Summer can be a season, a type of weather or a state of mind.
Summer weather has returned to the Roanoke and New River valleys in mid-September after some cooler mornings and less humid days to start
Summer as a state of mind is either comforting or exasperating as this relapse occurs, depending on your personal inclination for seasons.
Summer the season continues on the astronomical calendar for another week, ending next Wednesday with the autumnal equinox, but it ended on the meteorological calendar two weeks ago, with the end of August.
Which means we can look back at this summer’s weather statistics while summer continues — and try to unravel a mystery.
Fueled primarily by intense heat in the western third of the U.S., and persistent warmth relative to normal through the northern tier of the nation into the Northeast, summer 2021 is the nation’s hottest summer on record, barely beating out the Dust Bowl year of 1936, in preliminary data.
Locally, it was not the hottest summer on record — but it tied for third, with last summer.
Roanoke averaged 77.6 degrees from June 1 to Aug. 31, matching the summer average temperature of 2020, only trailing 2010 (78.2 degrees) and 2011 (78 degrees).
This may seem a bit perplexing at first, considering that this summer did not have any particularly extreme heat locally, Roanoke rising to no higher than 96 degrees, nor did it have a lengthy streak of heat remotely similar to last summer’s record 29 straight days of highs in the 90s. Our longest 90s streak was nine days, in late August.
By contrast, 2012, the summer of the derecho with four days over 100 and a run of 97+ for 10 of 11 days from June 28-July 8, was 1.3 degrees cooler as a whole and 14 spots farther down the list of hot summers than this one.
Part of the reason this summer ranked so highly for warmth is the theme we have discussed here repeatedly, the overwhelming trend toward warmer summer nights.
Based solely on the average of daily low temperatures, this was Roanoke’s sixth warmest summer on record, with an average low temperature of 67.1 degrees.
All of the 11 warmest summers by average low temperature, 14 of the 15 warmest and 16 of the 20 warmest have occurred since 2000, topped by 68.1 degrees in 2010. That is obviously anomalous, considering the local weather database goes back 109 years to 1912. Twenty percent of the period of record accounts for 75% of the 20 warmest summers by average low temperature.
Again, plowing well-tilled soil, both higher dew points related to advection from oceans warmed by climate change and urban heat island warming related to increased concrete and asphalt around the city’s airport weather station likely play a role in consistently warmer nights.
But a sixth-warmest summer based on low temperatures doesn’t get us to a third-hottest summer based on average temperature.
Based on average daily high temperature, 2021 ranked 15th hottest at Roanoke, at 88.1 degrees. That is a little more than half a degree warmer than last summer, even with its record 90+ streak, and the third warmest since 2000, trailing 2011 (88.7) and 2010 (88.4).
Unlike with nighttime temperatures, there has really been no local trend toward hotter summer days in recent years. 2011 ranks ninth, with seven of the eight hottest summers based on average daily high temperatures occurring in the 1920s through 1950s.
A reminder that we’re talking Roanoke here, not nationally or globally. Our summer nights are obviously and overwhelmingly getting warmer. Our summer days are not. The data is clear, locally. It differs elsewhere.
This summer had 42 days at or above 90, considerably more than the 109-year average of 28, but fewer than the last two summers (45 each), which are tied for 11th since 1912 and the most since 1987. Again, summers in the 1920s through 1950s dominate the top 10 list for most 90-degree days, topped by 54 in 1930. (These are figures for meteorological summer only, June 1 to Aug. 31, not annually — there are usually some additional 90-degree days in spring and fall.)
For 2021, a sixth-warmest summer for low temperatures, a 15th-warmest summer for high temperatures, with fewer 90-degree days than the last two summers still don’t seem to add up for this summer to end up tied for third warmest locally in 109 years.
So where is the hidden heat this summer to boost it so high on the list?
Or, perhaps, it just lost its cool somewhere.
Checking a statistic not often looked at in reviewing summer reveals something remarkable. There were only four days between June 1 and Aug. 31 with a high temperature below 80 degrees at Roanoke. Only 1987 with two and 1925 with three have had fewer.
The secret to this being such a highly rated warm summer locally wasn’t in extreme high temperatures, long runs of 90s, or even in especially warm overnight lows compared to recent years.
It was simply because the atmospheric pattern did not allow for many Canadian cold fronts to sweep out the heat, nor did those cool, damp wedge patterns, pushing in from the northeast and trapped against the Appalachians, develop as they have in some other summers. The unusually hot temperatures across the northern U.S. are a big clue as to why — high pressure up there blocked a lot of cooler pushes toward us.
After some days in the 90s, our high temperatures would retreat only back to the lower-mid 80s, mostly — not enough 70s highs to pull the average temperature down.
There were few cool breaks this summer. And, once again, we find ourselves awaiting the next such cool break as summer pushes toward fall on the calendar. The weather pattern of dominant high pressure over much of the central and eastern U.S. suggests we may be waiting a while.