Miata Mailing List: October 1994, Message #132
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   From: a.mccombs3@genie.geis.com
Subject: monoxide levels in the winter
   Date: Sat, 8 Oct 1994 13:09:40 -0400

RE: Meteorological inversions and emissions and all that:  In the DC area
(as well as in much of the northeast), temperature inversions are more
common in the summer than in the winter.  The temperature drop with altitude
increase is known as "lapse rate," and the "standard adiabatic lapse rate"
(i. e., the average drop in air temp as you increase altitude, without
moving the air up and down) works out to about 2 degrees Celsius (3.5 deg.F)
per thousand ft. of altitude increase.
 
If the lapse rate exceeds approximately 3 degrees celsius (5.5 deg. F) per
thousand feet of altitude increase (this is for air of less than 100 percent
humidity - unsaturated air), then rising air will tend to keep rising.  This
is because a given "parcel" of air will expand as it rises, due to the
pressure drop with altitude increase.  The pressure drop causes a
corresponding drop in temperature.  This averages out to the standard
adiabatic lapse rate noted above - BUT if the lapse rate in that particular
area, at that particular time, over that particular altitude range, is
GREATER than that of the rising parcel of air (i. e., it's cooler than the
rising parcel), well - then the parcel of air is relatively warmer than the
surrounding air through which it is rising.  If it's warmer, then it's less
dense (remember, it's free to expand in the open atmosphere), which means it
continues to rise.
 
The above is what makes those puffy little cumulus clouds you typically see
on summer afternoons, particularly along the gulf coast and in the midwest
and rockies.  To carry it to extremes, you get air mass thunderstorms.  This
has the effect of "turning over" the atmosphere, and cleaning out the crud
(like car exhaust) down at the bottom, and moving it somewhere else,
hopefully well diluted with fresh air so you don't notice it so much.
 
However:  When the air is pretty much at a uniform temp with altitude
increase (or worse, when it's warmer aloft - the inverse of the standard
situation, hence the term "inversion"), then that rising parcel of air will
still cool as it rises and expands, whereupon it will be more dense than the
relatively warmer surrounding air.  In actuality, it never starts to rise in
the first place, or may rise only a thousand feet or less before reaching
equilibrium again.  You can see this particularly on summer mornings in the
northeast; the crud is held so low to the ground, that you can see the
gray/brown layer of crud.  If you fly, it's even more apparent as you get
above the stuff and can see forever.  I've often wondered about breathing
that crap, when I look down through it when flying.
 
Now that we know what an inversion is, what it does, and what it looks like,
those of you around the major metro areas in the northeast will probably
recall seeing more of the above inversion scenario during the summer than in
the winter.  Inversions can certainly occur in the winter too, but as I
noted a while back, there are more frontal systems moving through the
temperate zones (the northern and central U.S.) in the winter, and bringing
fresh masses of air with them, than in the summer.  From what I've seen, the
inversion problem (with its accompanying smog and such) is worse in the
summer than in the winter.
 
Anyway, so much for the (very basic) meteorology lesson.  (Like I said a
while back, I ain't no meteorologist, but I've dabbled on the edges in
aviation for a while.)  So: Back to the original question - if the above
problem with inversions (or at least stable air) is worse in the summer, as
it appears to me, in major metro areas of the northeast, then why are we
getting "cleaner" gas in the winter?
 
--Jack (sorry to tell you more than you ever wanted to know) M. & Kansei--