Weather Blog

High pressure equals....rain?

High pressure equals....rain?
A drizzly day in Portland.
A few astute observers noticed Sunday that it rained a bit across the region, despite their home barometers solidly pointing into the "fair" category.

Indeed, atmospheric pressures right now are about as high as they get around here as a big ridge of high pressure builds in.  Typically, that spells a mix of fog and sunshine, but it rained enough to measure on Sunday.

For specifics: It rained in Portland around 5 a.m. Sunday morning with a current reading of 30.72" (1040.3 mb).  A few hours later, it rained in Seattle with a pressure of 30.65" (1038.7 mb). (Nice to know meteorologists will still be needed to help expand on those barometers' helpful forecasts of "stormy", "change" and "fair" :))

How can it do that? Isn't it supposed to be sunny when the pressure is this high? Most of the time, yes. And actually, most of the atmosphere was quite dry and primed for sun.

But in this case, we had some warm air from the budding ridge moving over some lingering cold air from the wake of our flooding storms. That created some low level clouds and just enough lift to squeeze out a little rain. But it was a near-surface event. A few thousand feet up, it was much drier.

From here on out, it'll be more typical high pressure weather -- fog and/or sunshine. The fog could be dense enough to squeeze out a little mist or drizzle (or, if you're reading this in your head, "drizzle or mist," lest you read that as "Mr. Drizzle"), but not enough to cause any issues.

As to why some fog on what would normally be sunny days, check out our Weather FAQ entry on inversions and I'll do a more in depth blog entry on inversions later this week.