At 2:52 p.m. this afternoon the thermometer at O'Hare Airport reached 102º making today the hottest day in the city since the mercury peaked at 104º on July 13, 1995 during Chicago's tragic killer heat wave. That same day Midway Airport broiled at 106º. Midway observer Frank Wachowski reports a high so far today of 104º recorded at 2:40 p.m. equalling the 104º high reached there on July 30, 1999.
Steve Kahn WGN-TV/Chicago Tribune Meteorologist
Drought
Dry weather that is withering Chicago and Illinois has prompted increasing mention of a word not often heard in this area - drought.
What is drought? In the United States the mention of drought brings to mind scorched lawns, parched fields and withered crops, dusty rural roads, dry wells, sluggish rivers, shrinking reservoirs, water restrictions and failing water supplies. In its extremes in some countries, drought means hunger, starvation and famine, human emaciation, mass migrations of peoples, even death. Sometimes drought has led to war.
There is no single definition of drought that is satisfactory for all situations. We may say truthfully that we scarcely know a drought when we see one. We welcome the first clear day after a rainy spell. Rainless days continue for a time and we are pleased to have a long spell of such fine weather. Dry weather continues and we become a little worried. Only when the dry spell stretches into weeks and months do we realize we are in trouble.
The first rainless day in a spell of fair weather contributes as much to a drought as the last, but no one knows how serious it will be until the last dry day is gone and the rains have come again.
In that sense, drought is a retrospective phenomenon. We cannot recognize that we are experiencing it until well after it has begun, and we cannot identify its end until after normal precipitation patterns have returned.
Drought is the deficit that results when the amount of water available in the soil is not sufficient to meet the demands of evaporation and plant usage for an extended period of time and over a large area.
What causes drought? Meteorologists and climatologists do not have satisfactory answers. Until the massive and costly drought of 1988, most textbooks on weather and climate mentioned drought only briefly. However, the economic impact of the 1988 event was so great that its effects were not forgotten, and serious inquiry into the causes of drought began.
Research continues, but answers are still not at hand.
The occurrence of drought seems to violate all the rules. Drought violates the law of averages in that it is a major and persistent departure from average precipitation. Drought seems to violate the theory of probability because consecutive weeks and months of far sub-normal precipitation is extremely unlikely in otherwise well-watered regions.
We cannot even explain drought in terms of climatic change because the rains always return -- eventually. The evidence is that drought has always been only a temporary condition.
Many definitions of drought are possible, and many schemes exist for classifying drought. Here's a classification that fits well for the United States:
Permanent drought: This is drought associated with arid climates. Adequate moisture is permanently lacking and vegetation is sparse. Permanent drought is a feature of the southwestern deserts of the United States and much of the intermountain west.
Seasonal drought: This occurs in climates with distinct, well-defined rainy and dry seasons. Vegetation, adapted to that climate, grows during the rainy season and becomes dormant during the drought period. Coastal California experiences seasonal drought -- its winters are wet and its summers are rainless.
Contingent drought: This is drought that results from the occasional failure of precipitation in areas that are otherwise well-watered. It occurs because, over time, rainfall is irregular and variable everywhere, even in moist climates. It is the kind of drought that Chicago and most of the United States experience, though rarely.
Richard Koeneman WGN-TV/Chicago Tribune Meteorologist
