From: Ronald Wong (ronwong@inreach.com)
Date: Thu Oct 14 1999 - 20:49:51 PDT
Message-Id: <l03102802b42add8400a2@[209.209.18.29]> Date: Thu, 14 Oct 1999 20:49:51 -0700 From: Ronald Wong <ronwong@inreach.com> Subject: Re: fog in the bay
Nancy Gordan asked:
>
>Could someone please explain the fog in the bay so I could explain the
>phenomenon to fifth grade students on the East coast who have never been to
>San Francisco. We are discussing the fog as part of our explorations unit.
>
Go to your library and look for an introductory book on "physical
geography". It'll tell you everything you would want to know about fog and
a lot of other things that you might like to cover with your students.
Fog is nothing more than a ground level cloud.
It comes in basically two types depending on the way it is formed:
1. Radiation fog - sometimes called tule fog - generally forms in low
lying areas on clear, calm nights. At night, the land radiates its
excess heat away and it becomes colder. Air immediately above it
becomes cooler and a temperature gradient is created where the air
is coldest at the bottom and warmer with increasing elevation. If the
temperature of this air drops below the dew point, fog is formed.
This fog is often quite shallow and, because of the temperature gradient,
is densest at ground level. Driving through this stuff can be
quite hazardous especially on small country roads which wind their
way through the countryside. It's not uncommon to drive into a dip in
the road on a cold, dark night and suddenly finding oneself deep in a
pool of tule fog not knowing whether the road continues straight or
curves to one side or the other.
When the heat of the morning sun raises the air temperature above the
dew point, the fog dissipates.
The "fog in the bay" that you are referring to is a different type of fog
and is called:
2. Advection fog. When warm, moist air moves over a colder surface (a cold
body of water or land) and is cooled below its dew point we get what is
referred to as advection fog.
Off the coast of California, seasonal winds produced by low pressure
systems create an offshore current that carries the warm surface water
further out to sea. This water is replaced by colder water welling up
from below. These same winds bring with them moist Pacific air. When
this air is cooled by the colder water to a temperature below it's dew
point, advection fog is produced.
Since our prevailing winds are out of the west, the fog moves in along
the coast line and pours through the Golden Gate. If the interior tem-
peratures are warm enough, the fog may not penetrate very far - dissipa-
ting just east of Twin Peaks and Mt Davidson and "stopping" at the Golden
Gate Bridge. At other times it will spread out onto the bay.
Over in your neck of the woods is a fine example of advection fog.
Unfortunately your class would have to go to sea to see it. The warm Gulf
Stream moving North off your coastline brings warm air with it. When the
Gulf Stream runs into the cold Laborador Current off of Newfoundland the
famous/infamous fog of the Grand Banks is formed.
If you happen to have the Exploratorium's Snack Book (try your library if
you don't) you might want to consider making the "Fog Chamber". Although it
is used to show how the lower pressure of the upper atmosphere causes
clouds to form there, the effect is still due to the cooling of moisture
laden air below it's dew point. What you see when you put a piece of "dry
ice" (solid carbon dioxide) on the table is the fog created by the mixing
of the moisture laden air with the very cold carbon dioxide gas coming off
the piece of "dry ice". A similar fog appears when you open the freezer
door on warm days (at least in a house that has no air conditioning).
You and your charges can probably can think of other examples.
Have fun - ron
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