It’s October and the weather is just about as boring as it can get.  Other than relatively chilly temps and the possibility of snow over the Cascade passes the next couple days, there’s nothing meteorologically or seasonally interesting going on:  no atmospheric rivers, no Indian summers, and no big early-season freezes either.

One thing that’s perfectly normal and iconic about October in Western Oregon and Washington, is the arrival of valley fog.  We already had a couple foggy mornings in Portland this month, the most recent being last Thursday.  Our fog season normally begins with October and continues through the end of February.  In fact, October & November are the two foggiest months at PDX airport (average 6 days per month), with the three winter months just slightly behind.  Most fog in the valleys and basins of the Northwest is of the “radiational” type: moist low-level air is cooled to the dewpoint when the skies clear out above, and that moisture from recent rainfall condenses into fog.  (The coast is a different story altogether.)

It makes sense that winter would be foggy here:  it’s our rainy season, and temperatures are generally cool.  Clear winter nights almost always lead to fog in the lowlands, unless we get enough dry air in the picture; then things cool further and we get frost instead.

But why is fall foggier than spring?  After all, spring comes at the end of our wet season, when the soils are more saturated after all the winter rains.  And it still gets pretty cool at night during the springtime.  There is also more new plant activity in the spring, which releases moisture into the low levels of the atmosphere.

The answer is not very intuitive.  In short, autumn gets more fog than spring because the airmass aloft is still fairly warm, but the angle of the sun is getting pretty weak.  That combination means more inversions in the lower levels of the atmosphere.

To illustrate this, let us take a look at the sounding climatology for Salem Airport, which is the best proxy we have for Portland.  The red and blue squiggles at the top and bottom represent the record high and record low 850mb temp for any given date.  The black curve in the very middle of the graph represents the normals, smoothed out to prevent too much day-to-day noise.  Notice how airmass temps peak in mid-August, not late July and early August like the surface.  The disconnect is especially pronounced in winter; instead of December/January being the two coldest months like at the surface, it occurs in February/March instead!

The normal 850 temps on October 15/16 are around +7 to +8 degrees Celsius, depending on the reckoning you use.  Notice how this is much warmer than either April or May.  In fact, normal 850s on Halloween are about the same as they are on Memorial Day!  Yet at the surface, temperatures in October are only about 1-2 degrees Fahrenheit warmer than April, and several degrees cooler than May.  Why doesn’t the warm October atmosphere manifest at the surface?

The sun angle in October is much weaker than in April or May; in fact, mid-October is the same as late February!  That means warm air aloft has more trouble mixing down to the surface when skies are clear. Instead, the long clear nights allow for cool air to pool in the valleys and basins, resulting in the onset of “inversion season.”  Now, October isn’t like November through early February, when the sun is so weak that the cool air and low clouds remain trapped all day long.  We can still get very nice weather in October when skies are clear.  However, weak sun and inversions are now hampering our ability to get really warm at the surface, no matter how warm the airmass.

This helps explain why October is relatively cool in the lowlands of the Pacific Northwest.  It also explains why our “surface heat potential” under a warm upper-level ridge drops off so rapidly as we move through September and October; the sun simply runs out of energy and radiational cooling begins to overwhelm the lower atmosphere.

Now, why do stronger inversions lead to more valley fog?  Inversions tend to cause the lower levels of the atmosphere to become stagnant.  The air (and water) under an inversion layer gets trapped, as do pollutants from human activity.  When there is strong high pressure in the atmosphere, it causes more sinking motion…trapping low-level air in even more tightly.

Ground fog can sometimes form on cool and clear spring nights, if there has recently been heavy rainfall.  But it is much quicker to leave once the sun comes up.  The stronger sun and relatively cooler air aloft, means inversions are less likely to form.  The residual moisture from rainfall mixes out of the lowlands more easily = less fog.  I have seen episodes of dense fog in the Willamette Valley before in March, notably in 2004.  That year we had a very warm airmass overhead for several days, yet not enough northerly/easterly flow to clear out the moist lower layers. On some days the fog didn’t break until early afternoon.

Will there be much fog over the next week?  We actually have several passing disturbances (cold fronts) coming up, for clouds and rain.  That is not ideal for fog formation. The best chance may be Monday night and Tuesday morning, if we can get skies to clear out.

For now though, prepare for more rain the next few days, with snow below Cascade passes by tomorrow night.  Expect to need your heaters this week as well…high temps will only be in the 50s most days.  Quite different from the beautiful warm Octobers I recall, with tons of sunshine and many days in the 70s!

-Karl