A heat wave is a period of unusually hot weather lasting two or more days. The temperature is hotter than average for the region. In an extreme heat wave, temperatures break records. The frequency, severity, and length of heat waves are increasing.
Heat and humidity already kill an average of 600 people in the U.S. each year. Deadly heat waves of at least 20 days affect 30% of the earth’s population. That could increase to 74% by 2100 if global warming isn’t stopped.
National Weather Service Warnings
The U.S. National Weather Service notifies residents of upcoming heat waves. It uses a Heat Index Value also called the apparent temperature. It factors in humidity with the air temperature in degrees Fahrenheit.
The NWS issues a Heat Advisory when the heat index reaches a certain level within the next 12 to 24 hours. The trigger point depends on average temperatures and is different for each locality. The advisory may be issued for lower temperatures if it is early in the season or during a multi-day heat wave. A heat advisory means that people could be affected by heat. It warns them to take precautions. It also triggers public safety regulations. These can include a ban on evictions and electricity shutoffs.
The NWS issues an Excessive Heat Warning when higher heat levels are expected within the next 12 to 24 hours. At those temperatures, some people can be seriously affected or die if they don't take precautions. The warning alerts hospitals to prepare for an increase in emergency calls. It activates programs that check on the home-bound. Some cities will open public air-conditioned centers. It also triggers the same public safety regulations as a heat advisory.
An Excessive Heat Watch is a warning issued one to two days in advance of the heat wave.
Heat Wave 2019
The World Meteorological Organization called July 2019 the hottest month in history. It beat the previous record in July 2016. That's without the El Nino warming effect that boosted temperatures in 2016. Cities across Europe hit record temperature, including Paris at 108.7 F. Greenland experienced one of the most significant melt events ever recorded. It poured 197 billion tons of water into the North Atlantic, enough to raise the global sea level by 0.02 inches. Wildfires scorched Alaska and Siberia
OnMemorial Day weekend 2019, a heat wave hit the Southeastern United States. Six cities tied or set record temperatures. A wobbly jet stream pulled hot air from the south. At the same time, it pushed cold air from the north onto the Southwest, sending temperatures down into the 70s.
Heat Wave 2018
In July 2018, heat waves set new temperature records all over the world. Death Valley had the hottest month ever recorded on Earth. The average temperature was 108 F. Caribou, Maine, reported that July was its warmest month ever. So did 22 counties and cities in China.
Several towns reached new highs, including Los Angeles at 111 F, Amsterdam at 94.6 F, and London at 95 F. On July 5, 2018, Ouargla, Algeria, reported 124.34 F, the highest temperature reliably recorded in Africa. Climate scientists were shocked by the sudden onset of these extreme events. The urban heat island effect makes city daytime temperatures 1-7 F hotter and nighttime temperatures 2-5 F hotter.
A heat wave is caused by a high-pressure system that hovers over an area. It traps heat beneath it like an oven. High-pressure systems force air downward. Hot air on the ground cannot escape into higher levels. Without rising air, there are no rain or clouds. The sun just bakes the area until a new pressure system is strong enough to push the high-pressure system away.
Climate change increases heat waves by increasing the Earth's average temperature. How much has it warmed? Since the 1880s, the earth’s average temperature has risen a little more than two degrees Fahrenheit or one degree Celsius. Global warming is occurring at a faster rate than at any other time in the Earth's history.
In the northern hemisphere, global warming is changing the jet stream. That’s a river of wind formed when cold Arctic air meets warm southern air. But the Arctic is warming faster than the south. The jet stream slows down and becomes wavy. That allows heat waves to linger.
Over the past 30 years, heat waves have killed more people than all other weather-related natural disasters combined. Heat waves kill in four ways:
- Heat stress causes dehydration and loss of body salt. That throws off the chemistry of the body.
- As the body tries to ol, it taxes the heart. That can lead to failure in people with heart conditions
- When the core body temperature rises beyond 104 F, organs fail. The gut leaks toxins into the body, creating a deadly inflammatory response called heat stroke
- People drown while trying to cool off in lakes and rivers.
Heat waves are most likely to affect people who work or live outdoors. The Bureau of Labor Statistics estimates that half of all U.S. workers have jobs that require them to be outdoors. Of those, 6.1 million are in construction, 4.6 million are in logistics, and 2.1 million are in agriculture. In July 2018, a United States Postal Service worker died on the job when the temperature hit 117 F.
Also at risk are those without air conditioning. Worker productivity declines by 2% for every degree Celsius above normal room temperature.
The elderly, injured, and children are most threatened. People on certain medications that curb their ability to sweat are also at risk. Alcoholic consumption worsens the effect of heat waves.
Heat waves contributed to the record wildfire season in the American West. The heat dried out vegetation, creating tinder for fires. Scientists were surprised that the heat was enough to overcome soil that was still moist from a wetter than normal winter. Wildfires are driven more by the temperature and moisture content in the air than by the moisture content in the soil.
Heat waves could be the reason behind the 45 to 75% decline in worldwide insect populations. Warmer temperatures make males less fertile by reducing sperm count, according to research by the University of East Anglia. It only took a 9 to 12 degree F spike over a five-day period to cut sperm count in half. A second spike almost sterilized the insects.
From 2002 through 2009, the health-related costs of heat waves was $5.3 billion. In 2019, health care costs were $3.8 trillion. That's 18% of the U.S. economy. That's up substantially from 1960 when costs were only $27.2 billion, or just 5% of gross domestic product.
Heat waves also lower food production. Between 1964 and 2007, drought and heat waves destroyed one-tenth of the world's cereal production.
Heat waves will increase in the West, Midwest, and Great Lakes areas of the United States by the mid-2020s. Montana and Wyoming will have almost 30 days of heat waves in 2030, compared to 10 in 2000. Michigan will have 35 and Nebraska will experience almost 40 days of heat waves.
The climate impact map shows you how much hotter your state or country will be during the next 10 to 100 years.
By 2028, heat waves and other climate change effects will add $360 billion per year. Much of this is due to health costs.
By 2030, heat waves will lead to a $2 trillion loss in labor productivity. It will kill five times as many people in the U.S.
By 2100, climate change could cost the U.S. government $112 billion per year. Heat waves alone will lower workforce productivity by 1.2 billion hours, costing $170 billion.