Unlike extended droughts which can last for years on end, summer droughts are seasonal droughts which can return year after year, and which are usually broken by a standard seasonal change in the local weather patterns. Many parts of the world which do not have strong temperature variations nevertheless have strong wet/dry seasonal cycles: where the dry season is designated “summer” and the wet season is designated “winter”. In India, however, the drought season corresponds with northern hemisphere spring: with the monsoons arriving near the beginning of June.
Extreme seasonal variances in rainfall thus are common in most parts of the world. Drought, however, means something more than just a normal seasonal variance. To be a drought, there must be signficantly less rainfall than is normally expected for that time of year. For most practical purposes, a dry spell becomes a drought when the amount of rainfall during the dry season is both significantly less than normal and falls significantly short of normal local needs.
Technically, it is not drought if the area is already designated as desert. Drought can only occur in places where the region normally gets cycles of precipitation above the baseline desert precipitation and evaporation levels: and then only when that expected precipitation fails to come.
In North America, most parts of the United States and Mexico between the mountains and the Mississippi are commonly subject to summer drought, especially in the prairie and plains adjacent to the Mojave Desert. Under normal conditions, areas outside the direct rain shadow of the Rocky Mountains and the Sierra Madre will avoid a full summer drought through periodic heavy thunderstorms. Shifts in the jet stream, however, can completely kink the normal frontal patterns to make the thunderstorms develop just a little further east or north.
East of the Mississippi and even as far west as Houston, summer drought used to be much rarer. Both thunderstorms and days of drenching rain or snow commonly develop year-round from the intersection of hot, humid Gulf of Mexico air with the Ozark and Appalachian mountain ranges. Even if weather conditions were such as to sidetrack normal storm development, an occasional drenching tropical storm could be counted on to restore the reservoirs in the deep south. Yet the Atlantic hurricanes have been largely avoiding the United States since the record season of 2005: and Gulf State reservoirs are noticing the hit. Even parts of the Everglades are now in danger of drying out, even catching fire.
Much of Europe and Asia south and east of the Alps also have recurring summer droughts, as do Spain and Portugal. Those seem to be getting worse. In 2011, nearly all parts of southeastern Europe have been “enjoying” unrelenting sunshine since late spring.
Indian summer droughts are particularly notorious. They are typically broken by the arrival of the monsoon season: but for weeks and months before the monsoon rains arrive, Andhra Pradesh, Uttar Pradesh, and the Thar Desert over the border into Pakistan will see blazing hot temperatures which are among the hottest on the face of the earth, without a single drop of rain. Normally the monsoon season comes as a welcome relief: but in 2010 the extended heat wave was followed by a devastating monsoon which flooded a third of Pakistan, completely disrupting the growing season and ensuring hunger in the year to come.
City folk and tourists frequently see droughts – in the absence of vicious heat waves – as a sunny vacation without rain and thus something to celebrate: but even for tourism, droughts can be a mixed blessing. Tourists to the Greek isles celebrate its almost perfectly sunny summertime weather on the beaches: yet both tourists and locals in Athens and other large cities may curse the lung-choking “nefos” that comes alongside the summer drought to rot away historic marble architecture. The classical Greeks who built the Parthenon never envisioned the modern smog caused by summer atmospheric inversions.
Summer drought can damage established agricultural patterns and destroy entire crops: but it does not have to. It depends on what is planted and whether the drought is expected. Desalination and irrigation from a reliable river can completely compensate for an expected summer drought. Even as far back as the ancient Egyptians, the Nile flood made all the difference: and sophisticated devices were built to measure the degree of flooding, which would determine the pattern of agriculture and trade for the coming year.
The truly devastating summer droughts are the ones which come unexpectedly: especially after a winter with low snowmelt. Even a few weeks of this kind of summer drought can wipe away an entire agricultural year. When such a summer drought hits an area whose water supply is already stressed from previous low-precipitation years, even irrigation may no longer be an option. Very few crops can survive on almost no rain at all.