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Fra, 115.—27 December 1966 surface weather map, 1260 CST
fluenced by the same high pressure area and had light
winds.
The 19th was not an isolated day of high pollution,
but the third or fourth of a series. Between 16 and 20
January, hourly averaged SOQ. values were above 0.4
ppm most of the time at station 4 of the City of Chi-
cago Department of Air Pollution Control Network of
Telemetered Air Monitoring (TAM) Stations. (TAM
station locations are shown in Figure 110). The hourly
SO. and wind averages at TAM-4 are given in Figure
106. We see that the wind speed was alwaysless than
10 kt, often less than 5 kt, during this period. Wind
direction was very constant from the northwest. The
figure also shows typical sunny day temperature variation on 17, 18, and 19 January.
Figure 106 shows some interesting details of the
SO. concentrations during this period. The generally
high levels, the gradual increase with time, and the
extremely high concentrations at TAM-4 on the 19th
and 20th are all significant, but the temporary drop
in SO. levels with northeast winds, especially pronounced at TAM-4, is particularly interesting. A drop
in SOz with a shift to northeasterly winds is not surprising, especially at a lakefront station, because
these winds bring in clean air from over Lake Michigan. What is surprising 1s that a temporary shift to
northeast winds occurred over part of the city. Such
occurrences are common in warm seasons because of
lake breeze effects, but this situation developed dur-
ing the middle of January on a day when the high
temperature at Midway was 24° F.
When comparing Chicago temperatures to those of
its surroundings during winter, it is important to con-
sider carefully the condition of the lake surface. The
temperature and heat transfer properties of a solid ice
surface are much different from those of a water sur-
face. Mr. Ivan Brunk of the ESSA Weather Bureau in
Chicago reports (1969) that some ice was present in