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If you want to figure out the heat index without the help of a meteorologist, there is some good news: You can just plug in the numbers in a mathematical formula.
HI = -42.379 + 2.04901523*T + 10.14333127*RH – .22475541*T*RH – .00683783*T*T – .05481717*RH*RH + .00122874*T*T*RH + .00085282*T*RH*RH – .00000199*T*T*RH*RH
In this equation to calculate heat index (HI), T is the temperature in degrees Fahrenheit and RH is the relative humidity percentage (which is itself derived from temperature and dew point).
Simple enough, huh? The formula actually gets more complicated under certain conditions, which you can read about by linking to a National Weather Service webpage here.
It would probably be simpler just to look at the chart below.
Or, even better, just plug numbers for temperature and humidity or dew point into this online heat index calculator.
Sticky heat coming back
Heat index has become a relevant topic for Southwest and Southside Virginia in July.
First, after a relatively cool June, temperatures rose to slightly above normal for several days in early July, while dew points, reflecting moisture levels and factored with temperature into figuring relative humidity, climbed into the steamy 60s and 70s, raising heat index values a few degrees above the 80s to lower 90s (and a few mid 90s) temperatures.
Then, slightly cooler, much less humid air moved in behind a cold front early this week. With dew point values in the 50s and relative humidity values in the 30-40% range, the heat index was at times 1-3 degrees below air temperatures that rose well into the 80s to near 90.
Stickier air will be returning to our region on southerly wind vectors off the warm Gulf of Mexico and Atlantic Ocean the rest of this week as temperatures rise back into the lower to mid 90s across much of our region’s lower elevations, and well into the 80s in most of the higher ones. That will again bring about some heat index values that could approach 100 degrees in sub-1,500-foot elevation areas like the urban floor of the Roanoke Valley, Southside and most locations east of the Blue Ridge.
The core of “heat dome” high pressure will park over the Desert Southwest, with many 110-120 temperatures, slightly hotter than normal highs there, even pushing toward 130 in Death Valley, California. Meanwhile, the core of cooler air will set up in an upper-level trough over the north-central U.S. We got a taste of it the past few days with some cooler mornings and not-humid afternoons, but the cool air will not overspread our region for longer, as appeared possible looking ahead from last week.
Squarely under neither the hottest nor coolest air, Southwest and Southside Virginia will be at the edge of lesser high pressure over the Southeast U.S. that will bring some hot, sticky days with scattered afternoon showers and thunderstorms bubbling in the afternoon heat. Cold fronts and disturbances moving southeast from the north-central states will increase those storm chances on occasion, and perhaps bring brief breaks from the hottest weather. (For now, it appears the trough in the north-central U.S. will circulate Canadian wildfire smoke in a way that we do not see much of it for several days.) This general pattern may hold most of the rest of July.
So what is the heat index?
The idea behind the heat index is to provide a number for an approximate temperature as it affects the human body – or what it “feels like” – under various combinations of temperature and atmospheric moisture content.
When temperatures rise, the human body sweats as a cooling mechanism. But this cooling mechanism only works to maximum benefit if sweat droplets can evaporate. When the air contains more moisture, evaporation becomes more difficult, and the body’s cooling mechanism is slowed the hotter and more humid it gets. (With less moisture in the air, like the last few days, this perspiration-evaporation process works more efficiently, and the air can “feel” a little cooler than the actual temperature, hence a lower heat index reading than the air temperature.)
Heat advisories and excessive heat warnings are based on the heat index. The exact line where advisories or warnings are issued by the National Weather Service varies region to region, generally a higher bar in southern and arid western parts of the nation, lower in northern and cooler higher elevation areas.
There is a dividing line in heat advisory criteria bisecting the Cardinal News territory encompassing Southwest and Southside Virginia.
Earlier this year, the National Weather Service office in Blacksburg changed the criteria for the western half of its forecast area, most locations west of the Blue Ridge. A forecasted heat index of 100 now triggers a heat advisory in these counties, while a predicted heat index of 105 or greater results in an excessive heat warning.
To the east, including Roanoke to Lexington and then everything east of the Blue Ridge, the criteria remain 105 heat index for a heat advisory and 110 for an excessive heat warning. Most summers a few heat advisories get issued, but excessive heat warnings are rare – the conditions that produce temperatures approaching 100 degrees in our region are often not quite humid enough for a 110 heat index.
Phil Hysell, warning coordination meteorologist at the National Weather Service office in Blacksburg, said that Virginia Department of Health data indicated there were many hospitalizations and urgent care visits for heat-related illnesses in the New River Valley with heat index values below those previously triggering an advisory. So the bar was lowered. (Luke Weir covered the health data reasoning behind the changes pretty thoroughly in The Roanoke Times earlier this year.)
Advisories and warnings both east and west of the Blacksburg office’s forecast area remain at 105 heat index for a heat advisory and 110 heat index for an excessive heat warning. The Morristown, Tennessee, office covering most Virginia counties west of Interstate 77 adds 103 temperature for heat advisory and 105 temperature for excessive heat warning, regardless of humidity and heat index values. The Wakefield. Virginia, office to the east, covering some Southside counties, issues more heat advisories and warnings than Blacksburg, with generally hotter temperatures in lower elevations and higher humidity values closer to the ocean.
The regional differences come down partly to what residents are used to. The National Weather Service office in the Dallas-Fort Worth area, for example, has the same numerical criteria for heat advisories and warnings as most of our region – 105 and 110 heat index – but requires that the criteria are met for two days in a row. The Blacksburg office only requires its criteria to be met for two consecutive hours.
Heat does not start becoming dangerous when heat indices reach triple digits. Even a heat index as low as 90, with extended exertion and/or other underlying health issues, can cause heat stress on the body that can lead to illness, exhaustion or even in rare cases, heat stroke.
You probably know the advice for hot days by now: Drink plenty of liquids, take lots of breaks, get in shade when possible, and delay exertion until cooler late-evening hours if possible (or do them early).
Sometimes it is asked. “Why don’t we just call the heat index the actual temperature if that’s what it feels like?” The reason is that temperature is based on the kinetic energy of air molecules in a physical process that is not affected by human perception. Temperature isn’t about you or me, it’s about molecules and physics. Heat index cares more about how we “feel.”
Hottest days globally; not so locally
Last week, you may have seen news that the University of Maine “Climate Reanalyzer” calculated that several days last week were the hottest for global average temperature in recorded history.
While the National Oceanic and Atmospheric Administration has not officially recognized those calculations on the basis that they used computer projected temperatures in addition to actually recorded ones, there have been several indicators of extreme heat over large swaths around the world, such as the warmest June on record according to preliminary data, drastically low Antarctic sea ice extent, extremely warm sea-surface temperatures in the North Atlantic, and heat waves in various locations.
Despite all these headlines and legitimately concerning trends in global climate, Virginia thus far has been spared any really extraordinary heat this summer. In fact, the month of June, despite apparently being the warmest globally, averaged 2 to 4 degrees below normal in much of the Southwest and Southside Virginia region that Cardinal News covers.
While the heat underneath high-pressure ridges is often broader and more extreme than in the past, boosting global averages, there are still troughs in between the ridges where cooler air settles, and Virginia ended up in such a trough much of May and June.
July may look a little different, having started out 1-3 degrees above normal at many locations in our region, and an apparent lean toward above-normal temperatures in days ahead. But there is no firm indication yet of any kind of weather pattern that would bring about our region sharing in some of the extremely high temperatures many parts of the planet are experiencing. It’s still a long time till fall, though.
Sunday storm reports
In addition to being spared extreme heat, our region has also been spared the historic flooding some parts of the Northeast have been having. But the cold front and low-pressure trough that helped create that heavy rain did affect our region with a quick push of showers and storms on Sunday.
The most significant effects on our region were several high wind reports with thunderstorms in Southside across Henry, Pittsylvania, Halifax and Mecklenburg counties. Downed trees partially blocked U.S. 58 for a while in Mecklenburg County. There were many more wind damage reports in northern and eastern North Carolina from the same cluster of storms.
Looking ahead over the next 7-10 days or so, there is not likely to be anything that would focus storms or rain in an especially strong or widespread manner across our region, but with so much heat and increasing moisture present, even relatively weak disturbances could help trigger locally heavy rain or severe thunderstorms, while others not far away stay sunny and sticky.
The peak of summer is now ours to enjoy or endure, depending on your perspective and exactly how the weather pattern plays out, as we hold our collective breath about whether the most extreme weather of various types continues to slide around us.
Journalist Kevin Myatt has been writing about weather for 19 years. His weekly column is sponsored by Oakey’s, a family-run, locally-owned funeral home with locations throughout the Roanoke Valley.
