- The heat index measures how hot it really feels outside.
- The heat index is calculated based on air temperature and relative humidity.
- The National Weather Service will issue alerts when the heat index is expected to exceed 105 degrees for at least two consecutive days.
It's hot and going to get hotter.
That's quite an understatement when it comes to Florida weather. After record-breaking heat last summer, 2024 is starting the same way.
Several cities broke records over the Memorial Day weekend, including Fort Pierce, which hit 98 degrees Monday, May 27. The temperature tied a nearly 125-year record high, which a meteorologist said the city has done only five times since record keeping starting in 1901.
And it's only May. We're coming off thehottest year on recordand there's every indication 2024 will break that record, according to the Farmers' Almanac.
So as we struggle to keep cool, here's how to interpret National Weather Service forecasts, including what the heat index means and how dangerous heat can be.
What is the definition of heat index?
The heat index measures how how it really feels outside, according to the National Weather Service. It's sort of the summer equivalent of the wind chill factor our more northerly neighbors watch in the winter.
If in full sun, heat index can be even higher
Heat index values were devised for shady, light wind conditions. Exposure to full sunshine can increase heat index values by up to 15 degrees, according to the National Weather Service.
How do forecasters figure out the heat index?
The heat index is calculated based on two factors:
- Air temperature
- Relative humidity
How to calculate heat index: Use chart or online calculator
To figure out the heat index, use the chart above to find the air temperature and the relative humidity. Where the two meet would be the heat index.
For example, if the air temperature was 90 and the relative humidity was 70, the relative humidity would be 105, putting it in the dangerous zone.
Areas in red indicate extreme danger of heat-related problems.
Another way to calculate the heat index would be to use an online calculator provided by the National Weather Service.
What is an unsafe heat index? What number is dangerous?
In general, dangerous conditions would occur as soon as the heat index hits 105 degrees.
Conditions are considered extremely dangerous if the heat index is 126 degrees or higher.
Again, remember the heat index is calculated if you are in the shade. So if the heat index is 105-110 for your area and you'll be in the sun, the heat index could actually be 120 to 125 degrees.
What is the heat risk for Wednesday, May 29, in Florida?
On Wednesday, May 29, NOAA calculated most of Florida is under a moderate level of heat, which could affect people sensitive to heat.
➤ Interactive map: See the heat risk for your area today
Some portions of the state, especially on the southeast and west coasts and portions of Central Florida, were under major levels, which could affect anyone without effective cooling or adequate hydration.
The NWS heat risk is an experimental product that uses high-resolution weather, climate, and Centers for Disease Control and Prevention heat health data to identify potentially dangerous heat.
When does the National Weather Service issue heat 'alerts'?
The National Weather Service will issue alerts when the heat index is expected to exceed 105 degrees for at least two consecutive days.
What types of heat warnings does NWS issue?
The National Weather Service issues four types of heat-related advisories:
- Excessive heat warning: 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 degrees or higher for at least two days and nighttime air temperatures will not drop below 75 degrees. These criteria vary across the country, especially for areas not used to extreme heat conditions.
- What you should do: If you don't take precautions immediately when conditions are extreme, you may become seriously ill or even die.
- Excessive heat watch: 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: 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 degrees or higher for at least two days, and nighttime air temperatures will not drop below 75 degrees. These criteria vary across the country, especially for areas that are not used to dangerous heat conditions.
- What you should do: Take precautions to avoid heat illness. If you don't take precautions, you may become seriously ill or even die.
- Excessive heat outlooks: The outlooks are issued when the potential exists for an excessive heat event in the next three to seven days. An outlook provides information to those who need considerable time to prepare for the event.
Criteria in Florida for excessive heat warning, heat advisory
In Florida, the criteria is a little higher than the heat index numbers listed above for an excessive heat warning or health advisory to be issued by the National Weather Service.
- Excessive heat warning: Except for Miami-Dade County, the National Weather Services offices around the state would issue an excessive heat warning if the heat index is expected to reach 113 degrees or higher. The National Weather Service Miami said Miami/Dade County wanted a lower threshold for a warning for its population, which was set at 110 degrees.
- Heat advisory: Except for Miami-Dade County, the National Weather Services offices around the state would issue a health advisory if the heat index is expected to reach 108 to 112 degrees. The National Weather Service Miami said Miami/Dade County wanted a lower threshold for an advisory for its population, which was set at 105 degrees.
Symptoms of heat illnesses: What's the difference between heat exhaustion and heat stroke?
When your body can't cool itself, your body temperature rises. If it goes too high, it can be life-threatening.
The Centers for Disease Control and Prevention provided a list of warning signs and symptoms of heat-related illnesses and what you should do.
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 dampskin, 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.
- Donot give fluids.
People at higher risk for heat-related illnesses
The CDC warned some people are at a higher risk of getting sick from the heat than others. Those include:
- People aged 65 years or older
- Infants and young children
- People with chronic medical conditions
- People without air conditioning
- Athletes or others who exercise in extreme heat
- Outdoor workers
- People who are pregnant
The Key Points at the top of this article were created with the assistance of Artificial Intelligence (AI) and reviewed by a journalist before publication. No other parts of the article were generated using AI. Learn more.
