What Is the Heat Index? What Does the "Feel Like" Temperature Mean?
Why Does the Heat Index Matter?
Until July, 2012, I dismissed heat index values as made up numbers used by local weathermen to sensationalize their programs. Nothing could be further from the truth! A brush with heat exhaustion that left me ready to vomit and incapable of function for the rest of the day convinced me it's really important to pay attention to those heat index values!
Particularly if you live in a humid area, the heat index can be even more important the thermometer's temperature reading. The heat index temperature doesn't simply "feel like" a number - it is a measure of how effectively your body can exchange heat. Because of this, the heat index is frequently also called the apparent temperature.
If you ignore the heat index value when planning your day's outdoor activities, you could place yourself in danger of heat exhaustion, or even heat stroke. Heat index calculators use a combination of factors, including the temperature, relative humidity, and the dew point, to accurately determine how the weather affects the body's ability to regulate its temperature.
Did you Know?
Each summer, the United States experiences over 1,300 heat-related deaths?
Protect Yourself From Heat Illness
- What Is Heat Exhaustion? Preventing and Treating Heat Illnesses
Don't wait until you suffer a heat illness - educate and protect yourself today.
What is Relative Humidity?
People say "It's not the heat, it's the humidity" for a reason. Humidity is a measurement of how much water vapor is present in the air. Warm air is capable of holding more moisture than cool air, so the summer tends to have higher humidity levels than the winter.
The relative humidity indicates the rate of water evaporation at the specific air temperature. When you sweat, your body is trying to use evaporative cooling to reduce your body temperature, but high humidity slows the rate of evaporation and leaves you feeling uncomfortable. Evaporative cooling is also why a quick swim feels fantastic on a hot summer day. Also, think about stepping out of a shower. Even if your room's temperature was comfortable before the shower, you will feel chilly before you towel dry.
Unfortunately, air can only hold so much water. The relative humidity tells you how much of the air's holding capacity is already 'used.' If the temperature were 75 degrees with a relative humidity of 100%, a pool of water that was also 75 degrees would not evaporate. If the relative humidity is 90%, the water evaporates very, very slowly because it, essentially, has nowhere to go. When sweat cannot evaporate off your skin, you are left feeling hot and sticky, even if the thermometer temperature is otherwise pleasant. 75 degrees can feel pretty gross with 90% relative humidity!
Dew Point Table
Temperature, Degrees Fahrenheit
Dew Point, Degrees Fahrenheit
What is the Dew Point
The dew point is related to the relative humidity. The dew point is the temperature at which the air would have 100% relative humidity and atmospheric water vapor would condense into actual water droplets. Calculating it takes some complicated math, but online dew point calculators exist. Because warm air holds more water than cool air, a very hot day with a high dew point can have a lower relative humidity than a cooler, more comfortable day. Look at the table to the right to see how increasing temperatures with the same dew point lead to lower relative humidity.
By the time the dew point reaches 70 degrees, most people are very uncomfortable, but humidity is perceptible even when the dew point is in the upper 60s. "Oppressive" is frequently used to describe days with a dew point in the 70s!
Use your patio or porch more often with a large misting fan to keep you cool.
The Heat Index
A scientist named George Winterling developed the heat index in 1978 and, just one year later, the National Weather Service adopted it for official use. The actual calculation sounds like a bunch of gibberish unless you are an atmospheric scientist. Suffice it to say the heat index takes the temperature, humidity, and dew point into consideration to figure how effectivly the human body can cool down outside on a given day. To calculate the heat index easily, visit the NOAA website.
The heat index scale was devised to describe the temperature in the shade with a light breeze. Placing yourself in direct sunlight or a lack of breeze can cause the heat index value to rise by up to 15 degrees Fahrenheit.
Temperatures vs Heat Index ValuesClick thumbnail to view full-size
Stay Safe in the Sun
A personal mister fan is ideal for staying cool or cooling down a heat exhaustion victim.
Heat exhaustion can 'build' in your body. If you are exposed to heat for several days in a row, you can slowly become increasingly dehydrated from day to day. Heat exhaustion can catch up to you days later, even if the temperature has dropped slightly. Protect yourself by staying hydrated!
Learn how to Protect your Family During a Heatwave Power Outage
- How to Survive a Power Outage
Anyone can tolerate a power outage for a few hours, but what about a sustained power outage. What are four ways to survive a power outage?
How to Use the Heat Index to Stay Safe
According to NOAA, a heat index of 105 or more means you need to limit outdoor exposure and activity, as much as possible. High heat index values, regardless of the actual temperature, can lead to heat exhaustion, and even heat stroke. Sunburn reduces your body's ability to lose heat, so wearing sunscreen can actually help you stay cool by preventing burns. Because your body's reaction to extreme heat changes as you age, a teenager could feel fine but someone over 40 could suffer heat stroke under the same conditions.
If you go outside in the heat, make sure to monitor yourself and others in your group for signs of heat exhaustion. Symptoms of heat exhaustion include: heavy sweating, nausea, cramping, headache, fatigue, and more. To see a comprehensive list of symptoms, visit the Mayo Clinic website. In a first aid class, I learned you should move a person displaying these symptoms to the shade or an air conditioned building, loosen their clothing, fan them, and give them cool (but not cold) water to drink. Do not give them anything alcoholic or with caffeine - both alcohol and caffeine are diuretics and stimulate urine production, which causes further dehydration. If the person does not visibly improve, or his/her condition deteriorates, call 911 before heat exhaustion becomes potentially deadly heat stroke. Heat stroke can lead to seizures and even death.
Heat injury and illness can have lifelong consequences. Even after cooling down and rehydrating, many people report sensitivity to lights, particularly sunlight, and smells for weeks after suffering heat exhaustion. More severe heat stroke can leave you sensitive to heat and humidity for years to come. Due to a heat stroke seizure many years ago, one individual I know cannot do many of her favorite outdoor activities for any length of time because direct sun exposure for more than 10 minutes leaves her with crippling headaches for days to come.
Don't fall into the trap of only looking at the big temperature on the weather page with the temperature reading. Take a moment to look at the dew point and the heat index to plan your day - it could literally save your life.