ARLINGTON, VA - MAY 27: Navy veteran Jim Summers, 56 of St. Augustine, FL cools off in a spray shower while waiting to ride in the Memorial Day Rolling Thunder event at the Pentagon parking lot in Arlington, VA on May 27, 2012. (Linda Davidson/THE WASHINGTON POST)

* D.C. government issues “hyperthermia alert” due to heat index of 95, activates heat emergency plan *

When stifling heat and oppressive humidity smack us down mid-summer, it certainly poses a hazard. But this suffocating combination sucks the life out of us the most when it strikes early.

Take Memorial Day, for example. Temperatures reached 90 for the first time this year in Washington, D.C. and the heat index - what it feels like with the humidity - soared to the mid-90s. While that’s a pretty typical day in July, WJLA reports more than a dozen people around Washington, D.C. were hospitalized by these conditions.

An instructive infographic in the Post’s Health and Science section explains why our bodies are so taxed by the heat at this time of the year: “[their] best methods of coping with heat haven’t been tested in three seasons.”

The good news is that we will progressively deal with these hot conditions more effectively as our bodies adjust, or acclimatize. “...if you’re slogging though your work or workout now, you are already starting the process of acclimatization, which will make you better able to withstand heat all summer,” the infographic says.

Scientific research has shown - in deadly heat waves - more people tend to die in early season heat waves compared to those later in the summer. The elevated early season fatality risk is not only due to lack of acclimatization, but also because there’s a larger pool of vulnerable members of the population. It’s a morbid subject, but heat tends to first prey on the frail and weak, leaving behind fewer susceptible people for future heat events.

Fortunately, I have not come across reports of heat-related deaths from this particular early season heat wave. (But this does not mean there haven’t been any - many heat-related deaths are either unreported and/or attributed to pre-existing conditions that heat may have aggravated.)

Nevertheless, the heat has been intense. On Sunday, Chicago hit 97 degrees, a record for the date, its hottest reading so early in the year, and its second highest May temperature on record. Chicago also set a record high of 95 on Memorial Day. Jeff Masters described a number of other notable records:

Detroit (95°F), Flint (93°), Cleveland (92°F), and Toledo, Ohio (96°) tied or set records for their hottest temperature ever recorded in May. On Saturday, at least nine airports in the Midwest had their hottest May day on record, and 58 out of 456 U.S. airports set daily high temperature records. On Sunday, at least sixteen airports in the Midwest had their hottest May day on record, and 68 out of 456 U.S. airports set daily high temperature records.

Since Saturday, 1138 warm weather records have been set or tied compared to 267 cool weather records.

Today, the core of the heat is over the mid-Atlantic and Northeast - but at less extreme levels compared to the Midwest. And late today and tonight, a cold front will sweep the hot weather away.