Heat waves and cold waves

They aren't the flashiest of villains, but hot and cold spells – when they're sufficiently intense and prolonged – are among the deadliest of all weather events. Everyone knows that it's risky for us warm-blooded humans to spend much time in temperatures far above or below our margins of comfort. Yet only in the past several decades have we fully realized the risks that heat and cold can pose to even the most developed cultures. An estimated 50,000-plus Europeans fell victim to the brutal heat wave of 2003). And many thousands of vulnerable people die each winter when bouts of cold sap their immune systems, leaving them more vulnerable to influenza and other cold-weather ills.

Part of the reason we haven't given heat and cold waves the respect they deserve is because they make for less-than-compelling video. In an image-driven world, it's much easier to grab people's attention with scenes of a tornado or hurricane. Hot and cold air do their dirty work far more stealthily, away from the klieg lights. Often the victims are elderly people living alone on tight budgets, unable to keep their homes temperate enough for mere survival, let alone comfort. Sociologist Eric Klinenberg refers to heat waves – in a comment that could apply to cold waves as well – as “invisible disasters that kill largely invisible people”

Although it's perpetually warm in the tropics and chilly near the poles, the systems we think of as heat waves and cold waves tend to strike the midlatitudes, especially North America, Europe and Asia. Much of the time, the polar jet stream keeps weather systems marching across these continents at a fairly brisk clip. Temperatures rise and fall, but they rarely stray far from the seasonal norm for long. At other times, the jet stream arcs around a massive dome of high pressure extending far up into the atmosphere. Like a stone in a river, this high-pressure ridge forces the usual stream of fronts and cyclones to flow around it, sometimes for a week or longer. Somewhat paradoxically, a blocking pattern like this can produce extreme heat in the summer and vicious cold in the winter. In the summer, the clear skies above high pressure allow for maximum sunlight to reach the surface. In the wintertime, the arcing jet can pull bitter surface air from the Arctic around the ridge and south-ward on the east side, resulting in a cold wave that can extend well southeast of the ridge itself (or to its northeast in the Southern Hemisphere).

No matter what the time of year, forecasters keep a wary eye out for these blocking patterns. Sometimes an entire season may be prone to several occurrences of the same block, often due to El Nino or other ocean-atmosphere cycles.

Too darn hot

In some parts of the world, hot spells are imported Mediterranean-style climates often have coastal cities that stay naturally air-conditioned, with intense heat lurking just inland. A small shift in circulation can push the torrid air seaward, producing a spell of sizzling heat in areas that may not be used to it. The typical summer day in San Francisco falls short of 20°C/68°F, but on occasion the city has soared above 38°C/100°F. Likewise, Melbourne gets many summer afternoons that top out close to a pleasant 25°C/77°F, but every so often the city can roast in readings that touch 40°C/104°F. Thankfully, such incursions of heat tend to be short-lived, usually lasting only a couple of days.

A truly dangerous heat wave encompasses far more than a toasty afternoon or two. When it comes to the toll exacted by a heat wave on people, animals and plants, the duration and the warmth of the nighttime lows are the real killers. A weeklong stretch of severe heat can be far more deadly when nights stay unusually warm, erasing the few hours of relief one might otherwise get. Uncomfortably warm nights are most likely when and where the humidity is unusually high. In highly urbanized settings, buildings and pavements release the heat at night that they've absorbed by day (part of the urban heat island effect). Many of the worst heat-wave disasters of recent years have occurred in older midlatitude cities that feature dense urban layouts and large numbers of residents who lack home air conditioning.

Even when dangerous heat is obviously at hand, it's not always clear when a disaster is unfolding. Many people die indirectly in heat waves – from pollution, for instance, or from pre-existing health conditions exacerbated by the heat. It can take days or even months before the full scope of the death toll is evident. And since many of the victims are elderly, it's often assumed that the heat simply claimed people who were about to die anyway. If that were the case, then mortality ought to dip below average in the months following a heat wave. Such dips do occur, but studies show that they account for only about 20-30 percent of the excess deaths observed in a typical heat wave.

Heat waves don't correspond to latitude in the way one might think- In the subtropics, it's got to be really hot to qualify as a heat wave. Many residents of cities like Phoenix, Arizona, and Riyadh, Saudi Arabia, hardly bat an eyelash until the mercury shoots above 45°C/113°F. Stretches of truly unusual heat appear to be less deadly in climates like these – where people are already adapted to torrid conditions. Poverty can change this equation, though. Just before the summer monsoon sets in across India, temperatures can soar well above 40°C/104°F across wide areas. Several pre-monsoonal heat waves in recent years have each killed more than a thousand people, many of them landless workers forced by circumstance to toil in the elements.

Social scientists have looked closely at recent heat disasters in the midlatitudes and learned much about how to save lives. The impetus in the United States was a 1995 heat wave that was especially intense in Chicago: daytime highs reached 41°C/106°F and two nights stayed above 27°C/81°F. More than 700 Chicagoans died. Yet some city officials downplayed the event, even blaming the victims themselves for not taking precautions. After the disaster in Chicago, Laurence Kalkstein of the University of Delaware teamed up with the US National Weather Service to launch a new watch-warning system for extreme heat. The scheme, which included assigning people to check on neighbours, saved an estimated 100 lives that decade in the city of Philadelphia alone. The system was exported to Italy just before its 2003 heat wave: Rome declared 18 heat-emergency days that summer. Yet even with the warning system in place, the sheer intensity of that summer's heat produced hundreds more deaths in Rome than Kalkstein and colleagues had anticipated.

The 2003 heat wave may have provided a sneak preview of what North America and Eurasia can expect in the century to come. As it raises the average worldwide temperature, global warming is also expected to boost the number of days that climb above local heat-wave thresholds. One study by British scientists found that the typical European summer of the 2040s could be warmer than the blistering summer of 2003 was.

Cold's dangerous grip

While it usually takes several days for a large-scale heat wave to set in, cold waves often arrive with stinging abruptness. The more quickly a frigid air mass can sweep in from polar latitudes, the colder the weather it's likely to produce, since there's less time for warm ground to temper the air mass. Some of the world's sharpest temperature transitions have occurred in the US Great Plains, where blue northers can send readings plummeting by as much as 25°C/40°F in less than two hours. One of the strongest such fronts on record occurred on 11 November 1911. That afternoon, just before a cold front arrived, Oklahoma City set a record high of 28°C/83°F. Before midnight, the city had plunged to a record low of -8°C/17°F.

When snow accompanies the quick onset of cold, the impact can be more brutal still on 12-13 January 1888, more than a hundred US children, many of them rushing home from school on foot, perished in a frigid snowstorm that swept across the Dakotas with blinding speed, killing several hundred adults as well. The great “children's blizzard” struck just two months before New York experienced its own epic snowstorm.

As with heat waves, the true measure of a cold wave is its persistence. Britain endured its coldest winter of modern times in 1962-63, when snow cover lay across most of the UK for over two months. The bulk of the nation hovered below freezing through most of January and much of February, with several spikes of cold that were even more extreme (temperatures in Yorkshire sank below -20°C/-4°F at one point).

Some of the world's coldest settled areas are in Siberia. The region's largest city, Yakutsk, manages a paltry -37°C/-35°F for its average high in January, and stretches of cold, clear weather can send the mercury even lower. Yet these hardy Russians are seldom caught unprepared, since the cold is so frequent. That's not always the case further south, where temperatures well above freezing can prove fatal to people without adequate clothing and shelter. More than 600 people died in Bangladesh when a cold wave in December 2002 and January 2003 sent temperatures below 5°C/41°F. It was one of several cold snaps that each have killed hundreds across South Asia over the past few years.

Even in relatively well-off Britain, it's believed that as many as two million people, largely elderly, suffer from “fuel poverty” – typically defined as when heating costs take up 10 percent or more of household income. Unable to keep their poorly insulated houses warm, many Britons endure the winter in perpetual chill, which increases their vulnerability to pre-existing health problems and to communicable diseases such as the flu. Each winter brings an estimated 20,000-plus deaths in Britain due to health effects stemming from too-cold housing. In a 2001 initiative, the British government set out to eradicate fuel poverty by 2016, with national and local governments issuing grants to needy homeowners to help them pay utility bills and stay warm.