Forty million people were under heat alerts Tuesday across the Central and Southern United States as a hot and sultry air mass led to dangerously hot conditions.

Houston hit 100 degrees Sunday and Monday, Dallas has reached 101 on three days since Friday, and Oklahoma City saw a high of 103 degrees Monday. Some locations set daily records for high temperatures.

It’s an impressive heat wave, but it’s nothing compared with what Europe endured two weeks ago when all-time national records fell in at least half a dozen countries. What’s more, few all-time daytime high temperature records are being broken.

Then why does it feel so awful?

It’s all about the heat index

Heat waves in Europe are seldom accompanied by the humidity we get in the United States. Our proximity to the Gulf of Mexico and the Atlantic Ocean opens the doors to sauna-like moisture streaming northward unimpeded, bringing the sultry mugginess over wide swaths of the Deep South and Southern Plains this time of year, along with the East Coast on occasion.

The oceans are absorbing more than 90 percent of the extra heat from global warming, with dramatic increases observed in ocean heat content in recent decades. This in turn can make more moisture available for weather systems.

Right now, the Gulf is running about a degree Fahrenheit above average this season, which means the adjacent air can hold even more moisture — 3.2 percent more, in fact.

Heat alone is bad, but accompanied by tropical moisture is brutal, since it increases the heat index.

The heat index is essentially a “feels like” temperature that takes temperature and humidity into account. Heat places more strain on your body if it is accompanied by high humidity, since it inhibits the body’s ability to perspire and cool down. Beads of sweat evaporate from our skin and extract thermal energy, taking heat out of our body and making us feel cooler.

As the air warms, it can hold more water. However, when the air is already near saturation, it can’t hold much extra water. So even though we’re still sweating, the air isn’t allowing the sweat to evaporate as quickly. That’s when our body temperatures start to warm to dangerous levels.

The ongoing heat wave is concerning because the overnight lows have remained unusually high, on top of dew points running in the 70s.

Galveston, Tex., has seen a heat index of 95 degrees or greater every hour since noon last Wednesday. The heat index was over 100 degrees for 42 hours straight ending at 2 a.m. Saturday. Last Thursday the heat index there hit 117. And Galveston hasn’t seen a dew point below 75 degrees at any point during August.

At night, there’s no respite. That’s what makes these conditions downright dangerous, since the human body needs some relief to be able to stave off heat-related illnesses.

The hot and humid conditions occurring now in parts of the United States place more strain on the human body than the heat wave in Paris did. That’s because Paris had a dew point of 51.8 degrees (giving a relative humidity of 16 percent) when they hit the 109.2-degree air temperature, an all-time record for the city.

So, despite much lower temperatures here in the States, the heat indexes are markedly higher because of the humidity. (It’s important to remember that few, if any, residences in Paris have air conditioning, while it’s ubiquitous in the U.S. Deep South.) With the current heat wave here, we have between 2.3 and 2.8 times more water in the air, compared with Paris’s peak heat day.

As the climate continues to warm, the prospect of increasingly humid heat waves will increase. That will make heat indexes worse. Moreover, an increase in soil moisture, often spurred by agricultural use, can add even more moisture to the air.

Let’s take New Orleans. Since 1970, its dew points have been increasing an average of 0.053 degrees per year. That sounds insignificant, but that corresponds to a 2.6-degree increase today, compared with 1970. And that means there’s 9.7 percent more moisture in the air now, on average, than there was in 1970.

That has enormous implications for increasing heat indexes in the Big Easy.

In New Orleans, the overnight summer lows are warming 280 percent faster than daytime highs. That makes warm, humid nights unbearable. That’s the killer with these heat waves. And it’s only getting worse.

A study published in Environmental Research Communications last month by researchers at the Union of Concerned Scientists hints at an alarmingly hot future across the U.S.

“By the mid-21st century (2036—2065) … the annual numbers of days with heat indices exceeding 105 degrees are projected to triple,” said the study, which focused specifically on heat indexes rather than actual air temperatures.

The group also estimated that, during the same time frame, a quarter of the United States would venture into “no analog” territory every year. In other words, 25 percent of the United States will be venturing into unprecedented heat-index territory regularly.

Furthermore, the duration of extreme heat index events could double as global average surface temperatures and atmospheric water vapor both spike, according to the study. That means episodes like what’s been happening in Galveston will become increasingly common.