Weather Blog

Logging a foggy blog today

Logging a foggy blog today
Fog is a way of life around here. Seattle averages a whopping 41 days a year with dense fog -- meaning the visibility dropped to 1/4 mile or less at some point of the day.  And October is the foggiest month, averaging 7.1 days of dense fog per month.

But what causes fog? And why is fall our foggy season? Let's dive in: Fog is just basically a cloud on the ground. It's formed when the air cools and condenses to saturation, at which point what you see as fog is just a high concentration of water vapor drops -- the same physics that's making that cloud at 15,000 feet.

As for how to get a grip on whether fog will form, it's best to keep an eye on the dew point. Also known as the "wet bulb" reading, it is the temperature at which the air would become 100% saturated.  The closer the temperature is to the dew point, the more humid the air is, and the higher the dew point, the easier it is to get fog. And when the temperature matches or is within a degree or two of the dew point, that's when fog is likely to form.

For example, the temperature in Seattle all morning long Tuesday was stuck at 45 degrees, with the dew point hovering between 43 and 45 degrees. And sure enough, they were in a dense fog.

A Traveling Fog

There's two main ways to get fog -- advection fog or radiation fog. Or what might be an easier way to think of them is "traveling fog" and "home grown fog."

Advection fog is when warm air is brought in over an area that has cold air pooled at the ground. This is very common off the coast during the spring through early autumn time when the warm sun heats the air over the cold Pacific Ocean, but then the cool ocean waters bring the temperature back down to the dew point, creating fog.

Depending on the weather pattern, that fog will then be carried inland via westerly flow overnight and brought into the I-5 corridor. That's known as a "marine push" -- a push of marine air and fog off the ocean and toward inland areas.

This is the deal in June when you go to sleep when it's clear and wake up and it's cloudy, only to have the clouds gradually burn off.  For the greater Puget Sound area, this is more of a low overcast day and not so much a dense fog.  But this is a good example of a "travelling" fog since the fog is formed elsewhere then brought in.

Home Grown Fog

The fog that typically forms in the fall and winter is the radiation fog, or fog that forms right here.

In this case, you typically want clear skies and night, and a recent rain doesn't hurt (as the wet ground adds more available moisture to the equation) but isn't required. As the day's heat radiates back into space (thus the name radiation fog) the temperature drops down to meet the dew point, and fog forms.

It's easier this time of year because the nights are longer and colder, so the temperature has an easier time reaching the dew point.  The reason Seattle was foggier on Tuesday morning than Monday morning as we had some moist air move in from the south, raising the dew points and making it easier for fog to form.  Monday morning dew points were around 40, while Tuesday they were around 45.

What About Inversions?

Inversions go hand in hand with radiation fog.  Normally in the lower atmosphere, the air gets colder as you go higher in altitude. But with an inversion, the air briefly gets warmer as you go higher.

This occurs when you have a pool of cold air near the ground that gets stubborn. A typical chain of events is it's a clear and calm night. The warm air gets radiated back into space leaving cold air near the ground.
 But cold air is heavier and denser than warm air, so it takes some good nudging to get rid of it. If there's little or no wind, that cold air will just get entrenched. And if the moisture content is right, it'll create a dense fog too due to radiation cooling. Meanwhile, the sun comes up and warms the air above that cold air mass.

In these cases, it can literally be warmer at Snoqualmie Pass than in Seattle. That was the case Tuesday -- at 1 p.m. it was 59 at Snoqualmie Pass and 53 in Seattle.

The sun will start to eat away at the inversion, warming the top layers of the cold air mass -- sort of like eating the top layer off a club sandwich, then the next layer of meat ,etc.

But in the fall and winter, the sun is low on the horizon and daylight is short, so it's not very hungry and inversions can become very stubborn, leading to several days of fog and possibly pollution problems as the air becomes very stagnant near the ground and pollutants have nowhere to go.  And if it's foggy, it can help sustain an inversion since that prevents a lot of the sun's warmth from reach the ground and breaking up that cold air mass.

Other factors are in play -- breezy winds can inhibit fog and inversion development because it mixes the air more and keeps the coldest air from pooling near the ground. An east wind is even better since it's typically a dry wind. That's why on clear days with an east wind, those that are windy are typically sunny while those living in areas sheltered from an east wind, like Pierce and Thurston Counties, tend to be much foggier.

But as foggy as it is here, it could be worse. Cape Disappointment, along the southern Washington coast, is considered one of the foggiest places in the world, thanks to the local terrain that can trap cold air in that spot. The Crescent City/Humboldt Bay/Eureka California area is also among the foggiest in the nation for the same reason.