MetSwift’s Head Meteorologist examines some distinct trends toward hot days occurring sooner and more frequently in the USA and Europe and explains a worrying link to increasing drought risk during the hot season.
After the chill of winter, a rise in temperatures during the spring is welcomed by many. It becomes easier, more pleasant, to venture out during the lengthening days, be it for leisure or practicalities.
This climb is rarely steady, though. In mid-March, I studied the frequency of springtime snowfall events, which can continue to feature well into the season. Recently, some parts of Europe were reminded of this, as falls of snow and graupel (a soft form of hail) accompanied a biting cold Arctic wind during 6th-7th April.
Some spots recorded their lowest April temperatures on record, including Beauvais, France, which dropped to -6.9°C on 6th.
Less than a week before then, it was a very different story. That same place saw it’s highest March temperature on record, a whopping 24.8°C!
This is the other side to spring – early ‘heatwaves’ bringing temperatures more fitting of summer.
For this blog piece, I’m going to focus on temperatures reaching set thresholds: 28°C for the USA and 26°C for Europe. These are only occasionally observed in March but become much more relevant in April and May. I could have used lower thresholds but felt that too few readers would consider those benchmarks to be ‘hot’. Even 26°C is pushing it, to be honest!
The Overall Trend: More Hot Days in All Months
Below, you’ll find graphs showing how the typical number of days per month reaching those thresholds has changed each decade since the 1970s. I’ve included one for 28°C in Europe to justify my choosing of a lower threshold there. With such low frequency, regional trends are suppressed and difficult to interpret.
Please click on a graph to view it in full resolution.
In this data, we see a distinct upward trend for all months. Comparing 2010-2020 with the 1970s, the USA is tended to see about one additional day in each of March and April, with two more in May. In Europe, there’s little difference for March, but April saw about half an additional 26+°C day (i.e. additional day every two years), while May saw close to 1.5 more.
A few additional points are worth noting:
- The USA saw nearly as many 28+°C April days in the 1980s as in 2010-2020, a demonstration of shorter-term variability on top of a long-term trend.
- In Europe, it’s clear that climate warming has yet to reach a point that March routinely starts seeing more than occasional 28+°C days, but the 2000s hint that this may become the case in the near future.
- April saw a big jump upward in 2010-2020, while May saw one in the 2000s. This suggests an earlier trend in the occurrence of the first 28°C for places that rarely observe such conditions pre-summer.
Spatial Breakdown: USA
Our attention now turns to how the trends are laid out across the land. How much contribution is there from the south versus the north, for example? Presented below is a series of maps showing the average decade-to-decade changes for each month of the season.
What stands out most here is a general spread northward and increase in size of the positive trends as you move through the season. This tells us that small trends (within 0.5 of zero) are mainly due to temperatures rarely, if ever, reaching the threshold.
The other striking feature is the scarcity of negative trends. We can only see that for a couple of locations in the southeast for April and a scattering of 6 places in May. It’s intriguing that the May ones are so spread out and isolated from one another. Possible explanations include localised wind pattern shifts, cloud cover increase, and changes in land use.
Projecting forward, it appears likely that within the next few decades, many locations in the northern USA will start to observe 28+°C temperatures by May.
Spatial Breakdown: Europe
On the other side of the Atlantic, seawater is much more abundant. Typically, you don’t have to travel nearly as far to reach a large body of water.
This has a moderating effect on the springtime heat build because it takes more energy to heat a unit of water versus the same unit of land. The seas warm more slowly and cooling breezes counteract the strengthening solar rays.
Consequentially, it’s not very often 26+°C is observed before May outside of Iberia and the southeast corner of Europe. In the maps below, it’s only where at least a few days tend to be seen each month that trends are evident. The same situation as in the USA analysis earlier.
Another similarity is the spread northward and increase in intensity of trends going through the season. It’s more subtle here, though, which again shows the constraints imposed by the proximity to many large water bodies.
Even so, the trends in the south are a big deal. Here, long-lasting areas of high pressure tend to build northward from Africa by the summer, bringing blue skies and hot sunshine for many days, sometimes weeks, at a time. Like with a large part of the USA, there’s an endurance element to this for those who reside there.
If such conditions set in sooner, odds are, it will be a longer hot season with more days of testing heat to manage. There’s also increased drying of the ground, which may well be teaming up with reducing rainfall (a topic for another day…) to seriously raise the likelihood of drought conditions in the summer.
Further north, we would need to set lower thresholds to assess the trends well. Those 26+°C days are now occurring far north as Estonia by May, but only 3 or 4 times each year. By the 2050s, we could well be seeing twice that many there, with several up in Scandinavia. This has some connection to reducing spring snowfall: Heat energy that once melted snow, now warms the ground.
The Main Negative: More Drying of the Land
Of the impacts I’ve mentioned, I consider the increased rate of moisture loss from the land to be most serious.
For most people, temperatures of 28°C (let alone 26°C) don’t cause much immediate trouble, but unless there’s plentiful rain or a large surplus of water reserves following a wet run of weather, an extension to the hot season can become a big problem during the following summer.
This is one reason why a reliable water supply is expected to become an even more valuable commodity in the decades to come.
James Peacock MSc
Head Meteorologist at MetSwift
Cover Photo by Unknown Author is licensed under CC BY-NC-ND