Weather Blog

Watch: Why there's a cool breeze just before a rain storm

Watch: Why there's a cool breeze just before a rain storm

We all know rain showers are about as common in the Puget Sound region as a corner coffee stand. But around here, even in the mundane, there is beauty -- at least in the eyes of this meteorologist.

Have you ever been outside and felt that cool rush of air before in the minutes before it starts raining? Ever wonder why that happens? I've got some great weather videos that will help demonstrate.

Greg Johnson, who runs two HD cameras 24/7 out his window and publishes current photos and time lapse videos at SkunkBayWeather.com, recently sent me a compiled time lapse video from Friday, April 4. It was one of those "showers and sunbreaks" kind of days we see often in the spring, and his point was to gaze at the bubbling cumulus clouds and passing showers with a colorful sunrise and sunset.

But there was something in there that caught my eye in the evening -- a cloud let go a rain shower just as it came into the camera's view, and it provided a fantastic illustration why we get that cool blast of air.

But first, let's start with a big water balloon exploding! It'll be fun but also important (and probably the only time you'll ever hear that water balloons have deep importance!) Just watch and take this in:

What do exploding water balloons have to do with meteorology? Let's watch Johnson's video from Friday and see if you notice any similarities.

First, let's watch the event unfold in time lapse, using both his camera views stitched together. Watch the rain shower fall, and the blast of air push out toward the camera:



Did you see it? That rain shower began falling just on the left side of the screen, and then a blast of air raced outward (as evidenced on the waters of Skunk Bay), hitting the American Flag and camera moments later.

Here it is zoomed in a bit and slowed down. Watch the wind come roaring in off the water:

The cool breeze comes from the downdraft of cooler air aloft that gets pushed down as the rain comes down, then spreads out as it hits the ground. Think about our exploding water balloon and how the water shot outward -- in this case, it's the air pushed out from our falling rain.

Closer review reveals how a passing shower can really change the weather on a dime.

Here is 6:20 p.m. -- moments before the downdraft hits -- you can see the ripple on the water is juuuuust about to reach the shoreline. It's 53.3 degrees with a calm wind:



And here it is just over 60 seconds later -- that initial burst of cool wind as the outflow from the shower reaches the camera. In this case it's 19 mph:



And then after the wind starts, so does the rapid cooling. About 30 minutes later, the temperature has dropped to 48.7 degrees -- a 4.6 degree drop!



But it's not just on the ground. You can see this happen in the air as well. Did you notice the clouds on the second camera form from apparently nothing? Watch again when this video switches to the second camera shot -- when the wind comes in from the left, clouds form in the sky and roll along, as if the rain shower sneezed or something:



Here is in still shot:



This is from the downburst creating its only little mini-convergence zone. Note in the time lapse video the general air flow is moving right-ish to left-ish (camera is pointing north, so clouds are moving southeast to northwest) but here comes the downburst draft flowing due west-to-east into that opposite flow, crashing together and briefly causing a convergence of winds. The small updraft then created its own clouds.

Now here is the whole thing from start to finish, first the full event, then zoomed in and slowed down a bit more so you can see all of these events in action -- all from just a little rain shower that fizzled not too long after.



On grander scales, these downbursts can cause major problems

What I've illustrated is a typical weak Seattle shower. No harm, no foul. But energize that storm into a super cell thunderstorm as occur in much of the rest of the nation, and now this process can intensify and do damage.

In the Desert Southwest, this is the process that creates those massive dust storms (or "haboobs"). The downdraft from an intense thunderstorm creates much stronger outflow winds -- sometimes 40-60 mph plus. Put that over the dust that is much of Arizona or New Mexico and now you've swirled up a major dust storm as that strong wind boundary expands outward.



In the not-so-dry parts of the nation, this is how you get damaging winds during severe thunderstorms -- for example, a super cell that went through Denton, Texas earlier this month brought a burst of wind measured at 82 mph followed by 0.60 inches of rain in 20 minutes and a temperature drop from 81 to 64. Now that's a major outflow from a downburst of rain. In those cases, it is very close to an actual water balloon letting loose from from thunderstorm -- a massive amount of intense, heavy rain that can bring instant flooding and hurricane-force winds rushing outward.

Here is one in action that hit Norman, Oklahoma. Note how it goes from relatively calm to "end of the world" in just a few seconds as a massive downburst hit the man's backyard. (Note, there is a little off-color language in this one):



And if you've heard of "derechos" like the one in 2002 that left a 700 mile trail of damage in the Midwest and East Coast in 2002, those too are related to the same physics we saw in our little shower on Friday. Only in this case, it's a line of intense thunderstorms causing a constant outflow racing out ahead of the storm as the line moves east. In that 2002 event, some wind speeds were clocked over 90 mph.

So next time you're out on a cloudy or mostly cloudy day and you feel a strong, cool breeze suddenly whip up, be ready for the pitter-patter of raindrops to soon follow. You'll be a few minutes ahead of your nearby peers. Just be glad our versions bring 20 mph winds and some light rain -- not the tree-busting drenchers the rest of the nation deals with.

And a special thanks to Greg Johnson for keeping those cameras going! I'm grateful that even our standard weak meteorological events don't go by unnoticed, and you never know what you just might learn from a typical showery spring day in April!