Life + Outdoors » Garlick's Notebook

Why do we experience summer fogs?



Sprinkle flour upon a lazy Susan and turn it steadily in a counter-clockwise direction to simulate the rotation of the Northern Hemisphere. Roll a marble straight across the rotating surface in any direction. You will find that the path recorded in the flour curves to the right. This apparent curvature of trajectories is known as the Coriolis Effect.

Warm tropical air rises, cools by expansion, produces much rain and then moves away from the equator. The Coriolis Effect deflects this dehydrated air to the East so it cannot reach beyond about 40 degrees latitude in summer. Much of it then descends in a region named the Pacific High. My first diagram shows that when a parcel of air descends and spreads out, its upper parts descend farther, thus warming more by compression. This results in an “inversion” of the normal temperature profile, trapping cool air, the Marine Layer, below warmer air above.

Now consider the daily baking of our Western states under a summer sun (2nd diagram). The air warms and tends to rise, causing an atmospheric low. Air from the Pacific attempts to flow east toward the region of low pressure. But Coriolis deflects that Pacific wind toward the South. This wind (good for kite flying) then drags upon the surface of the ocean, which responds by turning right again, thus moving in a Westerly direction, away from the coast. Cool surface water (of the California Current) is replaced by even colder water from depth, a process known as upwelling (good for our fishing industry).

That cold, upwelled water chills the air, which has by now picked up a little moisture from the ocean. Wind stirs the air within the marine layer and, as it rises and further cools, its moisture condenses into droplets. Those droplets form fog (good for growing redwoods). The consequent temperature contrast between foggy Eureka and sunny Redding is astounding. Eureka residents drive over the Coast Ranges to see the sun, while Redding residents drive west to escape the heat.

The trade winds eventually return air to the tropics, and the cycle continues.

Don Garlick is a geology professor retired from HSU. He invites any questions relating to North Coast science, and if he cannot answer it he will find an expert who can. E-mail’s Prof. Richard Stepp pointed me in the right direction (my mind may have veered south).

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