The sun produces heat from nuclear reactions deep inside it. All life on Earth depends on this heat.
Credit: NASA/SDO

This summer, the entire globe experienced a heat wave. Western states in America experienced a record-breaking heat wave in late August and early September of 2022.

A heat wave is a period over which the temperature rises well above normal for a particular region. Heat waves may be less dramatic than such natural disasters as hurricanes, earthquakes, or tornadoes, and heat waves tend to cause less damage to property. However, heat waves rank among the deadliest extreme weather.

The varying nature of local climate makes it difficult to establish a simple definition of a heat wave. The temperatures of a heat wave must be hot relative to the average temperatures at a place during a particular time of year. For example, a temperature of 90 °F (32 °C) during the summer is not out of the ordinary for the northern African country of Sudan. However, it would be an exceptionally high temperature for the northern U.S. state of Alaska. Meteorologists agree that high temperatures must persist for more than one full day to be considered a heat wave.

This chart can be used to determine the heat index (HI), a number that describes how people are affected by the combination of temperature and relative humidity. As the temperature and relative humidity rise, the heat index increases. The higher the index, the greater the likelihood that people will develop heat-related illnesses. The risk of illness is low when the index is below 80 °F.
Credit: World Book chart by Linda Kinnaman

Heat waves are sometimes accompanied by high relative humidity. Relative humidity is the amount of water vapor in the air compared with the amount the air can hold at saturation. High relative humidity interferes with the human body’s ability to cool itself through the evaporation of sweat. Thus, high humidity can make a heat wave more uncomfortable and dangerous. Some weather agencies make use of a measure called the heat index, which factors in relative humidity to give a measure of how hot the air feels.

Heat waves are generally created by areas of high air pressure. Meteorologists call these areas highs or anticyclones. Warm highs bring high temperatures and air that contains little moisture. These highs are large weather systems that may affect a region for an extended time. In the summer, a warm high sometimes stalls over North America, for example, causing a heat wave.

The high temperatures of a heat wave can cause dehydration and hyperthermia in people and in animals. Dehydration is a condition in which the body loses too much water. In severe cases, dehydration can lead to a fast heartbeat, low blood pressure, shock, and death. Hyperthermia is a condition that occurs when the body becomes overheated. Forms of hyperthermia include heat exhaustion, heat cramps, and heatstroke (also called sunstroke). Heatstroke is a serious condition that can cause permanent neurological damage and death.

Elderly people, infants, and people with long-term illnesses may suffer heatstroke more readily than others during a heat wave. Hyperthermia can be particularly dangerous for people with long-term medical conditions. Such conditions as asthma, diabetes, heart failure, and high blood pressure place the body’s organs under stress. The added stress from hyperthermia can induce heart attack, breathing failure, and other fatal medical events among such people.

The complexity of diagnosing heat-related deaths has made it difficult for experts to track the toll of heat waves. The World Health Organization estimated that heat waves killed more than 166,000 people between 1998 and 2017. Another study found that approximately 500,000 heat-related deaths occurred worldwide between 2000 and 2019. Despite the uncertainties, experts agree that heat waves are deadly. In 2003, for example, Europe suffered a catastrophic heat wave in which some 70,000 people died.

Heat waves can also put stress on electric power and water services. People’s use of air conditioning strains the electrical grid. Power plants may need more water for cooling. People may also use more water to try to keep cool. When demand for electricity exceeds generating capacity, officials may have to reduce voltage to prevent a blackout. Such a reduction, called a brownout, may damage electrical equipment or cause the equipment to operate less efficiently. High temperatures also buckle pavement and warp railroad tracks.

Drought kills crops. Two farmers examine a Kansas corn field that has been affected by a drought in 2012. Climate scientists suspect such droughts might become more common in some areas due to the effects of global warming.
Credit: © Mashid Mohadjerin, The New York Times/Redux Pictures

Heat waves are dangerous to plants and animals as well. Excessive temperatures can kill animals that cannot find ways to keep cool. Prolonged heat can wither plants. Droughts usually accompany heat waves, compounding the danger to living things. Heat waves and droughts dry out the ground and plants, increasing the likelihood and severity of wildfires.

A Russian firefighter sprays water on a wildfire in the Ryazan region, southeast of Moscow. In 2010, a devastating wave of wildfires wept across Russia following a record-setting heat wave.
Credit: AP Photo

Data suggest that heat waves are increasing in duration, frequency, and intensity. The time of year in which heat waves occur is also expanding. Most climatologists think that climate change brought about by human-caused global warming is driving these changes.

People must exercise caution during heat waves to avoid health problems. Experts advise wearing light-colored, loose-fitting clothing when outdoors and avoiding going out during the hottest part of the day. People should drink fluids regularly but avoid beverages with alcohol or caffeine, which can contribute to dehydration.

People who are homeless or who live in homes without air conditioning are particularly vulnerable to heat waves. During a heat wave, local governments may open air-conditioned cooling centers where people can stay to cool down.

People can help to protect their community during a heat wave. They can air condition their homes only to what temperature is necessary, conserving power. People might be asked to limit water use at certain times to prevent drops in their community’s water pressure. They may be asked to conserve water if the heat wave is accompanied by a drought. Because heat waves increase the risk of wildfires, people may be prohibited from starting fires outdoors.

Motorists stop to observe the Walbridge fire, part of the larger LNU Lightning Complex fire, from a vineyard in Healdsburg, California, on Aug. 20, 2020. A series of massive fires in northern and central California forced evacuations as flames quickly spread, darkening the skies and polluting the air.
Credit: © Josh Edelson, AFP/Getty Images

Wildfires have ravaged the U.S. state of California, killing at least 5 people and forcing thousands to evacuate (leave) their homes. Fueled by a heat wave and sparked by lightning strikes, the wildfires have set the state ablaze and covered the air in thick smoke.

Wildfire is the uncontrolled burning of large areas of vegetation. Wildfires may be ignited by human activities or by natural causes. Such natural causes include lightning, volcanic activity, or even intense sunlight. Wildfires occur on every continent except Antarctica. They are most common in the temperate forests and grasslands midway between the equator and the poles. They are less common in tropical and desert ecosystems.

These are far from California’s deadliest wildfires. Numerous major wildfires occurred in California in the early 2000′s. In 2003, wildfires in southern California destroyed thousands of homes and killed more than 20 people. In 2007, deadly wildfires forced the evacuation of about 500,000 people in San Diego County. In 2013, the so-called Rim Fire burned large areas of the Stanislaus National Forest near Yosemite National Park. Some of the largest wildfires in California history burned during the summer of 2018.

The unhealthy air quality caused by the wildfires is especially concerning, because of the pandemic (global outbreak) of the respiratory disease COVID-19. Smoke-polluted air can weaken lung function, reducing the body’s ability to fight infection.

The current round of fires was made worse by an intense heat wave baking the American Southwest. On August 16, temperatures in California’s Death Valley reached 130 °F (54.4 °C). If these measurements are confirmed, they would be the hottest surface air temperatures ever recorded on Earth.

December 23, 2019

Saturday, December 21, was the winter solstice in the United States and the rest of the Northern Hemisphere. In the Southern Hemisphere—which includes Australia, New Zealand, and other lands south of the equator—the solstice marks the beginning of summer. In Australia, summer is a season known for bushfires. Such wildfires are common in Australia, due in part to the country’s hot and dry climate. Many wildfires start in the remote countryside known to Australians as the bush. Bushfires can be extremely destructive, especially if they reach urban areas. The fires often kill people and destroy property and farmland. They also devastate forests, along with the koalas and other animals that live there.

Police in Jimboomba, Queensland, rescued this koala and her joey from a bushfire in late November 2019. Many other koalas were not so lucky. Credit: Jimboomba Police

A number of major bushfires have plagued Australia in 2019. Late last summer, in February, bushfires consumed more than 50,000 acres (20,000 hectares) of land in northern New South Wales. Far more destructive bushfires ignited again in New South Wales in September, burning nearly 5 million acres (2 million hectares) there and in neighboring Queensland. Some of those fires lasted for weeks, while others continue to burn, turning skies black or orange, causing severe damage, and killing a number of people. The bushfires also killed thousands of vulnerable and slow-moving koalas trapped by the rapidly spreading flames.

Bushfires are relatively common in Australia. These fire-damaged buildings are seen alongside a house that survived Christmas Day bushfires near Melbourne in 2015. Credit: © Julian Smith, EPA/Landov

Koalas live only in the forests of Queensland, New South Wales, and Victoria, in eastern Australia. They are found in significant numbers in only a few areas, and the animals are protected by law. Bushfires, however, ravage the koala’s natural habitat and its population. Koalas cannot escape the fast-moving fires, and their only defense is to climb to the top of a tree, curl into a ball, and hope the flames do not rise to reach them. Koalas sometimes survive this way, but they often burn their paws and claws descending charred trees, leaving them unable to climb properly again.

Koalas are not officially listed as endangered, but the population in New South Wales and Queensland has decreased by more than 40 percent since 1990. The animals are threatened by deforestation and habitat loss, as well as the effects of climate change, which is causing longer and more intense heat waves and droughts, leading to more frequent, intense, and longer-lasting bushfires. Beyond the flames and smoke of the fires, many koalas die from lack of water or exposure to prolonged periods of high temperatures. Australia just suffered through the driest spring in its history, and a brutal heat wave peaked on Dec. 19, 2019, when the highest ever nationwide average temperature—107.4 °F (41.9 °C)—was recorded.

February 11, 2019

Last week, monsoon rains dumped record amounts of water in northern  Queensland, Australia, flooding roads, swelling rivers above their banks, and causing landslides. The coastal city of Townsville received more than 3.3 feet (1.0 meters) of rain, raising the level of water held by the Ross River dam to dangerous levels far above its capacity. On February 3, city officials opened the dam’s gates, intentionally flooding several neighborhoods to ease pressure on the dam and keep it from collapsing. Crocodiles, snakes, and other wildlife followed the waters into the flooded streets, appearing in places they are not typically found. Boats and other vehicles capable of navigating high waters rescued many people and pets trapped by the flooding. Two people are known to have died in the flooding.

Floodwaters inundate the Townsville, Australia, suburb of Idalia on Feb. 4, 2019. Credit: © Ian Hitchcock, Getty Images

Some Queensland farmers welcomed the rainfall. Drought conditions have been widespread in recent months across Australia (where summer is from December to February), and the farmers hoped the rain would help their crops. The monsoon flooding came at the tail end of the hottest month ever recorded by Australia’s Bureau of Meteorology. On February 1, the agency reported that the average January temperature across the continent exceeded 86 ºF (30 ºC). The Australian Capital Territory, New South Wales, the Northern Territory, and Victoria all set new high temperature records for the month of January. Some parts of western Queensland and New South Wales had weeks of temperatures above 104 ºF (40 ºC). Numerous farm and wild animals died in the extreme heat, as did many fish in overheated lakes and rivers.

Despite the monsoon rains in Queensland, much of Australia remains in a drought. Scientists believe that such extreme weather events will become more frequent in coming years as a result of global climate change.

September 6, 2017

Over Labor Day weekend in southern California, the La Tuna wildfire expanded to become one of the largest fires in the history of the city of Los Angeles. The fire, burning in the Verdugo Mountains just north of downtown Los Angeles, has covered as much as 7,000 acres (2,830 hectares), consuming several houses and causing evacuations, injuries, and road closings. The bright hilltop flames could be seen throughout the giant metro area, which has roasted in 100 °F (38 °C) temperatures during an extended heat wave.

The La Tuna wildfire burns in the Verdugo Mountains in the eastern San Fernando Valley of Los Angeles, California, on Sept. 1, 2017. Credit: Scott L (licensed under CC BY-SA 2.0)

The fire began Friday night along a stretch of La Tuna Canyon Road and the 210 Freeway between Burbank, Glendale, and the Sunland-Tujunga neighborhood in the San Fernando Valley. More than 700 residents were forced to flee their homes, and the roads passing through the fire were closed, creating traffic headaches during the holiday weekend. There is no known cause for the fire, but investigators have seen no evidence of arson.

California Governor Jerry Brown declared a state of emergency for Los Angeles County, allowing Mayor Eric Garcetti to use state and federal assistance. On Sunday, cooler temperatures and brief showers helped more than 1,000 firefighters contain some 30 percent of the massive La Tuna fire. Bulldozers and hand crews also helped by clearing brush and other fuel from the remaining perimeter of the fire. The 210 Freeway has since partially reopened and people have been allowed to return to their homes—with words of caution that heavy winds could cause the fire to spread out of control once again. Under current conditions, firefighters hope to completely contain the wildfire in the coming days.

In Riverside County east of Los Angeles, firefighters were battling another giant wildfire over the weekend. The 3,800-acre (1,540-hectare) Palmer brush fire broke out Saturday and forced more than 400 people from their homes. The recent heat wave in Los Angeles is not unique to the city, either, as record-setting temperatures have broiled Eureka, San Francisco, San Jose, and Oakland in northern California.

In 1961, the Bel Air fire burned 16,000 acres (6,475 hectares) and destroyed more than 480 homes in Los Angeles. In 2008, the Sayre fire consumed over 11,000 acres (4,450 hectares) and hundreds of buildings in Los Angeles.

January 10, 2014

While areas of North America have endured record polar temperatures this week, the Southern Hemisphere, where it is summer, is experiencing the opposite extreme–record high temperatures. Temperatures in some parts of the sparsely populated Pilbara region along Australia’s rugged northwest coast hit 122 °F (50 °C) yesterday. The Australian newspaper The Blaze reports that the heat is so severe that an estimated 100,000 bats have died, “littering the ground and alarming authorities who warn locals not to touch the animals for fear of infection.” Large numbers of parrots, kangaroos, and emus have been found dead in the parched terrain around Winton, Queensland, the chief executive of the Winton Shire Council told media outlets.

Southern South America is sizzling as well. In Brazil, zookeepers in Rio de Janeiro are giving their charges ice pops to keep them cool in the 120 °F (49 °C) heat. An extended heat wave in Argentina recently overloaded the electric grid, resulting in blackouts that have left thousands of households without electric power. In the Argentine capital of Buenos Aires, record high temperatures have left hundreds of fish dead in city park lakes.

Australians often flock to Bondi Beach in Sydney to celebrate the Christmas holiday, which falls during the summer in the Southern Hemisphere. (© Mike Stone, Alamy Images)

Australia’s most recent heat wave follows its hottest year on record. The country’s  Bureau of Meteorology reported that in 2013, temperatures were 2.16 degrees Fahrenheit (1.2 degrees Celsius) above the long-term average, making 2013 the warmest year since record keeping began in Australia in 1910. The world’s driest continent also had its hottest day, hottest month, hottest winter’s day, and hottest summer in 2013. The hotter-than-normal temperatures, which began late in 2012, were so extreme that the Bureau of Meteorology was forced to change its official weather forecasting map to include two new colors—deep purple and pink—to show areas with temperatures above 122 °F (50 °C).

Additional World Book articles:

• Bushfires in Australia
• Australia 2009 (a Back in Time article)
• Global warming 2012 (a Back in Time article)
• The Great Meltdown (a special report)
• Meltdown: Climate Change in the Arctic (a special report)

January 30, 2013

Winds approaching 62 miles (100 kilometers) per hour blasted Sydney, Australia’s largest city, yesterday, driving huge swells onto Bondi Beach and drenching the center of the city.

Days of high winds, torrential rains, and powerful ocean swells along Australia’s east coast have left at least four people dead and thousands more displaced in the states of New South Wales and Queensland. According to Australia’s Bureau of Meteorology, the storm–the remnants of Tropical Cyclone Oswald–has shattered records in various parts of New South Wales, triggering flooding along hundreds of miles from north of Brisbane south to Sydney. State officials estimate that at least 23,000 people remain cut off by floodwaters. Massive flooding north of Brisbane forced the evacuation by military helicopter of approximately 1,000 people from the town of Bundaberg, where rapidly moving floodwater threatened to carry away entire buildings. The highest rainfall was recorded some 500 miles (800 kilometers) north of Brisbane, where 53.5 inches (136 centimeters) fell over the three-day period ending on January 27.

Massive flooding along Australia's eastern coast has extended from north of Brisbane south to Sydney. (World Book map; map data © MapQuest.com, Inc.)

The latest floods in Australia have come on top of a barrage of bizarre and destructive weather. Just two weeks ago, New South Wales and Queensland were in the grip of a searing heat wave that triggered dozens of enormous wildfires.

January 18, 2013

Sydney, Australia, is experiencing its hottest day on record, with temperatures reaching 114.4 °F (45.8 °C). The previous recorded high in Sydney–113.5 °F (45.3 °C)–was in January 1939. The heat has caused extensive damage to Sydney’s urban railway lines, causing delays to much of the network. Prime Minister Julia Gillard has advised Australians to “take the appropriate precautions to stay safe and monitor information from local emergency services as they work to protect lives and property.”

A record high of 104 °F (40 °C) set on January 8 prompted the Australian Bureau of Meteorology to add two new colors to its temperature charts to extend their range to 129 °F (54 °C) from the previous cap of 122 °F (50 °C). David Jones, manager of climate monitoring prediction at the Bureau of Meteorology, declared last week that Australia was undergoing the “largest heat event in the country’s recorded history.”

According to public safety officials, four months of record-breaking temperatures combined with widespread drought conditions and high winds have created “catastrophic” fire conditions along the heavily populated eastern and southeastern coasts of the country. These areas include Australia’s three largest cities, Sydney, Melbourne, and Brisbane, as well as the capital, Canberra. A bushfire in Victoria state has left one man dead. He was found in his burnt-out vehicle outside the town of Seaton, which is 120 miles (200 kilometers) east of Melbourne. Firefighters continue to battle dozens of wildfires sparked by the intense heat in Victoria and New South Wales, and whole towns and sheep ranches are reportedly cut off  by fires and endangered. (A bushfire is an uncontrolled fire in bush areas, especially the Australian bush.) Very similar conditions on Feb. 7, 2009, came to be known as Black Saturday, during which wildfires in Victoria state left 173 people dead and caused $4.4 billion in damage.

Additional World Book articles:

• Celsius scale
• Fahrenheit scale
• Why Fires Need to Burn (a special report)
• Australia 2009 (a Back in Time article)

January 10, 2013

The year 2012 was the hottest on record for the contiguous United States, with an average temperature of 55.3 °F (12.9 °C), which eclipsed 1998, the previous record holder, the National Oceanic and Atmospheric Administration (NOAA) announced this week. Temperature variations between years are usually measured in just fractions of a degree; 2012, however, was 1 full degree Fahrenheit (0.56 Celsius degree) warmer than the previous record temperature, set in 1998. In fact, the entire range between the coldest year on record, which occurred in 1917, and the previous warmest year on record, 1998, was just 4.2 Fahrenheit (2.4 Celsius) degrees.

The average 2012 springtime temperature in the lower 48 states was so far above the 20th century average–5.2 Fahrenheit (2.9 Celsius) degrees–that it set a record for the largest temperature departure for any season on record. July 2012, with an average temperature that was 3.6 Fahrenheit (2 Celsius) degrees above average, was the hottest month ever recorded in the contiguous United States.

Shades of red on the map indicate temperatures up to 8 Fahrenheit degrees warmer than average. Shades of blue indicate temperatures up to 8 Fahrenheit degrees cooler than average. The darker the color, the larger the difference from average temperature. (NOAA Climate.gov)

According to an unofficial count maintained by the Weather Channel, 34,008 new daily high records were set at weather stations in the United States in 2012, compared with only 6,664 new daily record lows. Fully 61 percent of the nation was engulfed in drought in 2012. In addition, with 11 natural disasters that exceeded a $1-billion damage threshold, 2012 turned out to be the second-worst year on record on the Climate Extremes Index, surpassed, again, only by 1998.

“Climate change has had a role in this [record],” noted Jake Crouch, a climate scientist at NOAA’s National Climatic Data Center in Asheville, North Carolina. Crouch stated his belief that it is unlikely such a record would have occurred without the long-term warming trend caused in large part by emissions of greenhouse gases.

Additional World Book articles:

• Climate
• National Weather Service
• Weather
• Weather 1998 (a Back in Time article)

January 9, 2012

Temperatures in Australia topped 104 °F (40 °C) on January 8, and the mean national temperature climbed to the highest in history, breaking a record set just the day before, on January 7, reported the Australian Bureau of Meteorology. The new record prompted meteorologists at the bureau to take the extraordinary step of adding two new colors to its temperature charts to extend their range to 129 °F (54 °C) from the previous cap of 122 °F (50 °C).

“If you look at yesterday, at Australia as a whole, it was the hottest day in our records going back to 1911,” noted David Jones, manager of climate monitoring prediction at the Bureau of Meteorology. “From this national perspective, one might say this is the largest heat event in the country’s recorded history.”

According to public safety officials, four months of record-breaking temperatures combined over the past week with widespread drought conditions and high winds have created “catastrophic” fire conditions along the heavily populated eastern and southeastern coasts of the country. These areas include Australia’s three largest cities, Sydney, Melbourne, and Brisbane, as well as the capital, Canberra. Firefighters and emergency crews in the state of New South Wales are facing a highly dangerous situation battling 130 separate bushfires. (A bushfire is an uncontrolled fire in bush areas, especially the Australian bush.) Very similar conditions on Feb. 7, 2009, came to be known as Black Saturday, during which wildfires in Victoria state left 173 people dead and caused $4.4 billion in damage.

Additional World Book articles:

• Celsius scale
• Fahrenheit scale
• Why Fires Need to Burn (a special report)
• Australia 2009 (a Back in Time article)