As National Weather Service meteorologist Cameron Batiste joked in his Memorial Day weekend forecast on Thursday, you can’t spell Houston without H-O-T. So has city hit 100 degrees already? Technically, no — but that’s not what your body thinks.

While the thermometer might have read only 90 degrees on Wednesday, the humidity made it feel more like 100 degrees. Is it better to know the actual temperature or how it feels to the body? Let’s start with some basics:

What’s the heat index?

Batiste warned in his holiday weekend forecast that high temperatures in the Houston metro area on Saturday will be in the low 90s with heat index values in the 101 to 106 range.

“We turn up the heat a bit higher for Sunday with most inland locations topping out in the mid-90s for air temperature with heat index values in the 102 to 110 range,” he said. “Keep in mind that our criteria for a heat advisory is heat index values at or above 108, so Sunday will likely feature our first one of the year.”

The weather service will likely issue another heat advisory, which alerts the public about dangerous levels of heat, on Monday, when Houston could have its hottest temperatures of the year to date.

ALERTS EXPLAINED: Houston prepares for intense heat this week. What’s a heat advisory? What do the heat alerts mean?

Why issue heat advisories when air temperatures are only in the 90s? Blame humidity and its worsening effect on heat for the human body, which is at the heart of what we know as the heat index.

Humidity, or the level of water vapor in the air, can make our perception of heat seem much hotter than the actual temperature. That’s because the body cools itself through the evaporation of sweat from our skin. But such evaporation isn’t as effective when there’s too much moisture already in the air. Think about how hard it is to get dry after a shower if you’re in a steamy bathroom. 

“If the air is unsaturated, there is more room for water to evaporate into the surrounding air,” the National Weather Service explains on its heat index website. “If the air is saturated, however, little to no additional liquid can evaporate into the surrounding air, even though your body’s heat is warming, the sweat — and so the heat — remains in your body.”

Because moisture levels in the air can intensify how heat feels to the body, meteorologists also focus on dew points, or the temperatures at which air is saturated with water vapor. Typically, Houston can expect oppressive, muggy and sticky air if the dew point is at least 65 degrees. This week, dew points range from 72 to 77 degrees.

The heat index takes all of that into account, which is why the actual air temperature might be 94 degrees, but a high humidity level (say, a dew point of 73 degrees) can make it feel like it’s 103 instead.

Why do we use the heat index?

The primary usage of heat index readings is for public health and safety, said Keith White, a senior meteorologist at the weather service’s office in New Braunfels, which oversees the Austin and San Antonio metro areas.

“There is an index called the wet-bulb globe temperature which has been used by the military and sports groups for decades that do strenuous outdoor activity that takes these (heat and humidity) factors into account,” White said. “It’s just not as readily understood by the general public, in part because WBGT values of 90 are extremely dangerous but 90 sounds comparatively cool when our temperatures are in the upper 90s and low 100s every day.”

TEXAS HEAT TRACKER: Use our interactive maps and charts to see current conditions and historical data for where you live

The weather service began producing wet-bulb globe temperature forecasts in recent years, “but the temperature and the heat index are still the primary drivers of our forecast messaging,” he said.

Because of its focus on health and safety, it’s often better to pay attention to the heat index values rather than actual temperatures. But the heat index does not take into account other weather factors that can affect human health, such as sunshine and wind.

“Exposure to full sunshine can increase heat index values by up to 15 degrees Fahrenheit,” the weather service says on its heat index website. “Also, strong winds, particularly with very hot, dry air, can be extremely hazardous as the wind adds heat to the body.”

What happens if the heat index is really high?

Extreme heat poses health threats to some of the most medically vulnerable in our community, including young children, adults older than 65, those who are pregnant and those with chronic illnesses, according to the weather service.

“During extremely hot and humid weather, your body’s ability to cool itself is challenged,” the weather service warns on its heat safety website. “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.”

HEAT SAFETY: Houston’s extreme heat is early. Here’s what it means for summer 2024 — and why it’s so dangerous

The national Centers for Disease Control and Prevention offers tips on how to spot someone suffering from heat-related illness and how to respond.

A person experiencing heatstroke may have these symptoms:

• A body temperature exceeding 103 degrees
• Hot, red, dry or damp skin
• A headache, dizziness or nausea.

The CDC recommends that you call 911 for the person right away and move the person to a cooler place.

Someone experiencing heat exhaustion may show these signs:

• Cold, pale and clammy skin
• Nausea or vomiting
• Muscle cramps
• Dizziness, headache or passing out.

For this, the CDC suggests moving the person to a cool place, loosening their clothes, putting a cool, wet cloth on their body and letting them sip water. But the CDC also urges immediate medical help if the person’s symptoms worsen or last longer than an hour.