A 15 yr (1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000) climatology of derecho-producing mesoscale convective systems (DMCSs) is presented in order to better delineate their spatial and temporal patterns. Several significant results emerged from the analysis, including the development of the NW flow corridor as the dominant derecho activity region in the climatology. Results suggest that, as the sample size of DMCSs increases (230 events), the prominent derecho activity corridors across the eastern US become located in the northern Plains through the Ohio Valley, with a secondary maximum in the southern Plains. Evidence further suggests that climatological factors strongly control the distribution of derechos. For example, an anomalously strong 500 hPa height gradient existed coincident with the northern US derecho activity corridor. Another aspect of derecho development is related to the temporal distribution. Evidence suggests that derecho systems tend to occur in groups or 'families', several events occurring within several days. The synoptic environment also appears to be responsible for activating these corridors and providing an environment conducive to DMCS grouping. KEY WORDS: Derecho · Climatology · Mesoscale convective systemsResale or republication not permitted without written consent of the publisher Clim Res 24: 129-139, 2003 warm season anticyclones appears to favour and subsequently activate some derecho corridors, particularly in the Great Plains and northern tier of the US. Once synoptic-scale conditions are favourable for DMCS development, the DMCSs, after being initiated by localized forcing, tend to group or 'train' over a similar geographic region. In other words, a specific region may experience a series of DMCSs over successive days. DATA/METHODOLOGYThe primary data source for this investigation is the Storm Prediction Center's Severe Plot 2.0 software (Hart 2000) containing a database of severe convective wind events (≥ 26 m s ) and documented wind-damage reports, taken from the NOAA publication Storm Data. In order to assure consistency, the identification criteria used in BM98 is also used in this investigation (Table 1). The derecho events were then mapped to facilitate visual inspection and ensure temporal and spatial continuity. Individual wind damage reports from each derecho event were then gridded on an 1°× 1°grid-cell domain using the Interactive Data-common Language (IDL) so that contours could be generated in order to discern the geographical and frequency distribution of DMCSs.Composite reflectivity values obtained from radar archives were also examined to ensure convective characteristics of many cool-season (September-February) DMCSs. These images were obtained from the National Climate Data Center's (NCDC) on-line radar archive (available at http://lwf.ncdc.noaa.gov/oa/ radar/radardata.html).Data from the NCEP/NCAR reanalysis were used to reconstruct the 500 hPa environment during months of DMCS activity. The reanalys...

Warm-season (1 May-30 September) hourly dewpoint data were examined for temporal changes at two weather stations in northeastern Illinois during a 42-yr period (1959-2000). This area has dense population (greater than 8 million), and shifts to more or less atmospheric moisture have major implications on cooling demands. The 42-yr period was analyzed as two separate arbitrarily chosen equally sized periods, the early (1959-79) and the later (1980-2000) periods. Analyses of data from Chicago's O'Hare International Airport and the Greater Rockford Airport showed a statistically significant increase in the number of hours with dewpoints greater than or equal to 24ЊC (an important cooling-plant threshold) in the latter period. Examination of heatwave periods indicated that later (especially 1995 and after) heat waves contained many more extreme dewpoint values. These increases in extreme dewpoint characteristics in northeastern Illinois affect the operation of, and suggest shifts in design criteria for, airconditioning systems and affect summer peak electrical loads.

Historical climate conditions were examined to determine whether climate information could improve decisions concerning the seasonal maintenance and operations of regional outdoor swimming pools in heavily urbanised northeastern Illinois. Key weather‐sensitive issues defined for pool companies concern time periods for maintenance (including pool painting), weather limits to pools being open, and hot days which extend hours of pool operation. The specific conditions included the frequency of spring and fall painting periods (defined as dry conditions for two or more days with Tmax ≥18°C (65°F) and Tmin ≥4°C (40°F)); summer inclement weather days (days with precipitation ≥2.54 mm (0.10 inch) with not more than 2 consecutive hours of dry conditions during open swimming hours and/or Tmax ≤21°C (70°F)); and hot days (Tmax ≥32°C (90°F)) were examined. Climatic records for the region going back 42 years revealed that there were nearly twice as many fall painting periods as those in the spring, indicating that greater emphasis should be placed on pool painting after the swimming season. The average number of inclement weather days was more than twice the number the company budgeted for at the beginning of each swimming season. Plans to extend swimming hours on hot days involves a risky decision since the number of hot days varies dramatically from year‐to‐year in the Chicago region. Studies showed that winter negative sea‐surface temperature anomalies in the central Pacific were related to an above average number of spring painting periods. Also, a higher number of hot summer days occur following El Niño‐classified winters. Results reveal that decision‐makers in the swimming pool management industry can improve several management decisions by including climate information. Copyright © 2002 Royal Meteorological Society.

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