Weather Blog

At least it's a dry rain...

under the umbrella
Raise your hand -- how many of you who have lived here for, say, five years or longer carry an umbrella with you when you go out? (OK, don't actually raise your hand, because I can't see you and you'll look goofy to anyone within eyesight of your computer, and if you're outside, you might accidentally hail a cab or bid in an outdoor auction). 

Anyway, I'll bet the number is pretty small, because long-time Northwesterners pride themselves on not needing an umbrella.  Part of the reason, I'm sure, is that you get used to the rain and carrying the umbrella would just be cumbersome every day.

But another part of it is -- our rain isn't all that heavy. In fact, dare I say that just like Arizona is a "dry heat", we have a "dry rain", in that many times, you don't get all that wet and you dry off shortly after going inside. A large part of that is that our raindrops aren't that large -- small enough that you don't get too wet if you're caught outside. Case in point was the drizzle this morning -- enough to wet the roads and make your windshield wipers go, but not really enough to soak anything.

Contrast that with showers in the Midwest or East Coast, where 10 seconds is enough to soak you to the bone.

So, why are our raindrops and showers usually not so drenching? Our updafts are typically very weak.

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.

That's why thunderstorms tend to have large raindrops, as they tend to also have strong updrafts.  Same story for hail -- the stronger the updraft, the larger the stone can grow before it's heavier than the force of the wind and falls to the ground.

Strong updrafts and strong storms are caused by large temperature differences. Look at the recent flooding and tornado outbreak in the Midwest, and you can see how that occurred with how cool it's been around here lately, and how hot it's been along the Eastern Seaboard.  Put 50 degrees on one side and 95 on the other, and that's one monster front with very strong updrafts.

Same with the tropics, where the intense sun heats the ground to much warmer temperatures than aloft, which aids in updrafts and thunderstorm development, and how a quick shower there is so wet.

(This also explains how come despite 160 days of rain here a year, we get less annual rainfall than many cities along the south and east, because their storms dump more rain in a shorter period of time. A 20 minute shower here might be 0.02" while in Miami, it might be 0.20")

Now, there are, of course, times when we do get heavy, soaking rains here. Typically those are during Pineapple Express events, where we have a very warm air mass tapping into heavy, tropical moisture, and on days after storm passes and cold air moves in aloft, allowing for greater temperature differences and stronger storm development (that's the deal when we talk about "unstable air")  Convergence Zones can also whip up some very heavy showers, but that's due to strong updrafts caused by the colliding winds.

So I wouldn't recommend putting your entire umbrella collection in the garage sale (besides, how many tourists would show, anyway?) but don't feel like you have to have it surgically attached, either :)