Day ap­proaches. There are other parts of the coun­try where Au­gust is the hottest month, though, es­pe­cially in the North­west and the western Gulf Coast.

3 Hu­mid air is denser than dry air. We all know the feel­ing: Step out­side into a hot, muggy day, and you’d think you could cut the air with a knife. Some say it feels like swimming. When the hu­mid­ity gets high, the air seems dense.

Ac­tu­ally, though, the air is less dense on those hot, hu­mid days than on the hot and dry ones. That may seem coun­ter­in­tu­itive — how can air be­come less dense if we add more wa­ter va­por to it?

Den­sity is de­ter­mined by two things: mass and vol­ume. Take the mass of a ma­te­rial and di­vide it by the vol­ume of the ma­te­rial, and you have its den­sity. The spe­cial thing about gas— or air, in this case— is that a fixed vol­ume at the same tem­per­a­ture and pres­sure will al­ways have the same num­ber of mol­e­cules in it, no mat­ter what kind of mol­e­cules we de­cide to add. Amedeo Avo­gadro dis­cov­ered this phe­nom­e­non in the early 19th cen­tury, and since then, it’s been known as Avo­gadro’s Law.

Let’s say we take one cu­bic foot of air at a tem­per­a­ture of 80 de­grees, and we add wa­ter va­por to it. Avo­gadro’s Law says the other mol­e­cules in the air, such as oxy­gen and ni­tro­gen, must leave that space. Wa­ter va­por mol­e­cules have a lower mass than other mol­e­cules in the air. Ni­tro­gen has a molec­u­lar weight of 28, and oxy­gen has a weight of 32. Wa­ter va­por, on the other hand, has a molec­u­lar weight of just 18. So if you re­place “dry air” mol­e­cules with wa­ter va­por, you have an over­all lower mass per vol­ume of air, and thus a lower den­sity.

4 Sum­mer is warm be­cause it’s when Earth is clos­est to the sun.

The sun is a mass of flame more than 1.3 mil­lion times larger than Earth, with an av­er­age tem­per­a­ture of nearly 10,000 de­grees.

That’s pretty hot. So it makes sense that sum­mer hap­pens be­cause Earth is clos­est to the sun at that point in its or­bit — right? Wrong. “This is by far the most com­mon mis­con­cep­tion about the re­la­tion­ship be­tween Sun and Earth,” NASA says, “one that is un­for­tu­nately per­pet­u­ated by lousy di­a­grams in most school text­books.”

It’s true that Earth’s or­bit around the sun is not a per­fect cir­cle; it’s an el­lipse. But it’s still closer to be­ing a true cir­cle than an oval. Andin fact, we’re far­ther from the sun dur­ing the sum­mer than we are in the win­ter.

The mech­a­nism be­hind our sum­mery heat is ac­tu­ally the Earth’s tilt. As we make the year-long jour­ney around the sun, there are times when each hemi­sphere is tilted to­ward and away from it.

This has two ef­fects. First, tilt­ing to­ward the sun means more hours of sun­shine dur­ing the sum­mer. That alone means there’s more time for the day to warm up. The sec­ond, and more sig­nif­i­cant, ef­fect is that the sun’s rays shine from a more di­rect an­gle. So more heat can hit the ground in the sum­mer than if the sun was very low in the sky, as it is in the win­ter.

On the sum­mer sol­stice, which usu­ally hap­pens on June 21 or 22, the North Pole is tilted as far to­ward the sun as it’s go­ing to be all year. The sol­stice is the long­est day of the year, in terms of day­light, in the North­ern Hemi­sphere, and we also con­sider it to be the of­fi­cial first day of sum­mer.

5 Rel­a­tive hu­mid­ity can help us judge how un­com­fort­able the day will be.

Fore­cast­ers are re­peat­edly told that we should re­port the rel­a­tive hu­mid­ity per­cent­age in the sum­mer. “I would ap­pre­ci­ate the Washington Post giv­ing some con­sid­er­a­tion to the in­tel­li­gence of its read­ers and re­port­ing hu­mid­ity as a per­cent­age,” one reader wrote in a re­cent, typ­i­cal e-mail.

But hu­mid­ity as a per­cent­age is a mis­lead­ing and not par­tic­u­larly use­ful met­ric.

Rel­a­tive hu­mid­ity tells you how close the air is to sat­u­ra­tion, but it does not tell you the over­all amount of mois­ture in the air. Hot air can hold a lot more wa­ter than cooler air, sowe of­ten have a moister at­mos­phere on a hot day with lower rel­a­tive hu­mid­ity than on a cool day with higher rel­a­tive hu­mid­ity. So it’s less hu­mid on a 65-de­gree day with a rel­a­tive hu­mid­ity of 100 per­cent than when it’s 100 de­grees with a rel­a­tive hu­mid­ity of 40 per­cent.

In Washington, when it’s both hot and hu­mid, rel­a­tive hu­mid­ity lev­els are usu­ally be­tween 40 and 50 per­cent, and very rarely higher than 55 per­cent. If fore­cast­ers re­port that the hu­mid­ity is “just” 40 per­cent when it’s 100 de­grees, the public may not ap­pre­ci­ate that that’s ac­tu­ally quite hu­mid.

The dew point, the tem­per­a­ture to which the air would have to fall to reach sat­u­ra­tion, is our pre­ferred way of quan­ti­fy­ing and de­scrib­ing hu­mid­ity. It pro­vides an ab­so­lute sense of the amount of mois­ture in the air.

A dew point be­low 55 in­di­cates low hu­mid­ity and com­fort­ably dry air, and one above 65 in­di­cates high hu­mid­ity and in­creas­ing lev­els of dis­com­fort. If you see a dew point re­port of 70 or higher, hu­mid­ity lev­els are ven­tur­ing into un­bear­able ter­ri­tory. In other words, it’s prob­a­bly sum­mer­time in D.C.

Twit­ter: @CapitalWeather

JAHI CHIKWENDIU/THE WASHINGTON POST

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