Plots below: Climate Division accumulated temperature and precipitation departures (ATD and APD) from the 30-year average for the current year, years with high numbers of human or mosquito WNV cases, and years with low numbers of cases.
Prolonged June-to-August periods with above-average temperatures (above zero line and/or long periods with increasing trend line) and below-average precipitation (below zero line and/or long periods with decreasing trend line) are most conducive to high numbers of human and/or mosquito pools testing positive for West Nile virus.
Few cases of WNV are found during wet summers that are either cool or hot. Cool, but dry, conditions often result in a late start to the WVN season.
Below: Area shaded in pale orange indicates range of past temperature (accumulated Departure from Normal) when high numbers of cases of WNV were observed. The weekly count of mosquito pools testing positive for WNV are obtained from the Illinos Department of Public Health. Variables can be turned off and on, scales can be expanded, and actual values can be viewed using a mouse.