You could be forgiven for thinking that Richmond’s weather this week has transported us to some other time or place.
June-uary? Seattle, Virginia?
Though the sun blazes high and long across the sky during these pre-solstice days, the calendar date was no match for the thick clouds, rain and cool maritime air swooping around a stalled low pressure system.
That weather pattern trapped Richmond’s temperatures in the 60s throughout Monday, Tuesday and Wednesday, instead of bringing the upper 80s that would feel right for mid-June.
While it’s not a very extreme place to land on the thermometer, there’s a case to be made that these were some of our most abnormal afternoon highs in a very long time.
The phrase “extreme cold” doesn’t really have a ring to it after the last freeze.
But there are several fun ways to put these odd temperatures into context.
To begin with, the recent daytime highs at Richmond International Airport — 68 degrees on Monday, 67 on Tuesday, upper 60s as of Wednesday afternoon — were much closer to our normal morning low of 65 for this week of the year.
But thanks to the moderating nature of cloudy weather, these days had just slightly below-normal nighttime lows in the lower 60s. But we’ll set those aside and focus on the cool highs.
Another comparison could slide us along on the calendar. Those highs are more typical of Richmond’s readings in early April or late October.
But summer weather isn’t all 90s and sunshine, despite the way we imagine it.
So we compare the past few days against the record cool highs for this same time of year.
A much cooler bout of weather during this week in 1965 set the bar way down with record-chilly highs only in the upper 50s and lower 60s.
So in our 123-year weather record, Monday went down as the second-coolest high for a June 15, while Tuesday was the third-coolest for June 16.
Or, we could play around with geography.
While there are plenty of places high in the Appalachians and Northeast where a 60-something June day is unremarkable, there aren’t any places nearby where it’s perfectly normal.
Along our latitude, upper 60s on a mid-June afternoon are normal in a place like Silverton, Colo. Despite enjoying similar amounts of daylight, they owe a cooler climate to sitting in a valley 9,300 feet above sea level. (The similarities end there: On a typical June night, they’d still be near freezing).
But there’s one common way of running the numbers where there’s much more than meets the eye: departure from normal.
These cool June highs were about 18 to 20 degrees below the daily normal high of 86 degrees.
But looking back, we had a similar-sized cool departure on May 20 when it was 59, and an even bigger departure on March 20 when the high of 88 was nearly 27 degrees above normal.
So why would these 60s in June be more abnormal than that record-setting heat in March or the chilly afternoon in May?
The raw departure doesn’t take into account that our weather variability changes throughout the year.
March is a very volatile month from a temperature standpoint. Sizeable swings and anomalies come with the territory during winter and early spring.
June? Not so much. Summer brings subtler changes thanks to higher humidity and a less-active jet stream.
So an 18-degree departure from normal is much more impressive in June than it is in January.
For the math-inclined, it comes down to the standard deviation.
We enjoy plenty of nice weather days, but few are ever perfectly normal. Instead, we see over time that daily temperatures are distributed around that average. Most of the sample will fall somewhere near, but slightly off from the long-term mean, while a few are outliers in either direction.
That is not a very useful way to predict what tomorrow’s weather will do, but it explains a lot about what we expect over time.
Historically, summer days show a tighter clustering than the other seasons. For example, the standard deviation for high temperatures in Richmond peaks at about 12 degrees in January, dips to 7 degrees by mid-June, and down to about 6 through July and August according to the National Oceanic and Atmospheric Administration.
Another way to interpret that is: Throughout the years, we’d estimate that 68% of the time, the high for June 15 will fall somewhere between 79 and 93. That’s the normal of 86 plus or minus that 7-degree standard deviation.
And 95% of the time, June 15 should wind up with a high between 73 and 100 (plus or minus two in either direction). So that shows the rarity of afternoons in the 100s or 60s at this time of year.
Tuesday’s high of 67 was nearly three standard deviations below normal, which is the farthest any high has strayed so far in 2020. Richmond’s last high to swing so far in that direction was in September 2017.
(For lows — which we haven’t mentioned yet — we only have to go back to that late freeze on May 10 to see an equally impressive outlier.)
When we deal with many consecutive days, like a whole summer or an entire year, there’s a much greater chance of encountering an abnormal reading.
Climate change means we’re shifting the averages slightly higher. So the fact that the planet is warming doesn’t rule out such cool summer days, but it may be putting them slightly farther out of reach than before while making heat extremes more frequent.
Sure enough, Richmond’s daily temperature scorecard for 2020 shows six daily records set or tied for warmth, and three for cold.
So the Tidewater-in-March feel of Tuesday was unusual all on its own, but not so shocking in the grand scheme of things.
A well-placed weather pattern just took us to the tail end of our weather variability for a few days.
Some might say it’s downright refreshing given the sultry summer default.
Whether 60s or 90s are your definition of pleasant, all will be right with the weather calendar again when summer’s heat returns early next week.