es oteMa lace walaR se ia i Radlk tiled eo Oe Ie we tat PD ene ae eee . ~ . + 151 These observed wind patterns suggest that we may have found a heat-island circulation much like that described by Lowry“ and pictured schematically in Figure 112, Two conditions are usually required for city-rural circulations to begin. The first is a temperature excess in the citv; the second is sufficiently light ambient winds. A plot of hourly temperatures at Midway, (YHare, and Argonne (Figure 113) shows a definite temperature excess at Midway Airport (in the city) relative to O’Hare Airport and Argonne (essentially rural sites). Figures 110 and 111 show that surface winds were light in the Chicago area on the 19th, and Figure 114 shows light winds aloft at Peoria, [linois and Green Bay, Wisconsin—the two rawinsonde stations nearest* to Chicago. Therefore, favorable temperature and wind conditions for a city-rural circulation did exist. The available data suggest a city-rural circulation somewhat different than the classical form. The classical case is for a somewhatcircular city. The corresponding circulation is a ring vortex, with air rising in the center of the doughnut, flowing outwardaloft, and returning to the surface at the outer perimeter. Chicago’s topography and demography are not classical because the city lies along a lake shore. The heat island is not circular, but is instead a northsouth band. Thus, heat island circulations in Chicago may have an aris of symmetry, rather than a center of symmetry. For analysis of this episode, we must also consider possible interactions with the synoptic scale air flow. This point is not clear; perhaps the outside air simply flows around or over the city air ‘dome’ with little interaction between the two. There is some evidence that interaction did occur in this case, however. The wind fields at 1500 and 1700 (Figures 110 and 111} show definite northerly components in both converging currents. This suggests that momentum is being transferred downward from the northerly winds alofty (Figure 114). The Chicago circulation observed on 19 January 1966 appears to have had the following basic characteristics: 1. Relatively warm city air rose over the axis of the heat island; 2. To preserve continuity, air from the edges of the | city was drawn inward; *Peoria is 210 km southwest and Green Bay is 300 km north of Chicago. *It is necessary to verify that the circulation reaches high enough altitudes to interact with the northerly winds. The depth of penetration of the rising city air was estimated from the 0600 temperature sounding at Peoria and the Midway high temperature of 24. The depth of penetration was about 1 km. This is high enough for the rising air to eneounter north winds, > ie lel cba a ea OMe ida etl tek 3. At the upper limit of convection, the rising air began to spread outwards, to the east and west, at the same time gaining momentum from northerly windsin this region; 4. The air was thus carried to the outer limits of the circulation, perhaps 10-20km away, and reached groundlevel at a point south of where it ascended; 5. From this point, it was drawn back into thecity, where it was repolluted. If this circulation pattern prevailed, air could have been recycled through the city several times before finally leaving at the south end of the metropolitan area, One additional item of evidence to support the existence of this kind of circulation is found in the recorded SO. concentration of air arriving at TAM-4 from the northeast. Figure 107 shows concentrations near 0.20 ppm in the northeast winds. These are un- usually high concentrations for winds off the lake. This 1s evidence of a recirculation of once-polluted air back into the city. One curious aspect of this episode is that the circulation died (about 1900} as the city-rural temperature difference increased. Theory predicts the oppo- site—the strength of the circulation should increase in proportion to the temperature difference. The explanation probably lies with some change in the external conditions (especially winds) that permit such loeal circulations. There is considerable latitude for speculation here, of course, but one thing is clear: an unusual circulation prevailed in Chicago on 19 January 1966. This event was associated with severe air pollution in Chi- cago, and as far as we know, has not, been described for this city before. From only one case, one should not try to identify critical parameters or their threshold values for heat island circulations in Chicago; however, two points are worth noting: 1. Surface winds outside the city were mostly less than 5 kt between 18 and 20 January. Winds aloft at Peoria and Green Bay were mostly less than 10 kt up to 700 mb on the dayof the circulation. 2. The city-rural temperature difference ranged from 3 to 10° F on January 19. The larger values occurred at night. 27 December 1966: A Wind Off Lake Michigan The surface weather map for 1200 on the 27th is given in Figure 115. The ridge line has just passed Chicago. This movement caused a surface wind shift to easterly. The storm over Oklahoma developed rapidiy and moved toward Chicago during the after-