Hot days can be so familiar to us that they can hide their danger. And they aren’t as visual or clear-cut as other kinds of extreme weather.
The scale of a snowstorm stands out with rulers and yardsticks. Rainfall and rivers have gauges, or sometimes a depth of water in a place it doesn’t belong.
Hurricanes and tornadoes have their tiered intensity categories, not to mention textbook signs on radar.
We meteorologists try to give context to hot weather with comparisons with historic records, or sometimes-abstract averages.
Then throw in the varying effect of humidity and comfort scales, and a heat wave can be an awfully nebulous thing to picture.
But we know hot weather greatly affects our quality of life when it’s excessive or comes out of season, and takes a heavier toll on the most vulnerable people.
And we know climate change isn’t making our hot Virginia summers any easier to deal with.
Why look at 90s?
A 90-degree day doesn’t sound extreme. It may be perfect for pools and cookouts.
But 90 happens to be a useful benchmark in Richmond. Our normal daily high peaks at that level in July. So no matter what day of the year we hit, say, 91, that’s statistically hotter than normal.
Counterintuitively, it’s also normal to gradually rack up dozens of 90s between April and October.
So maybe it makes more sense to think of 90-degree weather like a hill to climb each summer, based on an accumulating tally of hot days.
No year is ever perfectly average. But if it were, those 90s would creep in slowly by May and early June, add up neatly on every other day through July and August, then taper off in autumn.
The new 30-year climatology (based on the 1991 to 2020 era) puts Richmond’s normal count at 44.9 days at or above 90, according to the National Oceanic and Atmospheric Administration.
The previous 1981 to 2010 benchmark was 42.1 days per year. And between 1971 and 2000, normal was 40.7 days. Adding the 2000s and 2010s to our climatology while taking out the 1970s and 1980s yields four extra days of heat.
It may not sound like much.
But that shift is proof that the “hill” of hot weather is getting higher over time, and the climb is perhaps taking longer to finish over the course of the year.
When we graph out that metaphor using 2020 as an example, it got off to a slow start with no 90s at all until this stretch of early June.
Then, last July’s long streak of hot weather was a particularly steep section to climb. By September, we topped off our 90-plus count at an above-normal 49 days.
The year before, 2019, tied for second-most on record with a count of 70 days.
We don’t yet know where 2021 will end up, but we’re already out ahead of the typical pace thanks to May’s dry heat.
Through June 4, the year-to-date count was seven days. And we’re likely to stay ahead of that normal pace over the next week with several hot afternoons in store.
The year with the most 90s up to this point was parched 1941, at 16 days.
There is a connection between drier weather and hotter afternoons, and vice versa. Some of our recent summers were rather steamy and saturated, so 90s were harder to hit outright at times even though the heat index felt like it.
So if this summer continues to tilt toward drought across the region, don’t be surprised if 90s become more routine than 80s. And maybe more 100s will enter the picture if patterns allow.
More 90-degree facts for Richmond
Most 90-plus days: 78 (2010)
Fewest 90-plus days: 14 (tie between 1917, 1927 and 1971)
Last year with fewer than 40 90-plus days: 2009
Last year with fewer than 20 90-plus days: 2000
Earliest 90-day: March 17, 1945
Latest start to 90 season: July 2, 1972
Earliest end to 90 season: Aug. 13, 1949
Latest 90-day: Oct. 16, 1897.