The World Meteorological Organization, an agency of the United Nations, defines a heat wave broadly as a period of unusually hot weather over a region persisting for at least two consecutive days during the hot period of the year. They differ from warm spells, which can occur at any time of the year, it says.
The definition of a heat wave varies across countries. In most parts of the United States, temperatures must be above the historical average in an area for two or more days before the label is applied.
In Britain, the Met Office, the United Kingdom’s national weather service, classifies a hot spell as an official heat wave when at least three consecutive days have temperatures that meet or exceed a specific threshold.
To account for a warming climate, the Met Office recently changed the threshold. Where it once calculated a threshold based on temperatures from 1981 to 2010, it now relies on temperatures from 1991 to 2020.
In France, a heat wave is defined officially as a “level of very high heat” that persists through the day and night for at least three consecutive days. In Paris, that level is said to be when the daytime temperature, in the shade, exceeds 31 degrees Celsius (about 88 Fahrenheit), and when it is at least 21 degrees Celsius (about 70 Fahrenheit) at night.
Blisteringly high temperatures are becoming more common on every continent, and climate scientists have little doubt that the burning of fossil fuels is a significant driver. Some of the heat extremes that the world has experienced in recent years would have been virtually impossible without the influence of human-induced climate change, scientists have found.
The main reason for this is simply that, as heat-trapping gases in the atmosphere cause average temperatures to rise around the globe, the range of possible temperatures in many places shifts upward, too, making warmer days more likely. In cities, heat-absorbing roads, buildings and paved surfaces can also contribute to hotter temperatures, while in rural areas, increased irrigation might instead make the land cooler.
Temperatures are on average about 2 degrees Fahrenheit (1.1 degrees Celsius) higher than they were in the late 19th century, before emissions of carbon dioxide and other heat-trapping gases became widespread. So extreme heat takes off from a higher starting point.
But beyond that, there are other factors that may be at play. Scientists, for example, are studying the links between heat waves and the jet stream, the river of air in the upper atmosphere at middle latitudes. A recent study found that heat waves in Europe increased in frequency and intensity over the past four decades, and linked the increase at least in part to changes in the jet stream.
The researchers found that many European heat waves occurred when the jet stream had temporarily split in two, leaving an area of weak winds and high pressure air between the two branches that is conducive to the buildup of extreme heat.
Scientists are working to pin down how the meandering of the jet stream, which has long shaped weather patterns for billions of people, might be changing in this warming era. One factor that is being taken into consideration is the rapid warming of the Arctic, which narrows the difference in temperatures between the northern and southern bands of the Northern Hemisphere. But how exactly this might be affecting extreme weather is still a matter of debate.
Rising temperatures can put many at risk for heat-related illnesses, including heat stroke and heat exhaustion. Here’s guidance from experts on how to stay cool during the swelter — even without air-conditioning.
The most important thing is to avoid heat-related illness.
Staying cool and hydrating often are the two most important things you can do to avoid feeling sick and discomfort when it’s extremely hot. If you don’t have an air-conditioning unit, or if your A.C. has been on nonstop and you still feel hot, here are ways to cool your body and home:
Spritz your skin with a mist of cool or room-temperature water.
Block out the windows in your home with a blanket or a darker sheet during the day to keep the heat out.
If you don’t have an air-conditioner, keep your windows open and run fans to circulate the air.
Wipe your forehead with a cool cloth.
Avoid strenuous exercise outdoors if possible.
Put ice cubes in your water bottle, especially if you’re outdoors.
If you do plan to exercise outside, or need to exert yourself outdoors for work, drink a slushie beforehand or douse your head in cold water. Cold showers can also help you cool down.
Keep an eye out for signs of heat exhaustion.
For adults, the Centers for Disease Control and Prevention says to watch out for symptoms of heat exhaustion, which include heavy sweating; cold, pale and clammy skin; a fast, weak pulse; nausea or vomiting; muscle cramps; fatigue; dizziness; or headaches and fainting. If you are experiencing these symptoms, sip water, move to a cooler location if possible, loosen your clothes, try to take a cool bath or place cool, wet cloths on your body. Seek medical attention immediately if you vomit or if your symptoms worsen or last longer than an hour.
There are a few tips to keep in mind to stay hydrated during a heat wave, when people should be especially vigilant about keeping themselves safe and healthy.
1. Drink lots of fluids. The golden rule of hydration is to drink when you’re thirsty. But if you, like many others, find it hard to motivate yourself to drink plain old water, you can rest easy. All sorts of drinks are effective at providing your body with fluids, not just water. Juices, milk, teas and even sodas can all be great sources of hydration.
2. Limit caffeine (if you’re sensitive) and alcohol. If you’re sensitive to caffeine, or haven’t had it in a while, it may be best to steer clear of coffee and energy drinks, said Kelly Hyndman, a researcher at the University of Alabama at Birmingham who studies kidney function and fluid retention.
It’s also probably best to steer clear of beer during a heat wave. While it may be tempting to crack a cold one, research has shown that alcohol acts as a diuretic, causing your body to lose more water.
It may also be best to not drink only water. When you sweat, you not only lose fluids, but you also excrete salts. So replenishing those salts and electrolytes is also critical; you can do that with sports drinks, or through the foods you eat.
3. Eat hydrating foods. Any food or drink that has fluid in it will be hydrating, and fresh fruits and vegetables hit the mark as they contain a high percentage of water as well as other nutrients that will benefit your overall health.
Watermelons, peaches, berries, grapes and oranges are all great candidates. Juicy vegetables like cucumbers, celery and olives are also full of fluid. Meals that are liquid-based, like soups or porridges, can also help you stay hydrated.
4. Avoid foods that are hard to digest. Metabolizing food is an energy-demanding process that can raise the body’s temperature. While it is absolutely not good for your health to stop eating, heat wave or not, some foods are harder to digest than others. These foods are “thermogenic,” meaning they create heat inside your body during the intensive digestive process.
All forms of protein, for example, including meats, fish and eggs, are highly thermogenic — especially red meat. Spicy foods containing cayenne and ginger are also thermogenic, as are sweet potatoes and whole grains.
5. Take care with exercise. Exercising in alarmingly high temperatures means that you’ll lose fluids at a faster rate than normal. To stay safe and adequately hydrated, follow some common sense guidelines. Try not to exercise at the hottest points of the day. Know your limits, and be aware of how hard you are pushing. If things feel strenuous, that’s a sign to take it down a notch.
The symptoms of heat stroke, according to the Centers for Disease Control and Prevention, include a high body temperature (103 degrees Fahrenheit, or 39.5 degrees Celsius, or higher); hot, red, dry or damp skin; a fast and strong pulse; a headache; dizziness; nausea; confusion and passing out. If someone is experiencing these symptoms, call your local emergency number and try to move the person into the shade or a cooler area. You can also use cool cloths or a cool bath to lower the person’s body temperature. Do not give him or her anything to drink.
Making matters more confusing, a compromised ability to make rational decisions can also be a symptom of heat stroke, so be aware that people with heat-related illnesses may deny feeling ill. Watch out for other signs that might hint at a problem, such as if they start stumbling or appear less coordinated than usual. Ask them if they have a headache, nausea or dizziness. Talk to them about a variety of topics to see if they exhibit symptoms of confusion.
If you suspect someone is having a problem with the heat, err on the side of caution and insist he or she gets into the shade or somewhere cool. People thought to be experiencing heat stroke should drink plenty of water; you can also spray their body with cold water or rub them down with ice or a cold cloth. If they don’t cool down quickly, seek medical advice.
Children should be instructed that if their friends ever start acting confused or mumbling in the heat, they should alert an adult.
For adults, the Centers for Disease Control and Prevention says to watch out for symptoms of heat exhaustion, which include heavy sweating; cold, pale and clammy skin; a fast, weak pulse; nausea or vomiting; muscle cramps; fatigue; dizziness; or headaches and fainting.
If you are experiencing these symptoms, sip water, move to a cooler location if possible, loosen your clothes, or try to take a cool bath or place cool, wet cloths on your body.
Seek medical attention immediately if you vomit or if your symptoms worsen or last longer than an hour.
Young children are especially vulnerable to heat exhaustion and heat stroke. Parents and caregivers should monitor their activity during hot weather and plan activities that are less likely to cause them to overheat, like running through sprinklers or playing in a pool. They should also make sure their children are wearing lightweight, light-colored clothing, using sunscreen and regularly hydrating.
If your children are playing outside of the water, try to keep them in the shade and consider bringing a spray bottle to spritz their skin (and your own).
When it is humid and at least 90 degrees Fahrenheit (or 32 degrees Celsius), children should not play outside for more than 30 minutes at a time. Keep babies under 12 months out of the sun as much as possible.
Teens tend to be more active than adults in the summer and should plan activities like hanging out in a park before noon, when the heat will be less intense, said Dr. James Mark, an emergency medicine physician at Cleveland Clinic in Ohio.
What are the signs of heat exhaustion in children?
If a child develops heat exhaustion, she may start to feel dizzy and nauseated, suffer muscle cramps or begin vomiting. Her skin may feel cold and clammy to touch.
If you observe these symptoms in a child, bring her to a cooler place; ask her to sit still or lie down; remove excess clothing; apply a cool, wet cloth or water to her skin and give her water to drink.
What are the signs of heat stroke in children?
“In heat stroke, the skin is hot and dry instead of cold and clammy, and the child gets sleepy and maybe confused,” said Dr. Claire McCarthy, a pediatrician at Boston Children’s Hospital and an assistant professor of pediatrics at Harvard Medical School. Children with heat stroke may also experience a high fever or seizures.
Heat stroke can also creep up on young kids who haven’t exerted themselves at all.
“They’re either dressed too warmly in a hot environment, they’re left in a hot vehicle or in a room that doesn’t have any circulation, they’re out at the beach wrapped up in the sun,” said Dr. Tony Woodward, the medical director of emergency medicine at Seattle Children’s. “All of those kinds of things can lead to their temperature going up very quickly.”
As extreme heat blankets large swaths of the globe, pet owners have particular cause for concern.
“If it feels too hot to you, it’s even worse for your pet,” said Dr. Sarah Hoggan, medical director for VCA California Veterinary Specialists —Murrieta.
Heat stroke can be fatal for dogs, cats and other animals — but simple precautions can help them get through a heat wave. Here’s what you need to know to keep your pets safe.
Keep pets inside as much as possible.
Ideally, keep your pet in air-conditioned spaces for as much of the day as you can. If you don’t have air-conditioning, plop your pet near a fan. In general, you want to keep pets in an area that is not hotter than 80 degrees, Dr. Hoggan said.
If you need to leave your pets outside, make sure they have enough shade and fresh water. Do not leave them unsupervised for more than a few minutes, and make sure the animals are not left directly in the sun, said Dr. Jerry Klein, a chief veterinary officer at the American Kennel Club.
For animals that are normally kept in cages, like rabbits, positioning a small fan nearby or placing a frozen water bottle in the cage can help them cool themselves, said Dr. Hoggan.
Time your walks.
Especially in cities, where hot sidewalks and pavement can burn and blister paws, try to avoid the peak heat in the middle of the day. Set an alarm and walk your dog early in the morning, taking the shadiest route possible. Or save the stroll for after the sun goes down. You can also consider buying dog boots to protect your pets’ paws, said Dr. Lori Teller, incoming president of the American Veterinary Medical Association.
Be sure to not overexercise your dog, either. Take frequent breaks, and make sure to carry water with you.
Don’t leave your pet alone in the car, even for a few minutes.
Hundreds of pets die from heat exhaustion each year after they are left in parked vehicles. It doesn’t matter if you crack the windows or park the car in the shade, the interior of a car can still reach 120 degrees Farenheit (or 49 degrees Celsius) in a matter of minutes, said Katie Wilkes, a spokesperson at the American Red Cross.
“Every year, we say it, and every year people forget,” Dr. Klein said.
Know which pets are at highest risk.
An animal that is very old, very young or has underlying health issues is at higher risk for heat stroke and exhaustion when the temperatures climb, Dr. Hoggan said. Dog and cat breeds with thicker coats, like huskies, golden retrievers and Siberian cats, are also at risk.
Animals with shorter snouts and “smushed” or squished faces — like pugs, English and French bulldogs, Boston terriers and Persian and Himalayan cats — are extremely susceptible to heat, Dr. Teller said. These pets are not able to pant as effectively, and so they can struggle to regulate their temperatures. Make sure they spend as little time outside as possible, she advised.
Know the signs of overheating, and act quickly.
If your pet is panting excessively, with thick, ropy saliva, acting unusually lethargic, vomiting or experiencing diarrhea, get care immediately. In dogs, signs of heat stroke also include a deep red tongue and brick red gums. For cats, open-mouth breathing can indicate that they are too hot.
Animals with heat exhaustion or heat stroke may also act confused, Dr. Teller said. Cats and dogs might not respond to their names or simple commands, and some may stagger. “They may seem to not fully be with it,” she said.
Don’t delay care if your pet shows these signs. Moisten towels with cool water — not ice water, which can cause blood vessels to constrict — and wrap them around your pet. You can also buy cooling vests in pet stores, Dr. Teller said. Take your pet to a vet or animal hospital as soon as you can. The Red Cross has a pet safety app with instructions for animal first aid, as well as a directory of local animal emergency resources.
“You don’t want to make a mistake that could potentially cost you your pet’s life,” said Ms. Wilkes.
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