Why Seattleites (usually) don't need to carry an umbrella

A Seattleite braves a rainy day in the city -- without an umbrella. (Photo: Tim Durkan Photography)

Sure, it rains a lot around Seattle, but on those rainy days, you'll be hard-pressed to find a lot of people seeking shelter under an umbrella. In fact, it's a badge of honor around here, and the locals will tell you it's easy to spot the tourists on the rainy days because they're the only ones huddled beneath the bumbershoots.

But why is that? Is it that Seattle locals have some sort of built in water-repellent skin? We joke we have webbed feet but no, our skin is the same as those from drier lands.

It turns out, it's the size of our raindrops that allow us to mostly get off scot free. A typical rain in Seattle features relatively small raindrops, which don't have quite the cumulative soaking effect that a big, bad Oklahoma thunderstorm will bring.

Our raindrops are small for roughly the same reason our storms are relatively small (as far as intensity goes) compared to the rest of the nation: Our moderate, marine climate puts a damper on intense atmospheric dynamics.

The size of raindrops comes down to vertical winds. Large raindrops mean that there are strong updrafts inside the clouds above your head. These upward blowing winds can hold raindrops inside the clouds for a longer period, allowing them to continue to grow in size until they finally become heavy enough for gravity to finally win the battle over the updraft, allowing the raindrop to fall to the ground.

The stronger the updraft, the larger the drop can grow. (Plus the warmer the air, the more moisture it can hold, so imagine places that are typically hot most of the year.)

That's why thunderstorms tend to have large raindrops, as they tend to also have strong updrafts that can support those larger raindrops.

Meanwhile, updrafts around the Puget Sound region are typically very weak, allowing small raindrops to easily succumb to gravity. And thus when it rains, it doesn't really pour, and we go about our walks knowing we'll dry out pretty quickly once we get inside.

This video shot at Seattle's Pike Place Market on a typically rainy day in October pretty much paints the picture:

But then again, it's not always light rain. There are a few rare instances where even the most die hard Seattleite *might* want some rain protection.

Case in point, a particularly intense rain storm on Sept. 16 that soaked people in Edmonds in just a few seconds -- just as school got out. Many didn't have their umbrellas (because, why would they?) but an exception might have been needed here:

(Although I love the teen soldiering on at about the 22 second mark. He earned his lifetime Seattleite pass with that walk home!)

The raindrops were so large here because strong updrafts were aplenty, thanks to an intense Puget Sound Convergence Zone. The convergence zone is formed when northerly winds from the Strait of Juan de Fuca turn south over the I-5 corridor, only to meet up with southerly winds that wrapped around the southern side of the Olympics. Their collision forces strong updrafts, which leads to convection, rain, thunder, hail, and can also lead to REALLY big rain drops, if the colliding winds are strong enough. Then the showers that hit the next morning were back to normal and were barely noticeable to the walking public.

How can I tell if it might be a "no problem" rain or a "I'll risk looking like a tourist to stay dry from that deluge" rain?

Here are a few scenarios and key words that you can look for in a forecast to get a clue of the impending soak factor of the forecasted rains:

Typical large drop/deluge type storms:

* Strong autumn/fall storms that bring Wind Advisories or High Wind Warnings

* "Pineapple Express"-type storms

* "Puget Sound Convergence Zones" (just in the zone area itself; not necessarily showers outside the zone). Although zones run the gamut and a collision of weaker winds will lead to more gentle, less-noticeable rains.

* "Unstable Air": The days -- usually one or two days -- after a strong storm system has passed and we talk about possible afternoon thunderstorms or hail, of if you see those towering cumulus clouds bubbling up -- those signify convection and stronger updrafts, for potentially "wetter" rains. Those showers tend to be more scattered than widespread and thus can sneakily fall amid sunbreaks, so those are most apt to catch you off-guard.

Usually any forecast that would feature conditions for large raindrops tend to get a bit more attention anyway and will typically be specified -- so watch for these terms and get an early heads up that the GoreTex might be something to toss in the car.

Typical small drop/"Is it even raining?" type storms:

* "Strati-form rain" -- not a term we'd really use in a forecast, but those frequent days with low, gray clouds that seem to be flat and take up most of the sky are usually going to bring just light to moderate rain that we call know and love.

* "Warm fronts" -- they typically have weaker dynamics and smaller rain drop sizes.

* "Patchy fog" -- that can bring drizzle or mist. You'll barely get wet.

* "Weak cold fronts" -- those generally don't have much dynamics either.

Then again, with the soggy way this autumn has started, even if it's raining heavily, it might be time to just surrender

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