Heat: Everything You Need to Know About Hot Weather

Throughout this guide we’ll cover heat-related topics such as heat basics, heat safety, heat-related illnesses, heat lightning, and more.

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Heat Basics


Yellow sun icon with rays over a grey circle backgroundHeat Definition

We all know what being hot feels like, right? If you close your eyes and think of the word “heat” you probably think of the sun beating down on you, sweat dripping down your face, and you might even get a little thirsty.

But what is heat? If you look it up in the dictionary, heat seems pretty simple: “The quality of being hot, high temperature.”

But extreme heat and heat safety aren’t that simple. In fact, extreme heat often results in the highest number of annual deaths among all weather-related hazards.

Extreme heat often results in the highest number of annual deaths among all weather-related hazards ☀️ #HeatStress #HeatFacts Share on X

Yellow thermometer and sun icon on a grey backgroundExtreme Heat Facts

But what is extreme heat? In the United States, we refer to extreme heat as a long period (2 to 3 days) of high heat and humidity with temperatures above 90 degrees.

Extreme heat can occur quickly and without any warning. It’s dangerous because extreme heat slows down evaporation and the body must work extra hard to maintain a normal temperature.

This can lead to overworking the human body. For that reason, older adults, children, the sick, and overweight people are at a heightened risk from extreme heat.

It’s also important to note that extreme heat can happen anywhere. When you think of heat in the United States, you probably think of the beaches of the Gulf Coast or the deserts of the West. Extreme heat is most dangerous in places where you least expect it because you are probably less prepared.

Extreme Heat in the United States

How many extreme heat days do major cities around the country experience each year? Our meteorologists took a look and it turns out the hottest city in the country is Phoenix, AZ. On average, Phoenix sees 168 days of 90+ degree temperatures per year.

The hottest city in the country is Phoenix, AZ. On average, Phoenix sees 168 days of 90+ degree temperatures per year ☀️ #HeatFacts Share on X

In second place is San Antonio, TX with 113 average days of 90 degrees or above per year. In third place is Orlando, FL with 106 average days. You can take a look at the full list below.

Average 90+ degree days in major cities in the United States

Heat wave icon, yellow sun with a blue wave on a grey circle backgroundHeat Waves

Some people refer to extreme heat as a “heat wave.” The National Oceanic and Atmospheric Association (NOAA) considers a heat wave as a period of unusually hot weather that typically lasts two or more days.

To be considered a heat wave, temperatures must be outside the historical average for a given period.

As you can see, extreme heat and a heat wave are a little different because extreme heat refers to sustained temperatures above a certain degrees while a heat wave compares current temperatures to historical averages.

How Do Heat Waves Form?

Heat waves typically happen when air gets trapped as opposed to cycling the globe. When air stays put it can warm like air inside of an oven.

A lot of times, heat waves in the U.S. are due to high pressure systems that force air downward. This force prevents air near the ground from rising. Since the sinking air acts like a cap, it traps warm ground air in place. Without rising air, there cannot be rain or anything to prevent the hot air from getting even hotter.

Blue and yellow thermometer icon on a grey backgroundTemperature Definition

As we talk about heat, it’s important to review temperature and different temperature measurements.

Temperature is the degree or intensity of heat present in a substance or object, especially as expressed according to a comparative scale and shown by a thermometer or perceived by touch. We can also definite temperature as a measure of the warmth or coldness of an object or substance with reference to some standard value.

What is the Temperature?

When we talk about the weather, we measure temperature with a thermometer. A thermometer measures the air temperature. Most thermometers are closed glass tubes containing liquids such as alcohol or mercury. When the air around the tube heats the liquid, the liquid expands and moves up the tube. A scale next to the tube then shows what the temperature is.

According to our team of meteorologists and weather safety specialists, temperature is a basic reading that is the first step to predicting heat stress and heat dangers.

A blue and yellow tape measure iconDifferent Temperature Measurements

Sometimes people question their thermometers when the instruments read the same temperature on two days that feel very different. Have you ever had this happened to you? The thermometer can read 85 degrees two days in a row, but maybe it feels extra hot the first day and cooler the second day.

That’s because there are other factors that go into what the temperature feels like. Besides temperature, there are two other ways most people use to describe how much heat our bodies feel from the weather.

Heat Index

The first measurement besides temperature that people use to measure heat and its affect on the human body is the heat index. What is the heat index? The heat index is a measure of how hot it really feels based on two factors: The temperature on the thermometer and relative humidity. This is a more advanced metric than temperature alone.

In order to find out the heat index on a given day, you can use a heat index chart or look at data from a weather station that measures heat index for you. When using the chart, you find the temperature on one axis and the relative humidity on the other axis to find what it really feels like.

For example, if the temperature is 90 degrees and the relative humidity is 50%, it really feels like 95 degrees. However, if the humidity goes up to 85% on that same 90-degree day, you’ll feel like it’s 117 degrees. Factoring in relative humidity makes a big difference!

Man running with speed behind him yellow iconWet Bulb Globe Temperature

Relative humidity isn’t the only additional factor that influences how hot we really feel outside. What about the sun or the wind?

This is where wet bulb globe temperature becomes such an important measurement. Referred to as WBGT for short, wet bulb globe temperature is an apparent measurement used to estimate the most accurate level of heat stress in direct sunlight. It takes five factors into consideration, including: Temperature, humidity, wind speed, sun angle, and cloud cover (solar radiation).

WBGT helps produce the most accurate temperature readings. That’s why organizations like the U.S. military, industrial hygienists, and sports programs trust WBGT when it comes to heat safety and important weather safety plans.

Wet bulb globe temperature is the generally accepted standard for heat stress management.

You can learn everything you’ll ever need to know about wet bulb globe temperature on our WBGT page.

Our weather stations measure temperature and wet bulb globe temperature now.

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Air Quality

It’s no secret that air quality is vital to good health. Air quality measures how clean or polluted the air is. Are you aware of the invisible dangers of air quality, such as air pollution? According to NOAA, air quality is determined by the quantities and types
of gaseous and particle pollutants found in the air we breathe. There are two main critical pollutants that affect air quality in the United States:

  • Ground-level ozone: A gas typically produced from other air pollutants reacting in the presence of sunlight. Ozone is a major constituent of smog.


  • Fine particulate matter (PM2.5): Small particles (with a diameter of 2.5 microns or less) emitted into the air or formed by atmospheric reactions of other pollutants.


Air quality affects crops, forestry, aquatic life, and ultimately humans. Smoke, dust, and smog are some primary pollutants that lead to poor air quality. Consequently, poor air quality from pollution can lead to respiratory and cardiovascular health problems and environmental damage.

The World Health Organization declared that air pollution kills approximately seven million people worldwide every year. Breathing in air with high levels of pollutants causes premature death due to stroke, heart disease, chronic obstructive pulmonary disease, lung cancer and acute respiratory infections. It is critical to monitor the air quality both inside and outside your home, to minimize exposure to toxic pollutants.

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Heat Threats

Most people love the summer and the return of warmer weather, but extreme heat can be very dangerous. There are a large variety of heat-related illnesses and they mostly all fall under the umbrella of heat stress.

Heat Stress

What is heat stress? Heat stress occurs when your body cannot cool itself enough to maintain a healthy temperature. There are many effects of heat stress. Some of them are very minor while others can be deadly. The effects of heat stress include:

A dark blue "1" in a red circle An inability to concentrate
A dark blue "2" in a red circle Muscle cramps (also known as heat cramps)
Heat rash
A dark blue "4" in a red circle Severe thirst
a dark blue "5" in a red circle Fainting
a dark blue "6" in a red circle Heat exhaustion
a dark blue "7" in a red circle Heat stroke

Heat stress is a major health risk in sports, outdoor recreation, athletics, and outdoor labor.

Mild Heat Related Illnesses

Human head with stars around it signifying dizzinessThe first few symptoms of heat stress on the list above are mild compared to things like heat exhaustion and heat stroke. You can’t overlook these symptoms because they can be dangerous on their own or can be warning signs that a more serious type of heat illness is about to occur.

The first sign is confusion or an inability to concentrate. You can also feel dizzy as a result of reduced blood flow to the brain.

Heat Cramps

Another common heat related illness is known as heat cramps. Heat cramps are muscle spasms that result from losing a large amount of salt and water through sweat. These cramps typically happen in the abdomen, arms, and calves.

Heat Rash

Heat rash is another uncomfortable heat related illness. A heat rash occurs when sweat ducts become clogged. Once sweat ducts are clogged, then sweat cannot get to the surface of the skin. Instead, it becomes trapped beneath the skin’s surface. This causes a mild inflammation or rash. Some of the main causes of heat rash are hot climates, physical activity, and overheating.

women athlete doing a sit upiconDehydration / Severe Thirst

Dehydration can be a very serious side effect of extreme temperatures. You may start to notice you’re dehydrated by a sudden thirst. Dehydration is a serious heat related disease that can be life-threatening if left untreated.

Under normal conditions, we all lose body water daily. This happens in a number of ways, like through sweating, crying, and using the bathroom. When we can’t replace this water fast enough by drinking enough fluids or eating foods with water in them, we risk dehydration.

Some other common symptoms of dehydration besides severe thirst include less frequent urination, dry skin, fatigue, dizziness, dry mouth, and increased heat rate or breathing.

We think of dehydration and its symptoms as the turning point for heat related illnesses. Once you get to this point things can get very serious very quickly.


Fainting is a serious heat related side effect. This is a more severe result of reduced blood flow to the brain. Heat causes an increase in blood flow to the skin and pooling of blood in the legs, which can lead to a sudden drop in blood pressure. This drop in blood pressure is what might make you feel light-headed or faint.

When people faint as a result of extreme heat, they can hit their heads or do physical harm to other parts of their body.

Heat Exhaustion

Human head vomitting One of the most serious side effects of heat stress on the body is heat exhaustion. Heat exhaustion is a serious condition that can develop into heatstroke. It occurs when excessive sweating in a hot environment reduces blood volume. This causes the body to go into overdrive to keep itself cool.

As a result, your body loses fluids faster, your blood pressure rises, and disorientation might set in. Other side effects of heat exhaustion include cool, pale, or clammy skin, nausea or vomiting, rapid, weak pulse, and muscle cramps.

Heatstroke (It’s one word, not “heat stroke”)

Worse than heat exhaustion is heatstroke. What is heatstroke? Heatstroke is an absolute medical emergency.

Heatstroke definition: Heatstroke occurs when the core body temperature rises above 104 degrees and the body’s internal systems start to shut down.

When someone suffer a heatstroke a plethora of organs can suffer, even if the body temperature is reduced quickly. Most victims will have profound central nervous system changes like delirium, coma, and seizures. The victim may stagger, appear confused, collapse, and become unconscious. Besides nervous system effects, heatstroke can also damage the liver, kidney, heart, and other muscles. These are all heatstroke symptoms and effects.

A lot of the symptoms are the same as heat exhaustion except those suffering from heatstroke may have a throbbing headache, their pulse will be strong, and their skin will be dry.

Types of Heatstroke

Running icon red personHeatstroke can happen when someone is exposed to a hot environment. This is call nonexertional heatstroke. This is when a hot environment alone raises the core body temperature. It occurs most often in older adults and in people with chronic illnesses who are exposed to hot, humid weather for prolonged periods.

The second type of heatstroke is called exertional heatstroke. Exertional heatstroke (EHS) is caused by an increase in core body temperature brought on by intense physical activity in hot weather. Anyone exercising or working in hot weather can come down with exertional heatstroke. It’s most likely to occur if you’re not used to high temperatures and/or haven’t gone through a proper heat acclimatization program.

Without prompt and adequate treatment both types of heatstroke can be fatal.

Heat Stress Statistics

A lot of people don’t take the dangerous of extreme heat seriously.

You can understand the reality of heat stress by simply looking at some heat stress statistics.

How many people die a year as a result of heat-related illnesses? The National Weather Service reports a yearly average of 170 direct and indirect heat related fatalities in the U.S. alone.

A study by the Environmental Protection Agency (EPA) found an average of 2,8000 yearly heat related hospitalizations per year. Want to hear another worrying heat stress statistic? The EPA also reports that heat related deaths are on the rise nationwide.

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Heat Safety

In this section you’ll find heat safety tips, popular heat warning types and how to access them, and life-saving treatment tips for all heat-related illnesses.

Basic Hot Weather Tips

Besides paying attention to heat watches and warnings, there are some basic hot weather tips you should know to increase your overall heat safety.

Heat Wave Safety Tips:

A dark blue "1" in a red circle Stay Hydrated: Drink plenty of fluids. You should drink more than you think you need to in hot weather. We recommend about 16 ounces before heading outdoors and then 5 to 7 ounces every 15 or 20 minutes. Insulated water bottles that keep water cold are a great piece of heat safety equipment! Make sure you avoid liquids that will dehydrate you like coffee, tea, and soda.
A dark blue "2" in a red circle Wear Protective Clothing: The sun can burn your skin and make you feel hotter. You should wear protective clothing include lightweight, light-colored, and loose-fitting items. If you sweat through your clothing you should change into dry clothes.
 Wear Sunscreen: To further protect yourself from the sun, you should wear sunscreen or sunblock. We recommend something full spectrum that protects you from both UVA and UVB rays.
A dark blue "4" in a red circle Take Frequent Breaks: If you can, take frequent breaks from hot temperatures in a shaded or air-conditioned area.

Transparent exclamation point in a red text bubbleHeat Warnings

There are four types of heat wave warnings that the National Weather Service (NWS) uses to inform the general public of excessive heat: Excessive Heat Outlooks, Excessive Heat Watches, Heat Advisories, and Excessive Heat Warnings.

Excessive Heat Outlooks

Excessive Heat Outlooks are issued when the potential exists for an excessive heat event in the next 3-7 days. These outlooks give the general public a warning ahead of time that conditions could be dangerously hot in the upcoming days. These are especially helpful for those who need considerable lead times for the event, like the elderly.

Excessive Heat Watches

When an Excessive Heat Watch is in place you should be prepared. The NWS issues these watches when conditions are favorable for an excessive heat event in the next 24 to 72 hours. They use a watch when the risk of a heat wave has increased bit its occurrence and timing is still uncertain.

Heat Advisories

Red alert iconA 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 degrees or higher for at least two days. It’s important to note that the nighttime air temperatures will not drop below 75 degrees. During a Heat Advisory you should take action and take precautions to protect yourself from dangerous heat.

Excessive Heat Warnings

The most serious heat warning is an Excessive Heat Warning. The NWS issues these 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 degrees or higher for at least two days and night time air temperatures will not drop below 75 degrees. If you do not take precautions when Excessive Heat Warnings are issued you could become seriously ill or even die.

Heat Illness Prevention & Treatment

Red fan iconPart of heat safety includes knowing how to treat heat related illnesses. Here is a small list for how to treat many of the heat related illnesses we mentioned in the “Heat Dangers” section earlier in this guide. If you want to become a heat safety expert, especially when it comes to heat safety and student athletes, you should read our in depth blog post on the subject.

Heat Rash Treatment

First move the victim to a cooler, less humid environment. Then you should work to keep the affected area dry. We recommend using unperfumed talcum powder to make the person suffering from heat rash feel a little better. Don’t use ointments or creams, though, because they keep the skin warm and moist. They’ll make the conditions worse!

Heat Cramp Treatment

lower half of body running iconWhen it comes to treating heat cramps you should first stop activity and move to a cool place. Another good tip is to increase fluid intake. Even if you start feeling better you should rest for a few hours before returning to activity. If there is no improvement you should seek medical help.

Dizziness and Fainting Treatment

If you or someone you know is feeling dizzy or faints because of heat you should get the person to a cool area and lay them down. When they are fully conscious, increase their fluid intake. If they are unconscious you should seek medical help.

Heat Exhaustion Treatment

The first thing you should do if someone is suffering from heat exhaustion is to move them to a cool area and lay them down. Then you should remove outer clothing and wet the skin with cool water or cloths.

Next you’ll want to increase their fluid intake if they are fully conscious. No matter what, it’s imperative to seek medical advice if someone is suffering from heat exhaustion.

Heatstroke Treatment

Red mobile phone iconIf someone is suffering from the most serious heat related illness, heatstroke, you should treat the situation as a medical emergency. It requires urgent attention. First call for an ambulance. Then get the victim to a cool, shady area and lay them down. As with any first aid emergency, have someone stay on the phone with medical help until the ambulance arrives.

Remove the victim’s clothing and wet their skin with water. You should also fan them continuously. It’s very important that you cool them down.

Unlike those suffering from heat exhaustion, those suffering from heatstroke should not ingest fluids. If they are unconscious, lay them on their side and ensure their airway is clear.

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Heat Lightning

What is heat lightning?


Well, there is no such thing as heat lightning.

What some people call “heat lightning” is just a thunderstorm that’s too far away to hear the thunder. Some nights in the summer, you can see distant lightning from a storm over the horizon, maybe 200 miles away. This is lightning flashes off of clouds high up in the sky.

This is a pretty common misconception so, to reiterate, there is no such thing as heat lightning! What you are seeing is a distant thunderstorm too far away to hear the thunder. But that storm could be heading your way so you should always take precautions when you see lightning. If you have access to a lightning detection and alert system, check it to see how far away the lightning activity is and if it is coming towards you.

What Are the Differences Between Heat Lightning and Regular Lightning?

Yellow human with hands up iconWhat some people call “heat lightning” or “lightning without rain” or a “dry thunderstorm” is just cloud-to-cloud and cloud-to-ground lightning from a distant storm. It’s the same kind of lightning. Lightning is lightning. A key thing to remember is that you can only hear thunder if a storm is within 7-8 miles of your location. Anything beyond that, you’ll still see the flashes but won’t hear the bang. And yes, thunder always means lightning is nearby.

Is heat lightning dangerous? Heat lightning isn’t directly dangerous. The fact that you can see the lightning flash but not hear the thunder means that the storm is at least 10 miles away or more. It should be a warning that the atmosphere is right for thunderstorms. Some storms may be rumbling in your area within the next few hours.

Can heat lightning strike you? It would be rare for heat lightning to strike you, but there are occurrences where a “bolt from the blue” has struck 10 miles away or more from a thunderstorm. It’s useful to be aware and take precautions whenever you see lightning.

For more general lightning facts, check out our comprehensive guide to lightning.

When Does Heat Lightning Occur?

People tend to see “heat lightning” during the spring and summer months. People are out late in the evening and more people are out in the nighttime hours when it’s warm and pleasant. Even though it’s possible year-round, especially in the south where we have frequent thunderstorms year round, it tends to be noticed more during the summer. Again, there is no such thing as heat lightning.

Why Don’t We Hear the Sound of Heat Lightning? 

It’s basic physics around how sound propagates throughout the atmosphere. Eventually, the thunderclap dies off with distance. You can sometimes, in rare instances, hear a storm from 10 miles away. But under the right conditions, you can see lightning flashing from up to 300 miles away, over the horizon.

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Heat and Sports

Since heat can cause critical illnesses, especially in those exerting themselves physically, it causes a lot of problems for athletes. In some cases, those problems can be as serious as death.

Heat Stress Prevention in Student Athletes

A solid heat stress management plan is a great way to protect student athletes. There are a few pieces that must be necessary in any heat stress prevention toolbox.
A dark blue "1" in a red circle Heat Acclimatization Policy
A dark blue "2" in a red circle WBGT Monitoring
Heat Safety Protocol

2018 Student Athlete Heat Illness

In 2018 there were unfortunately a lot of heat-related illnesses in high school and college athletics.

The most serious incident that year happened on May 29 when a football player suffered from heatstroke at the University of Maryland. He was hospitalized and later died as a result of this critical illness.

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Heat Wrap Up

Heat is a welcome sign of summertime but it can also be extremely dangerous to your health. When exercising or exploring the great outdoors, always be prepared to manage the risks of severe weather. Stay alert and check the forecasts before you go outside. Heed weather watches and warnings and take action when needed.

And if you have any heat-related questions after reading this guide, let us know by reaching out on social media. We have a team of meteorologists on staff to help with weather-related decision-making.

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