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"Wow, a ton of rain fell in Seattle" -- how much rain is that, really?

A rainy November day at Seattle's Pike Place Market. But did it rain a ton? (Photo courtesy: Tim Durkan)

We've all glanced out the window and thought "Wow, a ton of rain is falling right now," but what about in the literal sense?

Greg Johnson, who runs the awesome web site SkunkBayWeather.com decided to do the math after over 4" of rain fell across his backyard back during Thanksgiving week.

"I did a little exercise, just out of curiosity. How many pounds of water have fallen in my front yard since (last) Tuesday?" Greg wrote on his Facebook page on Nov. 22. "My front yard is roughly 100x115ft. Water weighs 0.036127 pounds per cubic inch. We have received 4.26” since Tuesday (and counting)."

How much has all that rain weighed, then?

"My front yard now weighs 254,860 lbs. more than it did before Tuesday," he said (although much of that rain likely was runoff). "Sheesh… That’s way more weight than I gained over Thanksgiving."

But he wanted to point out just how heavy rains can create landslides in our region's prone areas.

That got me to thinking just how much water has to fall to make a literal "ton" of rain on various spots.

I decided to start with a standard 2-car driveway as some sort of general reference point. According to this link, that's about 20x20 feet.

That's 400 square feet, or 57,600 square inches. So one inch of rain comes out to... almost exactly a ton (2,080 pounds to be exact). Or on the other side, 0.96 inches of rain = 1 ton of rain, at least on a driveway of that size.

What about a roof? Taking a 40x70 foot roof, it works out to 0.13" of rain is 2,000 pounds sitting on that roof if it didn't ever drain. Of course, it's a lot more if the roof is larger...and has drainage issues during a spectacular Convergence Zone like what happened at Lynnwood High School in September:


Incidentally, if you want to think snow since snow has been in the forecast about as much has rain has the past several weeks, it depends on the water content of the snow. But average snow is about a 1:10 ratio to rain, as in 1 inch of rain is equal in moisture to about 10 inches of snow. Thus, even less than an inch of snow on your roof may weigh a combined ton, depending on your roof size. Then again, the weight is spread out so it's more the weight per square foot that would cause problems than the overall weight of snow on the roof.

Some other fun Seattle area facts for what would take to get a ton of rain?

Space Needle: Their site says the upper halo is 138 feet in diameter. That's an area of 14,958 square feet or 2.15 million square inches. That is 77,815 pounds per inch of rain, or a "ton" of rain falls on the Space Needle after a little 0.03" rain shower.

Washington State Ferry (M/V Tacoma): The Tacoma, among WSF's larger ferries, is 460 feet long by 90 feet wide or 5.96 million square inches. An inch of rain collected on the ferry without drainage would add 215,450 pounds to its weight, or the other way, a ton of rain after just under 0.01" -- barely qualifying as measurable rain. (If you're wondering, even if an inch of rain all stayed on the boat, the 215,000 pounds would be a a "drop in the bucket" compared to the 2.4 million pounds added weight the ferry takes each run when it's full of cars and people, according to a WSF official.)

CenturyLink Field: Just the field of play (120 yards long with the end zones by 50 yards wide) is 54,000 square feet or 7.77 million square inches. That is a whopping 280,923 pounds of rain per inch, or a ton of rain falls on the football field with just 0.007" of rain -- officially a Trace measurement, but spread out over an entire football field -- it's a ton of rain!

Tacoma Dome: It's 530 feet in diameter, or 31.7 million square inches. An inch of rain is 1.14 million pounds! Ton of rain? Just 0.0017" worth.

New 520 Floating Bridge: 144.3 million square inches on the deck, according to the WSDOT. So 1" of rain falls on the bridge, it'd weigh 5.2 million pounds more! Good thing that water doesn't all pond! It also means just a tiny trace of rain, if it was all added up across that much space would weigh a ton (0.00038" to be exact)

Fremont Troll: Hah! Trick question. He sits under the Aurora Bridge. It never rains on him.

What about an umbrella? A standard umbrella is 40 inches in diameter, or 1256 square inches (assuming a perfectly circular umbrella). That means your umbrella would carry (had the water not run off) 45.3 pounds per inch of rain. So it'd need 44 inches of rain to have felt a ton of rain (cumulative, since again, the rain is obviously not collecting on the umbrella.)

How much is 44 inches of rain? Believe it or not, it doesn't even cover what Forks received in October and November last year:

Thus, had you been standing out on the beach (in the cold) with an umbrella (tourist!) for those 60 days (and not been blown away by our occasional windy storms), your umbrella would have collected a little more than a ton of rain!

So next time someone looks out the window and says: "Wow, that's a ton of rain" you can reply, "it is a pretty heavy rain, but I'm not sure it's a standing-out-on-the-beach-near-Forks-under-an-umbrella-in-autumn-for-60-days rain."

Compute the rain yourself

Greg has made a handy Excel sheet where if you change the numbers highlighted in yellow, you can make similar calculations. You can download the spreadsheet here.

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