A heat wave is a period of unusually hot weather that typically lasts two or more days. To be considered a heat wave, the temperatures have to be outside the historical averages for a given area.

Dangerous Heat

For some, a heat wave might sound like an excuse to run around with a hose or into some sprinklers. In reality, though, heat waves are no laughing matter. They are serious weather phenomena that can be quite dangerous.

You see, there’s hot and then there’s HOT. One example of a HOT time was in late June and early July 2012 in the United States. During that time period, more than 8,000 warm temperature records were broken or tied. Many of these records had been unchallenged since the 1930s. Roads actually buckled in Chicago. Many deaths were attributed to the heat nationwide.

How Do Heat Waves Form?

Heat waves are generally the result of trapped air. During the 2012 heat wave, air was trapped above much of North America for a long period of time. As opposed to cycling around the globe, it simply stayed put and warmed like the air inside an oven.

The culprit? A high-pressure system from Mexico. Between June 20th and June 23rd, this system migrated north. It grew in size, and it parked itself over the Great Plains of the United States.

High-pressure systems force air downward. This force prevents air near the ground from rising. The sinking air acts like a cap. It traps warm ground air in place. Without rising air, there was no rain, and nothing to prevent the hot air from getting hotter.

But that wasn’t all. A weather pattern that normally pulls air toward the east was also weaker at the time. That meant that there was little that could be done to push this high-pressure cap out of the way.

Heat Wave Safety

Heat waves like this may be less exciting or dramatic than other natural disasters like tornados, hurricanes, flooding, or even thunderstorms, but heat waves kill more people in the United States than all other weather-related disasters combined.

Stay safe during a heat wave with these helpful tips.

• Take it easy. Slow down and avoid doing strenuous work.
• Dress for the weather. Wear lightweight, light colored clothing.
• Avoid high-protein foods and meat. Eating these can increase your body’s heat production.
• Drink lots of water. Unless you have a condition that is affected by changing how much fluid you drink, it’s a good idea to keep as hydrated as possible.
• Find an air-conditioned place to hang out. Don’t have an air conditioner of your own? Find a friend with one, or go to a public place like a library that has one.
• Avoid getting too much sun. Sunburns make it harder for your body to cool off.
• Be very careful in cars. Cars heat up extremely fast. You should never spend time in a parked car with windows closed, and you should always be wary of hot metal objects both inside (like seatbelt buckles) and outside the car.

Each National Weather Service Forecast Office issues some or all of the following heat-related products as conditions warrant. NWS local offices often collaborate with local partners to determine when an alert should be issued for a local area. For instance, residents of Florida are much more prepared for 90°F+ weather than residents in Alaska.

• Excessive Heat Warning—Take Action! An Excessive Heat Warning is issued within 12 hours of the onset of extremely dangerous heat conditions. The general rule of thumb for this Warning is when the maximum heat index temperature is expected to be 105° or higher for at least 2 days and night time air temperatures will not drop below 75°; however, these criteria vary across the country, especially for areas not used to extreme heat conditions. If you don't take precautions immediately when conditions are extreme, you may become seriously ill or even die.
• Excessive Heat Watches—Be Prepared! Heat watches are issued when conditions are favorable for an excessive heat event in the next 24 to 72 hours. A Watch is used when the risk of a heat wave has increased but its occurrence and timing is still uncertain.
• Heat Advisory—Take Action! A Heat Advisory is issued within 12 hours of the onset of extremely dangerous heat conditions. The general rule of thumb for this Advisory is when the maximum heat index temperature is expected to be 100° or higher for at least 2 days, and night time air temperatures will not drop below 75°; however, these criteria vary across the country, especially for areas that are not used to dangerous heat conditions. Take precautions to avoid heat illness. If you don't take precautions, you may become seriously ill or even die.
• Excessive Heat Outlooks—Be Aware! The outlooks are issued when the potential exists for an excessive heat event in the next 3-7 days. An Outlook provides information to those who need considerable lead-time to prepare for the event.

The National Weather Service (NWS) has multiple tools to assess the potential for heat stress due to extreme temperatures. The following tools can inform the issuance of NWS official heat watches, warnings, and advisories. Each of these tools integrate other weather parameters to provide a deeper level of information beyond what the actual air temperature can tell us.

Heat Index

The Heat Index is a measure of how hot it really feels when relative humidity is factored in with the actual air temperature. As an example, if the air temperature is 96°F and the relative humidity is 65%, the heat index--how hot it feels--is 121°F. The red area without numbers indicates extreme danger. The National Weather Service will initiate alert procedures when the Heat Index is expected to exceed 105°-110°F (depending on local climate) for at least 2 consecutive days.

NWS also offers a Heat Index chart for areas with high heat but low relative humidity. Since heat index values were devised for shady, light wind conditions, exposure to full sunshine can increase heat index values by up to 15°F. Also, strong winds, particularly with very hot, dry air, can be extremely hazardous.

Wet Bulb Globe Temperature (WBGT)

WBGT uses temperature, humidity, wind, solar radiation, and other weather parameters. It’s a particularly effective indicator of heat stress for active populations such as outdoor workers and athletes.

WBGT can be used to inform activity modifications during exercise or outdoor work. For instance, The American College of Sports Medicine bases its guidelines for the intensity of sport practices on WBGT, and it is therefore utilized by athletic programs in many school districts.

The Occupational Safety and Health Administration (OSHA) recommends protective measures for outdoor work:

• Acclimatize workers starting the first day working in the heat and after any extended absences
• Provide shade for outdoor work sites
• Schedule work earlier or later in the day
• Use work/rest schedules
• Limit strenuous work (eg., carrying heavy loads)
• Use relief workers when needed

Heat Risk

• The National Weather Service (NWS) prototype HeatRisk forecast provides a color and numeric value that places forecast heat for a specific location into an appropriate level of heat concern, along with identifying groups potentially most at risk at that level. The HeatRisk is accompanied by recommendations for heat protection and is a useful tool for planning for upcoming heat and its associated potential risk.
• Based on the high resolution NWS national gridded forecast database, a daily HeatRisk value is calculated for each location from the current date through seven days in the future.
• The HeatRisk takes into consideration:
  • How significantly above normal the temperatures are at your location,
  • the time of the year (for example, is this early season heat that you likely haven't become used to, or late season heat that you have become more used to),
  • Duration of unusual heat (for example, are temperatures overnight at levels that would lower heat stress, or will warm overnight low temperatures continue to add to heat stress into the next day), and
  • If those temperatures are at levels that pose an elevated risk for heat complications, such as heat stress, based on peer reviewed science.
  • While humidity is not directly taken into account, an approximation is used by considering:
  • How unusually warm the overnight temperatures are (more humid air usually leads to warmer overnight low temperatures than are typical for an area)
  • How large the difference is between overnight lows and daytime high temperatures (the difference tends to be smaller the more humid the air is).

What is a heat wave? A heat wave is a period of abnormally hot weather generally lasting more than two days. Heat waves can occur with or without high humidity. They have potential to cover a large area, exposing a high number of people to hazardous heat. Heat can be very taxing on the body; check out the heat related illnesses that can occur.

Extreme heat also impacts our infrastructure - from transportation to utilities to clean water and agriculture. High heat can deteriorate and buckle pavement, warp or buckle railway tracks, and exceed certain types of aircraft operational limits. Electricity usage increases as air conditioning and refrigeration units in homes and offices work harder to keep indoors cooler. Transmission capacity across electric lines is reduced during high temperatures, further straining the electrical grid. Water resources are also strained as conventional power plants require large quantities of water for cooling and crops may need increased water consumption, and people increase water consumption to stay hydrated and cool. Heat can have lasting impacts as crops may be damaged, reducing production which leads to short supply and or increased cost to the farmers and consumers.

How to Stay Safe During Excessive Heat Events

Outdoor Activities

• Slow down. Reduce, eliminate or reschedule strenuous activities until the coolest time of the day. Those particularly vulnerable to heat such as children, infants, older adults (especially those who have preexisting diseases, take certain medications, living alone or with limited mobility), those with chronic medical conditions, and pregnant women should stay in the coolest available place, not necessarily indoors.
• Dress for summer. Wear lightweight, loose fitting, light-colored clothing to reflect heat and sunlight.
• Minimize direct exposure to the sun. Sunburn reduces your body's ability to dissipate heat.

Eating and Drinking

• Eat light, cool, easy-to-digest foods such as fruit or salads. If you pack food, put it in a cooler or carry an ice pack. Don't leave it sitting in the sun. Meats and dairy products can spoil quickly in hot weather.
• Drink plenty of water (not very cold), non-alcoholic and decaffeinated fluids, even if you don't feel thirsty. If you are on a fluid-restrictive diet or have a problem with fluid retention, consult a physician before increasing consumption of fluids. 
• Do not take salt tablets unless specified by a physician.

Cooling Down

• Use air conditioners or spend time in air-conditioned locations such as malls and libraries.
• Use portable electric fans to exhaust hot air from rooms or draw in cooler air.
• Do not direct the flow of portable electric fans toward yourself when room temperature is hotter than 90°F. The dry blowing air will dehydrate you faster, endangering your health.
• Take a cool bath or shower.

Check on Others

• Check on older, sick, or frail people who may need help responding to the heat. Each year, dozens of children and untold numbers of pets left in parked vehicles die from hyperthermia.  Keep your children, disabled adults, and pets safe during tumultuous heat waves.
• Don't leave valuable electronic equipment, such as cell phones and gps units, sitting in hot cars.
• Make sure rooms are well vented if you are using volatile chemicals.

Safety Tips for Parents and Caregivers

Children and Heat Vulnerability

• Newborns, infants, and young children are particularly vulnerable to heat-related illness and death. Their bodies are less able to adapt to heat than are adults.
• Children under four years of age experience higher hospital admissions for respiratory illnesses during heat waves.
• The effects are more severe on children because their bodies warm at a faster rate than adults.

Protect and prepare: Keep children cool by having them drink plenty of water, take lots of breaks, wear light-colored and lightweight clothing, and limit playing outdoors to cooler times of the day. Make sure fluids are not very cold or high in sugar/sweetener content.

Heat Safety in Vehicles

Even on mild days in the 70s, studies have shown that the temperature inside a parked vehicle can rapidly rise to a dangerous level for children, pets and even adults. Leaving the windows slightly open does not significantly decrease the heating rate. A dark dashboard or car seat can quickly reach temperatures in the range of 180°F to over 200°F. These objects heat the adjacent air by conduction and convection and also give off long wave radiation, which then heats the air trapped inside a vehicle. 

• Touch a child's safety seat and safety belt before using it to ensure it's not too hot before securing a child
• Never leave a child unattended in a vehicle, even with the windows down, even for just a minute
• Teach children not to play in, on, or around cars. They could accidentally trap themselves in a hot vehicle.
• Always lock car doors and trunks--even at home--and keep keys out of children's reach.
• Always make sure children have left the car when you reach your destination. Don't leave sleeping infants in the car.

Community Interventions

If you are a public official, please download the Excessive Heat Event Guidebook for best practices during heat waves and to learn about options that communities can use to develop their own mitigation plans to prepare for and respond to extreme heat. The Guidebook was developed by the Environmental Protection Agency (EPA) in 2006, in collaboration with the National Oceanic and Atmospheric Administration (NOAA), the CDC, and the Department of Homeland Security (DHS). Municipal officials in both the U.S. and Canada provided vital information you can use to help the public cope with excessive heat. The guidebook highlights best practices that have been employed to save lives during excessive heat events in different urban areas and provides a menu of options that officials can use to respond to these events in their communities. 

• Assess locations with vulnerable populations, such as nursing homes and public housing
• Staff additional emergency medical personnel to address the anticipated increase in demand
• Shift and expand homeless intervention services to cover daytime hours
• Open cooling centers to offer relief for people without air conditioning and urge the public to use them.
• Provide toll-free numbers and Website addresses such as this one for heat exposure symptoms and responses
• Open hotlines to report concerns about individuals who may be at risk
• Coordinate broadcasts of excessive heat events response information in newspapers and on television and radio.

Heat Safety for Outdoor Workers

Outdoor workers can be at a higher risk to the effects of excessive heat. The Wet Bulb Globe Temperature (WBGT) is a good measure of heat stress during outdoor activities. See Occupational Safety and Health Administration (OSHA) resources and recommended practices when working under hot conditions.

• Drink water often
• Rest and cool down in the shade during breaks
• Gradually increase workload and allow more frequent breaks for new workers or workers who have been away for a week or more
• Know symptoms, prevention, and emergency response to prevent heat-related illness and death  
• Check weather forecasts ahead of time to be better prepared.

During extremely hot and humid weather, your body's ability to cool itself is challenged. When the body heats too rapidly to cool itself properly, or when too much fluid or salt is lost through dehydration or sweating, body temperature rises and you or someone you care about may experience a heat-related illness. It is important to know the symptoms of excessive heat exposure and the appropriate responses. The Centers for Disease Control and Prevention (CDC) provides a list of warning signs and symptoms of heat illness, and recommended first aid steps. Some of these symptoms and steps are listed below.

Heat Cramps

Heat cramps may be the first sign of heat-related illness, and may lead to heat exhaustion or stroke.

• Symptoms: Painful muscle cramps and spasms usually in legs and abdomen and Heavy sweating.
• First Aid: Apply firm pressure on cramping muscles or gently massage to relieve spasm. Give sips of water unless the person complains of nausea, then stop giving water. 
  
  Seek immediate medical attention if cramps last longer than 1 hour.

Heat Exhaustion

• Symptoms: Heavy sweating, Weakness or tiredness, cool, pale, clammy skin; fast, weak pulse, muscle cramps, dizziness, nausea or vomiting, headache, fainting,
• First Aid: Move person to a cooler environment, preferably a well air conditioned room. Loosen clothing. Apply cool, wet cloths or have person sit in a cool bath. Offer sips of water. If person vomits more than once,
  
  Seek immediate medical attention if the person vomits, symptoms worsen or last longer than 1 hour

Heat Stroke

• Symptoms: Throbbing headache, confusion, nausea, dizziness, body temperature above 103°F, hot, red, dry or damp skin, rapid and strong pulse, fainting, loss of consciousness.
• First Aid:  Call 911 or get the victim to a hospital immediately. Heat stroke is a severe medical emergency. Delay can be fatal. Move the victim to a cooler, preferably air-conditioned, environment. Reduce body temperature with cool cloths or bath. Use fan if heat index temperatures are below the high 90s. A fan can make you hotter at higher temperatures. Do NOT give fluids.

Using a fan to blow air in someone’s direction may actually make them hotter if heat index temperatures are above the 90s.

Heat is one of the leading weather-related killers in the United States, resulting in hundreds of fatalities each year. Heat can be very taxing on the body; check out the heat related illnesses that can occur with even a short period of exposure. Everyone can be vulnerable to heat, but some more so than others. According to The Impacts Of Climate Change On Human Health In The United States: A Scientific Assessment the following groups are particularly vulnerable to heat; check in with friends and relatives who fall in one of these populations, especially if they don’t have air conditioning.

• Young children and infants are particularly vulnerable to heat-related illness and death, as their bodies are less able to adapt to heat than are adults. 
• Older adults, particularly those with pre existing diseases, take certain medications, are living alone or with limited mobility who are exposed to extreme heat can experience multiple adverse effects.
• People with chronic medical conditions are more likely to have a serious health problem during a heat wave than healthy people.
• Pregnant women are also at higher risk. Extreme heat events have been associated with adverse birth outcomes such as low birth weight, preterm birth, and infant mortality, as well as congenital cataracts.

It is NEVER safe to leave a child, disabled person or pet locked in a car, even in the winter. If you have a toddler in your household, lock your cars, even in your own driveway. Kids play in cars or wander outside and get into a car and can die in 10 minutes! Deaths routinely are reported as early as April and tragedies continue into December in southern states.

Exposure to extreme heat can have many direct effects on human health (heat stroke, reduced labor productivity), as well as indirect effects (promoting air pollution and increasing asthma attacks, overloading power grids requiring rolling blackouts). Negative health outcomes occur if an individual is exposed to the hazard and has not sufficiently adapted to reduce sensitivity.

What are the signs of heat related illness?

Heat-related illnesses happen when the body is not able to properly cool itself. While the body normally cools itself by sweating, during extreme heat, this might not be enough. In these cases, a person’s body temperature rises faster than it can cool itself down. This can cause damage to the brain and other vital organs. Types of heat related illnesses include:

• Heat Cramps - Muscular pains and spasms due to heavy exertion. Although heat cramps are the least severe, they are often the first signal that the body is having trouble with the heat.
  • What to look for: heavy sweating, muscle pains or spasms
  • Actions to take: stop physical activity and move to a cooler place, drink water or a sports drink. Seek medical attention if cramps last longer than one hour,
• Heat Exhaustion - Typically occurs when people exercise heavily or work in a hot, humid place where body fluids are lost through heavy sweating. Blood flow to the skin increases, causing blood flow to decrease to the vital organs. This results in a form of mild shock. If not treated, the victim's condition will worsen. Body temperature will keep rising and the victim may suffer heat stroke.
• Heat Stroke - A life-threatening condition. The victim's temperature control system, which produces sweating to cool the body, stops working. The body temperature can rise so high that brain damage and death may result if the body is not cooled quickly. A person who is experiencing heat stroke needs medical attention.

Safety Tips

Staying Safe Before a Heat Wave

• To begin preparing, you should build an emergency kit and make a family communications plan.
• Gather food, water, medications in advance in case of store closures or having to relocate to a cooler location
• Know those in your neighborhood who are older, young, sick or have pre-existing medical conditions. They are more likely to become victims of excessive heat and may need help.
• Be aware that people living in urban areas may be at greater risk from the effects of a prolonged heat wave than are people living in rural areas.
• Get trained in first aid to learn how to treat heat-related emergencies.
• Hydrate with water throughout the day, and avoid sugary beverages or drinks with caffeine
• Check air-conditioning ducts for proper insulation.
  • Install window air conditioners snugly; insulate if necessary.
• Install temporary window reflectors (for use between windows and drapes), such as aluminum foil-covered cardboard, to reflect heat back outside.
• Weather-strip doors and sills to keep cool air in.
• Cover windows that receive morning or afternoon sun with drapes, shades, awnings, or louvers. (Outdoor awnings or louvers can reduce the heat that enters a home by up to 80 percent.)
• Keep storm windows up all year

Tips For Staying Safe During a Heat Wave

• Never leave children or pets alone in closed vehicles.
• Drink plenty of water; even if you do not feel thirsty. Avoid drinks with caffeine. Persons who have epilepsy or heart, kidney, or liver disease; are on fluid-restricted diets; or have a problem with fluid retention should consult a doctor before increasing liquid intake.
• Check on family, friends, and neighbors who do not have air conditioning and who spend much of their time alone.
• Check on your animals frequently to ensure that they are not suffering from the heat.
• Go to a designated public shelter if your home loses power during periods of extreme heat. Stay on the lowest floor out of the sunshine if air conditioning is not available.
• Check the weather/listen to NOAA Weather Radio for critical updates from the National Weather Service (NWS).

Tips If You Have To Be Outside

• Avoid strenuous work during the warmest part of the day. Use a buddy system when working in extreme heat, and take frequent breaks.
• Dress in loose-fitting, lightweight, and light-colored clothes that cover as much skin as possible. Avoid dark colors because they absorb the sun’s rays.
• Protect face and head by wearing sunblock and a wide-brimmed hat.
• Postpone outdoor games and activities.
• Stay indoors as much as possible and limit exposure to the sun.

Tips For Staying Safe After a Heat Wave

• In the event that you lose power in your home, it is important to throw out any food that became wet or warm without proper cooling.

Additional Safety Tips

• Eat well-balanced, light, and regular meals. Avoid using salt tablets unless directed to do so by a physician.
• Limit intake of alcoholic beverages.
• Avoid extreme temperature changes.
• Consider spending the warmest part of the day in public buildings such as libraries, schools, movie theaters, shopping malls, and other community facilities. Circulating air can cool the body by increasing the perspiration rate of evaporation.
• Download the FEMA App for heat advisories and safety tips.
• Text SHELTER + your ZIP code to 43362 (4FEMA) to find the nearest shelter in your area (example: shelter 12345).

Familiarize yourself with these terms to help identify an extreme heat hazard:

• Heat Wave - Prolonged period of excessive heat, often combined with excessive humidity.

• Heat Index - A number in degrees Fahrenheit (F) that tells how hot it feels when relative humidity is added to the air temperature. Exposure to full sunshine can increase the heat index by 15 degrees.

• Heat Cramps - Muscular pains and spasms due to heavy exertion. Although heat cramps are the least severe, they are often the first signal that the body is having trouble with the heat.

• Heat Exhaustion - Typically occurs when people exercise heavily or work in a hot, humid place where body fluids are lost through heavy sweating. Blood flow to the skin increases, causing blood flow to decrease to the vital organs. This results in a form of mild shock. If not treated, the victim's condition will worsen. Body temperature will keep rising and the victim may suffer heat stroke.

• Heat Stroke - A life-threatening condition. The victim's temperature control system, which produces sweating to cool the body, stops working. The body temperature can rise so high that brain damage and death may result if the body is not cooled quickly.

• Sun Stroke - Another term for heat stroke.

• Excessive Heat Watch - Conditions are favorable for an excessive heat event to meet or exceed local Excessive Heat Warning criteria in the next 24 to 72 hours.

• Excessive Heat Warning - Heat Index values are forecast to meet or exceed locally defined warning criteria for at least 2 days (daytime highs=105-110° Fahrenheit).

• Heat Advisory - Heat Index values are forecast to meet locally defined advisory criteria for 1 to 2 days (daytime highs=100-105° Fahrenheit).

• Adaptability: The ability of a person to take measures to reduce exposure and sensitivity - for example, avoiding outdoor activities during the day or wearing personal protective equipment (PPE) that is designed to mitigate heat buildup. When exposure is not preventable, adaptability can help reduce the impact of heat.

• Exposure: The extent to which an individual is exposed to extreme heat. Going outside on a hot, humid day and working in direct sunlight constitutes high exposure, while reducing exposure includes avoidance of these activities. Sometimes exposure is not preventable.

• Sensitivity: Inherent characteristics of a person that make them disproportionately affected by heat, such as pre-existing conditions, age, or occupation.