• Answers to fog questions from KIRO 7 Meteorologist Brian Monahan

    By: Brian Monahan


    What causes fog?

    There are two types of fog: advection fog and radiation fog. Radiation fog is by far the most common type. This happens when the temperature falls to the dewpoint temperature, usually after sunset. The dewpoint is a measure of the temperature to which air must be cooled for condensation to form; when the air temperature is equal to the dewpoint, the humidity is 100%. This has been the pattern we’ve been in the last few days: morning fog lifts to just low cloud cover then, as the air cools after sunset, redevelops as the humidity reaches 100%. Advection fog forms when warm, moist air aloft moves over cool land (more often water) below. This is what generally gives rise to sea fog. The conditions for fog development are ideal when the wind is light.

    Is there a time of day that fog happens more?

    More often than not, the late evening and early morning hours are foggiest times of the day. This is when temperatures are at their coolest and the humidity is the highest. In a typical pattern, with the heating of the day the temperature increases enough that it rises above the dewpoint (thus lowering the relative humidity). Also, wind picks up during the daylight hours and this also helps scour out low cloud cover and fog. This week, we’ve been in a very benign pattern in terms of low level wind: high pressure aloft has allowed for only light wind this week. This is a large part of why the fog has been so persistent over the last several days.

    How long will the fog in Seattle last?

    Today, fog will burn off early this afternoon but plenty of low clouds will linger through mid-afternoon. By late afternoon, just like yesterday, a light, drying downslope wind (northeast), will help mix up the atmosphere enough for a partly cloudy to mostly sunny sky for an hour or two before sunset.

    Why has the fog been so heavy lately?

    A large dome of high pressure aloft has given rise to the stagnant weather pattern. This high is creating a low level temperature inversion; the air temperature is actually increasing rather than decreasing with height. With a lower sun angle this time of year, there’s less of the sun’s energy available to heat the low levels of the atmosphere and remove the inversion (or lid on the atmosphere – this lid traps moisture in the lowest couple thousand feet of the atmosphere). It’s not until late in the day that we get enough of a breeze and enough heating to burn off the low cloud cover (if at all). An offshore – or northeast – wind makes this process a bit easier, which is why we had some sun yesterday and will have some again on Wednesday. Tomorrow the wind gets very light, if not slightly onshore, and this will mean low cloud cover sticks around most, if not all, of the day.

    Is there pollution in fog?

    Not in the fog, per se, but in this stagnant weather pattern, pollutants certainly collect under the atmospheric “lid.” A little mist and drizzle actually helps air quality in this pattern just a bit, as it helps knock some pollutants out of the air. I checked with the Puget Sound Clean Air Agency and there are no air stagnation advisories in place. Air quality is only an issue right now for the most sensitive groups.

    What’s the best way to drive in fog?

    Leave plenty of distance between you and the car in front of you and be sure to turn your low beams on, even during the day. This will help other motorists immensely in seeing you.

    Does fog mean that another weather pattern is coming?

    Actually, in this pattern, fog is a sign that nothing is changing. High pressure is sitting overhead and the atmosphere is stagnant. In the case of advection fog, that IS a sign of the pattern changing: warm, moist air is moving in over top of cooler air/land/water and this usually signals the arrival of a warm front.

    Why is it fogy in some areas and not others?

    Just a matter of where the inversion (or lid) is. The top of the inversion, by the way, is the part of the atmosphere where the temperature resumes decreasing with height. It is above the inversion that the sky is clear (mountains).

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