In the District, fire hydrants are green. (John Kelly/The Washington Post)

I drive on Lee Highway through Arlington every day and only recently noticed that all the fire hydrants are yellow with pale blue tops. What gives? This doesn’t strike me as a particularly noticeable color scheme, not like a bright red hydrant against green grass.

— Claire Williams,

This may seem like a tangent, but bear with Answer Man here: You wake up, groggy, and find yourself sprawled on a sidewalk with a dull ache in your temples and no memory of the last few hours. Obviously, you were drugged, bundled into a sedan, driven around town and then dumped unceremoniously by the side of the road. (That’s the last time you’ll ever go to back-to-school night.)

Where the blazes are you?

Then you realize you are next to a fire hydrant. Having read Answer Man, you are able to ascertain your whereabouts. You know that in Fairfax County, fire hydrants have a silver barrel and a red top, or bonnet. In Alexandria, they have a yellow barrel and white bonnet and outlet caps. The District has green fire hydrants. In Prince George’s and Montgomery counties, they are gray with green bonnets.

You focus your eyes and examine the hydrant. It is yellow with a pale blue top. “Aha!” you say, “I am in Arlington County.”

You would also be in Arlington County if the hydrant had a green top, orange top or red top. That is because in the 1990s, the county adopted the hydrant coloring system recommended by the National Fire Protection Association. The bonnet colors represent the flow of water at that hydrant: Blue is above 1,500 gallons per minute; green is between 1,000 and 1,500 gpm; orange is 500 to 1,000 gpm; and red is below 500 gpm.

“We have formulas we use for figuring out how much water is needed to extinguish a fire,” said Capt. Gregg Karl of the Arlington County Fire Department. If the flow is low, for example, several hydrants may need to be used, with a relay set up.

But does any of that matter if the firefighters can’t find the nearest hydrant? That is not a problem.

“All of your drivers know where the hydrants are,” Karl said. “It’s a requirement of the job. You learn the water system when you become a driver. Part of the test to get off probation is a test of your territory. You have to know your streets, your first due, your second due, your third due.”

First due are locations where your crew is expected to be the first responder. Second and third dues are where you will come later, if needed.

When Karl was beginning his career, his captain used to take the rookies out on weekends to touch every single fire hydrant in their territory.

“I can tell you where they all are around Clarendon,” Karl said.

Good drivers do something else, too: They compile hand-drawn map books that note the locations of hydrants, along with any quirks of address-numbering, one-way streets and obstructions that might block a fire truck. When a hydrant temporarily goes out of service, they note that, too. When the alarm sounds and the rest of the crew is getting dressed, the driver consults the map book.

“I still have mine from when I was a rookie 13 years ago,” Karl said. “It was passed down to me.”

Ken Willette of the National Fire Protection Association was a firefighter and fire chief in Massachusetts for 30 years. He said that even when he was off duty, he couldn’t help scanning the streets for fire hydrants. “That gets embedded,” he said. “Believe it or not, that is something that stays in your mind for many, many years.”

Technology is helping. For the past year, the Washington Suburban Sanitary Commission, which maintains hydrants in Montgomery and Prince George’s, has been testing a computer program called HyLo. It allows firefighters to punch an address into a laptop and pull up a map detailing the closest hydrants. It will also work on iPads and iPhones.

Some other color notes: Hydrants on private property are generally painted red. Also, on a recent drive down Lee Highway, Answer Man noticed many hydrants that were entirely yellow. Karl said those are new or replacement hydrants whose flow has yet to be measured. The bonnets will be painted later.

Finally, of the 3,548 fire hydrants in Arlington, one is extra special: It’s at the Air Force Memorial. Because the designers of the swooping, metallic memorial thought a yellow/blue hydrant would clash, they had it chromed.

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