ye 149 9 9 PP gh oh C ef C) ~ & & & s ye | 1016 1018 , ore 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