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.

October 22, 2018

In the early 1900′s, as millions of European immigrants arrived at Ellis Island in New York, people from Asia, the Pacific Islands, and Latin America were beginning their American experience at Angel Island in San Francisco Bay. Hundreds of thousands of immigrants entered the United States through the Angel Island Immigration Station from 1910 to 1940. Many Americans today can trace their family roots to Angel Island, where people disembarked from ships crossing the Pacific Ocean.

Many thousands of immigrants entered the United States through Angel Island Immigration Station in San Francisco Bay. Credit: © Wayne Hsieh78/Shutterstock

Angel Island lies about 3 miles (4.8 kilometers) north of San Francisco and 1 mile (1.6 kilometers) southeast of California’s Tiburon Peninsula. The 1.2-square mile (3.1-square kilometer) island is the largest natural body of land in the bay. The U. S. Army established Camp Reynolds on Angel Island in 1863, during the American Civil War. In the 1890’s, the U.S. Marine Health Service opened the Angel Island Quarantine Station. The station fumigated foreign ships and held in isolation sailors and immigrants who were suspected of carrying disease. In 1900, Camp Reynolds was renamed Fort McDowell. The army used the facility to quarantine U.S. soldiers returning from duty overseas.

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Angel Island, in California’s San Francisco Bay, is best known for its role as a United States immigration station. Thousands of immigrants—most of them from Asia—entered the country through facilities on the island from 1910 to 1940. Credit: WORLD BOOK map

About 1910, the island became the main processing center for immigrants arriving via the Pacific Ocean. Over the next 30 years, hundreds of thousands of immigrants—including at least 175,000 from China and tens of thousands from India, Japan, Mexico, the Philippines, and elsewhere—passed through Angel Island. Because of laws that limited Chinese immigration, officials gave the newcomers’ applications added scrutiny. Most arrivals were held at the facility for periods ranging from a few days to a few months, and about 18 percent of prospective immigrants were deported. For its role in enforcing the nation’s immigration limits, Angel Island became known as the “Guardian of the Western Gate.” In 1940, a fire destroyed the station’s administration building, and immigrants were relocated to processing centers on the mainland.

In 1954, the California State Park Commission acquired a portion of the island. In 1962, the federal government turned over most of the remainder of the island to the state. Today, visitors to Angel Island State Park enjoy such recreational activities as hiking, boating, camping, and sightseeing. The park also offers tours of the historic immigration station and military barracks, as well as other noteworthy island sites.

August 3, 2018

For many people in the Northern Hemisphere, the summer of 2018 has been touched by fire and obscured by smoke. Summers are typically fire seasons in many areas, but climate change has caused hotter and drier conditions in recent years, and wildfire outbreaks have become more frequent and more destructive. Other causes of increased fire activity include the building of more developments in fire-prone areas, a lack of proper forest management, and, of course, simple human carelessness. In the past weeks, fires have killed more than 100 people and destroyed homes and large swaths of land in Asia, Europe, and North America.

A firefighting helicopter flies over a wildfire in the Greek town of Rafina near Athens on July 23, 2018. The Greek wildfire was the deadliest yet this year, claiming the lives of 92 people. Credit: © Angelos Tzortzinis, AFP/Getty Images

In southeastern Greece, a wildfire erupted July 23, destroying or damaging some 3,500 structures and killing 96 people along the Attica coast. The giant wildfire, just 18 miles (30 kilometers) east of Athens, the Greek capital, burned out of control for several days, gutting seaside resorts and holiday homes. Fanned by high winds, the fast-moving blaze trapped and killed groups of people, including entire families as they huddled between the flames and steep cliffs overlooking the Aegean Sea. The high death toll prompted criticism of the the area’s lack of access roads, warning systems, and other civil protection measures in areas surrounded by forest and at high risk of wildfires.

In far northern Europe, dozens of wildfires have consumed more than 62,000 acres (25,000 hectares) this summer in Sweden, which is experiencing an abnormally lengthy and intense heat wave. Neighboring Finland and Norway are also suffering from unusual heat and wildfire outbreaks, as are Estonia, Latvia, and Lithuania on the other side of the Baltic Sea. Wildfires also torched drought-stricken areas of Germany, the Netherlands, and Poland.

This week in northern California, firefighters continued to battle the Carr Fire that has killed 8 people, destroyed over 1,500 buildings, and consumed some 125,000 acres (50,000 hectares) of land. The wildfire started on July 23 along Carr Powerhouse Road in Shasta County near the city of Redding. The “mechanical failure of a vehicle” ignited dried grasses and trees, and soon flames were roaring through the forested hills around Whiskeytown Lake west of Redding. The wildfire, aided by hot and dry weather, high winds, an abundance of natural fuel, and steep terrain, has since been chased westward by more than 4,200 firefighters—2 of whom have died fighting the blaze. Reports of  “firenados” (huge, rotating whorls of smoke, flame, and ash) encouraged nearly 40,000 people to obey evacuation orders and leave the fire area (which included parts of nearby Trinity County). Many people will return to find their homes and property in ashes.

The Carr Fire is the largest of 17 wildfires currently burning throughout the state of California. Collectively, wildfires in the western United States have scorched 4.6 million acres (1.86 million hectares) so far this year, a 24 percent increase over the annual average for the past decade. Wildfires have also stricken large areas of western Canada and parts of Mexico this summer.

Wildfires have also consumed vast amounts of territory in Siberia of eastern Russia. Since May, firefighters have been battling immense wildfires in the Amur Oblast region along the Chinese border, where towering pyrocumulus clouds have injected smoke and pollutants high into Earth’s atmosphere. Pyrocumulus clouds are formed by rising heat from wildfires or by plume emissions from fossil fuel-burning industrial plants, and they sharply increase the levels of carbon dioxide and harmful aerosols in the atmosphere.

October 11, 2017

On Monday, October 9, multiple wildfires swept across northern California, destroying some 2,000 homes and other buildings, killing 21 people and displacing tens of thousands of others. California Governor Jerry Brown declared states of emergency in Butte, Lake, Mendocino, Napa, Nevada, Sonoma, and Yuba counties, where the destruction is worst. Dozens of people remain missing as the fires continue to burn.

Firefighters hose the flames of a burning home in the Napa wine region of California late on Oct. 9, 2017. Credit: © Josh Edelson, AFP/Getty Images

Unusually hot and dry weather this summer has created ideal conditions for wildfires across northern California. At least 15 separate blazes broke out over the weekend, and strong winds whipped the flames across some 114 square miles (295 square kilometers). Napa and Sonoma counties, the heart of California’s wine country, were hardest hit. The status of the grape harvest remains unclear, but damage from fire and smoke is almost certainly widespread. Fires devastated parts of the city of Santa Rosa, where entire neighborhoods have been left in ashes and cinders.

Skies from San Francisco to Sacramento to Eureka remain choked with smoke and haze, and an orange glow illuminates the night. Numerous roads, schools, and business are closed, and several hospitals in the fire zone have been evacuated. Thousands of firefighters and numerous firefighting aircraft—including the massive Global SuperTanker (a converted Boeing 747 airliner)—are battling the blazes. Amy Head, the fire captain spokesperson for Cal Fire, the state agency responsible for fire protection, said the fires were probably linked to a warming climate. “It has been hotter, it has been drier, our fire seasons have been longer, fires are burning more intensely, which is a direct correlation to the climate changing,” she said.

Another state of emergency was declared on Monday in southern California’s Orange County, where a wildfire destroyed several homes in the affluent Anaheim Hills neighborhood and forced the evacuation of hundreds of residents.

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.

May 26, 2017

Tomorrow, Saturday, May 27, is the 80^th anniversary of the opening of the iconic Golden Gate Bridge connecting San Francisco with Marin County, California. One of the largest and most spectacular suspension bridges in the world, it spans the Golden Gate Strait at the entrance of San Francisco Bay. The Golden Gate Bridge opened to pedestrians on May 27, 1937, and thousands of people took in the spectacular views as they crossed the bridge on that first day. The next day, May 28, a caravan of cars crossed the bridge—which is part of U.S. Route 101—for the first time.

Northern California’s Golden Gate Bridge opened 80 years ago on May 27, 1937. Credit: © Shutterstock

The Golden Gate Bridge, which has a total length of 8,981 feet (2,737 meters), connects northern California to the peninsula of San Francisco. It contains about 88,000 tons (75,000 metric tons) of steel, 390,000 cubic yards (300,000 cubic meters) of concrete, and 160,000 miles (260,000 kilometers) of wire in its two main cables.

Towers stand on either side of the bridge, about 1,120 feet (340 meters) from the ends. They hold up the two steel cables, 36 ³/8 inches (92 centimeters) in diameter, from which the bridge hangs. The section between the towers is 4,200 feet (1,280 meters) long, one of the world’s longest spans. The floor is 220 feet (67 meters) above the water and 90 feet (27 meters) wide. It has a six-lane road and sidewalks.

The Golden Gate Bridge is one of the world’s most famous bridges. Credit: © Thinkstock

Prior to the Golden Gate Bridge, ferry service—which began in the 1800’s and continues today—was the only way to cross San Francisco Bay. In 1916, San Francisco journalist James H. Wilkins began the modern push for a bridge, issuing a challenge to politicians and engineers alike. Wilkins called for a suspension bridge with a center span of 3,000 feet (915 meters)—nearly twice the length of any such span at that time. After considerable debate, numerous designs, court rulings, and acts passed by the California legislature, the U.S. War Department, which owned the land around the strait, approved the project in 1924.

The Golden Gate Bridge and Highway District was formed in 1928. Joseph B. Strauss was appointed chief engineer of the project and designed the bridge. The bridge was planned and constructed to withstand strong tidal forces, wind, and earthquakes. Irving F. Morrow, an architect hired by Strauss, designed the towers and selected the bridge’s distinctive “international orange” color. The color blends well with the span’s natural setting, but—from the deck of a passing ship—it stands out clearly from the gray fog banks, white clouds, and blue skies in the background, allowing for easier navigation.

In 1930, Bay Area voters approved a $35-million bond measure to finance the Golden Gate Bridge, but the bonds did not sell during the tough times of the Great Depression. In 1932, however, local banker A. P. Giannini, founder of the Bank of America, bought the entire bond issue to help the local economy. Construction of the Golden Gate Bridge then began on Jan. 5, 1933, and continued for over four years.

Remarkably, the much longer and more expensive San Francisco-Oakland Bay Bridge was constructed at the same time as the Golden Gate Bridge (the mammoth bridges are within easy sight of each other). Begun in July 1933, construction on the Bay Bridge (which received federal funding, making paying for the Golden Gate Bridge even tougher) continued until its opening in November 1936.

September 9, 2016

Once teetering on the brink of extinction, the rare island fox of California’s Channel Islands has made the quickest recovery yet for a North American mammal in the history of the Endangered Species Act. In 2000, only 55 island foxes lived on Santa Cruz Island, and another 15 of the animals lived on the chain’s two northern islands. In 2004, the island fox was formally listed as endangered and was given a 50 percent chance of becoming extinct within a decade. An intense recovery program, however, achieved the difficult feat of recovering the species. The program included breeding the island fox in captivity, vaccinating the foxes, removing feral pigs from the islands, and relocating a number of golden eagles, which are invasive predators from the California mainland. The conservation efforts restored the Channel Islands’ fox population to more than 4,100, and the animal was removed from endangered species protection in August 2016.

California’s island fox was recently removed from the endangered species list. Credit: National Park Service/U.S. Fish & Wildlife Services

The island fox is closely related to the common gray fox of the southern United States. Like the gray fox, the island fox has gray coloring on the back, rust coloring on the sides, and is white underneath. The face has distinctive black, white, and reddish-brown patterns. The island fox weighs just 3 to 5 pounds (1.4 to 2.3 kilograms) and stands about a foot (30.5 centimeters) tall. It is one of the smallest canid species in the world. The Canidae family includes coyotes, foxes, jackals, wolves, and the domestic dog. Island foxes exist only on six of the eight Channel Islands off the southern California coast. The only carnivore unique to California, island foxes feed on beetles, crickets, earwigs, mice, and the occasional crab. They also eat the fruits of cactus, manzanita, saltbush, and other plants.

The Endangered Species Act of 1973 protects endangered and threatened wildlife and plants in the United States from hunting, collecting, and other activities that harm them or their habitats. Since this law was enacted, the numbers of such endangered animals as alligators, bald eagles, and peregrine falcons have recovered enough to be reclassified as threatened or removed from the endangered list altogether. Prior to the speedy recovery of the island fox, Steller’s sea lion was the quickest species to recover under the Endangered Species Act. Steller’s sea lion spent 23 years on the endangered list; the island fox took just 12 years to recover.

Thanks to the intensive efforts of the Nature Conservancy, the Institute for Wildlife Studies, and the Channel Islands National Park (plus hundreds of individual volunteers), the island fox has been saved from extinction—and in record time. The fox-saving efforts also had a side benefit: The removal of feral pigs allowed many rare plants native only to the Channel Islands to gain strength as well.

September 21, 2015

California wildfires have been spurred on in 2015 by dry conditions and high winds. Credit: © N. F. Photography/Shutterstock

Over the past weekend (ending on September 20), the 2015 wildfire season in northern California continued as two major fires put the number of homes destroyed over 1,400 . Although largely contained by Monday, the Valley and Butte fires in Lake and Napa counties have so far destroyed more than 2,600 buildings. Spurred on by hot, dry, and windy conditions, the wildfires are now the third most destructive in California history. Although the two fires are now largely contained by firefighters, at least six people were killed in California wildfires over the weekend, and the fires continue to threaten other properties.

A wildfire is the uncontrolled burning of large areas of vegetation (plant life). Depending on where they occur, such fires may also be called brushfires, bushfires, or forest fires. Wildfires may be ignited by human activities or by natural causes. The size and intensity of a wildfire depends on the type of vegetation fueling it, the terrain, and the weather. Since January 1, 2015, wildfires in the United States have burned more than 8.9 million acres (3.6 million hectares), mostly in the Pacific Northwest and Alaska. This year’s fires have already burned more land than in any other year over the last decade. The hardest hit areas are about 50 miles (80 kilometers) west of California’s capital Sacramento.

The Valley and Butte fires were only two of at least 10 major wildfires currently burning in California. U.S. Forest Service officials believe the wildfire season this year is so bad because the vegetation in the area has been rendered bone dry after severe drought over the last four years, and more recently weeks of extreme summer heat. Data from the Forest Service indicates that the fire season, typically occurring over the summer months from June through August, now extends almost year round, averaging 78 days longer compared to fire seasons in the 1970’s. Areas of Lake and Napa counties had been evacuated last week, but people have been allowed to return as more than 4,000 firefighters managed to contain the worst fires after midweek rain. However, the weather forecast calls for continued hot and dry conditions that increase the danger of wildfire outbreaks over the next few weeks.

September 15, 2015

Over last weekend, September 12th and 13th, California once again became the top priority for the United States Forest Service, as destructive wildfires broke out in several regions in the northern part of the state. Spurred on by hot, dry, and windy conditions, the wildfires burned through about 61,000 acres (24,690 hectares) of tinder-dry forest within about 24 hours. By Monday, the fires had caused at least one death.

A wildfire is the uncontrolled burning of large areas of vegetation (plant life). Depending on where they occur, such fires may also be called brushfires, bushfires, or forest fires. Wildfires may be ignited by human activities or by natural causes. The size and intensity of a wildfire depends on the type of vegetation fueling it, the terrain, and the weather. Since January 1 of this year, wildfires in the United States have burned more than 8.7 million acres (3.5 million hectares), mostly in the Pacific Northwest and Alaska. This year’s fires have already burned more land than in any other year over the last decade.

In the hardest hit areas, about 50 miles (80 kilometers) west of California’s capital Sacramento, the fires consumed at least 400 homes and buildings and displaced thousands of residents. Many people had to flee from their homes with little or no warning. A separate fire centered about 30 miles (48 kilometers) north of Napa has devastated about 78 square miles (202 square kilometers) so quickly that it left fire officials astonished. Forest Service officials believe the wildfires, which were probably started by lightning, spread so fast because the vegetation in the area has been rendered bone dry after severe drought over the last four years, and more recently, by weeks of extreme summer heat.

Firefighters are trying to protect an ancient grove of trees in Sequoia National Park that are threatened by the raging wildfires in California. (National Park Service)

On Monday, California Governor Jerry Brown declared a state of emergency for Lake and Napa counties due to the raging wildfires. Because these fires are so close to major cities and towns, more than 1,000 firefighters have been deployed to combat the blazes. The firefighters also worked to protect an ancient grove of Giant Sequoia trees at Sequoia National Park. This grove includes the General Sherman Tree, the largest tree in the world by volume.

Another Behind the Headline article

• Wildfires Scorch Western United States (Aug. 24, 2015)