Long-lived heatwaves in the mid-latitudes are typically the result of an atmospheric pattern known as a “blocking high.” In a blocking high, the jet stream bends in a north-pointing ridge (in the Northern Hemisphere, anyway), creating an area of high atmospheric pressure as long as it stays bent. High pressure means air tends to sink toward the surface rather than rise, making it hard for any clouds to break up the blue sky. The blocking high also distorts the average wind directions as it brings warmer air from the south up to the north.

That’s the pattern that produced Russia’s incredibly deadly 2010 summer heatwave. Instead of westerly winds, warm air coming from Kazakhstan moved in on Moscow. One reason Russia got so hot during this time is that it was already in a drought. Similar to how the human body cools itself by producing sweat that evaporates off your skin, soil moisture limits how quickly the land surface can heat up. With the soil already dry, Russia lacked this cooling buffer.

A group of researchers led by Dominik Schumacher at Ghent University have now extended this idea upwind, showing that the heatwave was linked to drought in Kazakhstan’s neck of the woods as well.

To break apart the different contributions to the event, the team tracked heat in a model of the observed June and July weather. (They also repeated the analysis—with similar results—for Western Europe’s 2003 heatwave.) They were able to compare this heatwave to the last few decades of weather patterns, which provided context.

The researchers found that a significant share of the heat imported from the southeast was due to the drought there, as the sun-baked dry land in Kazakhstan heated the air even more. For the region around Moscow, July 2010 saw double the average amount of imported, land-sourced heat on the wind (going back to 1980). Of the warmth carried up to Moscow by the change in wind direction, about 30 percent of it was due to the drought around Kazakhstan.

This also has a sort of amplifying effect, as the warm, dry air that reached Russia helped make the local soil even drier, further boosting air temperatures.

This can be seen from a timeline of the heatwave. The researchers found that the southern heat actually arrived in sudden “torrents” starting with the week of June 21. After this, temperatures slowly climbed for several weeks as the dry local land surface provided additional heat. During the week of July 26, the winds shifted a bit and a second burst of southern air arrived, causing temperatures to jump to their ultimate peak.

All this means that trends in subtropical drought may also have a separate amplifying effect on the worst mid-latitude heatwaves. There isn’t often a confluence of local drought, imported southern air, and drought in the source region of that air—1981 was the only other summer since 1980 where it occurred for western Russia—but it’s a dangerous combination.

The researchers point out that land-use adaptations to reduce the loss of soil moisture would be valuable not just locally but would also help limit problems downwind.

Nature Geoscience, 2019. DOI: 10.1038/s41561-019-0431-6 (About DOIs).