Chronicling the hottest, coldest, windiest, and rainiest weather

Popular culture loves extremes. Whether there’s a new fastest time in the Olympic 100-meter dash, an update about the world’s oldest living person, or reports of extreme weather, we tend to pay attention. Like weather, Earth’s climate can also exhibit extremes globally, regionally, and locally, albeit on longer timescales, so climate change reports often focus on long-term averages in sea level rise, atmospheric warming, and the like.

Yet short-term extremes are sometimes visible symptoms of larger changes in climate. Numerous local high-temperature records in recent years, for example, collectively point to Earth’s ongoing warming: Globally, 2016 was the hottest year on record, with 2020 a close second. Similar patterns in record high precipitation totals—five U.S. states recorded their wettest year ever in 2019, which was the second wettest for the country overall, for example—and tropical storm intensities are also increasing.

Many (but not all) nations have agencies that collect and keep records of extreme weather. Until fairly recently, however, no global, hemispheric, or continental-scale efforts existed for officially investigating, documenting, and verifying extremes in temperature, precipitation, wind, and other facets of weather and climate. In 2007, as part of its mandate, the World Meteorological Organization (WMO), which operates under the auspices of the United Nations, created an online listing of officially recognized weather and climate extremes. This listing—which contains the highest and lowest recorded global temperatures, the strongest wind speed, and the deadliest tropical cyclone—is contained in the WMO World Weather and Climate Extremes Archive.

The Need for a Weather Extremes Archive

There are many reasons for maintaining an online archive of global weather extremes. Most important is that knowledge of past and current weather and climate extremes is critical in helping establish baselines so that we can determine exactly how much and how fast climate is changing. For example, in 2015, a massive heat wave along the Antarctic Peninsula led to the highest temperature (17.5°C, 63.5°F) ever recorded for the continental area of Antarctica and its nearby islands. Then, in 2020, Argentina’s Esperanza Base, near the tip of the Antarctic Peninsula, measured a new record temperature: 18.3°C (65°F).

Knowledge of weather and climate extremes is critically important for engineering safe infrastructure and for public health.

Knowledge of weather and climate extremes is also critically important for engineering safe infrastructure and for public health. If a person is designing a building or bridge, knowing the maximum wind speeds and the ranges of temperature and other weather variables to which it could be exposed is essential for ensuring the structure’s safety. Similarly, doctors and health officials use information about meteorological extremes to understand the conditions that communities around the world may experience and to assess how public health could be affected.

Evaluating weather and climate extremes also helps advance our basic understanding of atmospheric science. For example, a recent WMO investigation confirmed the longest lightning flash (768 kilometers over Mississippi and Texas in 2020) and the longest-duration flash (17.1 seconds over Argentina and Uruguay in 2020) ever observed. This new report followed just 5 years after an investigation on lightning extremes that prompted a reevaluation of the long-standing meteorological definition of “lightning” as having a flash lasting less than 1 second [Lang et al., 2017].

Many locales commemorate and recognize the occurrence of major weather events, which can be a source of local pride—and can attract tourists and other visitors. For example, a huge sign at Mount Washington Observatory in New Hampshire acknowledges the site’s long-held record for the highest measured wind speed of 372 kilometers per hour, recorded in 1934. (This record was exceeded in 1996 by a 407-kilometer per hour gust at Barrow Island off Australia [Courtney et al., 2012].)

Because so many people are fascinated by weather and its extremes—the hottest, coldest, windiest, and so on—an archive of these extremes can also help engage and better inform the public. Since the inception of the WMO World Weather and Climate Extremes Archive, we have found that children especially are interested in hearing about weather extremes, as is evident from the numerous emails and other messages we have received. Grabbing the attention of younger generations promotes longevity and continued innovation in the study of atmospheric science by helping to ensure there will be motivated and qualified meteorologists and climatologists in the future. By providing official and accessible records of extremes, the archive also helps the public and the media, which has an unfortunate tendency to overhype individual events, to put weather and extremes into proper perspective.

Since 2007, we have evaluated more than 15 potential records, and over the past decade we have codified the process by which we carry out evaluations.

Vetting Candidate Events

Because no one had maintained an archive like this before, at the time of the archive’s creation in 2007, we anticipated that we would issue a formal evaluation of a new record every few years. Since then, however, we have actually evaluated more than 15 potential records, and over the past decade we have codified the process by which we carry out evaluations.

Reports of new potential extremes are initially assessed by leadership within the WMO and by the rapporteur of weather and climate extremes—a designated position within the United Nations—on the basis of available evidence. Next, an ad hoc international “blue-ribbon” evaluation committee of atmospheric scientists is assembled. Since 2007, we have had more than 30 evaluation committees comprising more than 100 scientists from more than 40 different countries. Currently, there are two such committees either being formed or actively evaluating events.

The members of these ad hoc committees are selected to represent a range of expertise. The areas of expertise sought include local climate knowledge and understanding of factors contributing to an extreme occurring at a particular location or about specific climate phenomena in general, as well as specialized knowledge about, for example, meteorological instrumentation or calibration or observation practices.

The rapporteur, in conjunction with a committee member from the area where the potential extreme occurred and others, constructs a background report of the available information and data regarding the extreme observation. This report includes the exact geographic position of the observation, the type of equipment used to record it (and specifics on the equipment’s calibration, maintenance, and operation), the synoptics (regional weather) surrounding the event, and any notable unusual or unique information concerning the observation.

The committee then reviews the report and discusses all aspects of the potential extreme. Members address five key questions:

• Is there need for more raw data or documentation on this event to determine its validity or invalidity? Are there other data or other analyses corresponding to this time/place extreme event?
• Are there any concerns about equipment, calibration, measurement procedures, or other processes/procedures associated with the measurement of the event? (This is to ensure that the equipment and procedures are consistent with accepted standards.)
• Are there any concerns associated with the nature of the event that would raise questions regarding the validity of the record?
• Are there any other concerns associated with the event?
• Fundamentally, does the documentation support or refute this current world weather record?

The committee members discuss and deliberate the available documentation via email, moderated by the rapporteur. After their deliberations, the committee recommends their finding to the rapporteur for final judgment, at which point the rapporteur either accepts the observation for inclusion into the archive or dismisses it.

A Hot Day in Verkhoyansk

The World Meteorological Organization (WMO) has listed high and low temperature extremes for the Antarctic region; however, similar categories were not listed for polar regions at or north of the Arctic Circle.

A recently concluded investigation aptly demonstrates the evaluation procedure for potential new records. Since the archive’s initiation, the WMO has listed high and low temperature extremes for the Antarctic region (i.e., the area at or south of 60°S, corresponding to the land and ice shelf areas included in the Antarctic Treaty). However, scientists and members of the media noted that similar temperature categories were not listed for polar regions at or north of the Arctic Circle (66.5°N).

This deficiency was particularly evident during the Siberian heat wave of summer 2020, when Verkhoyansk, a town in northeastern Russia 115 kilometers north of the Arctic Circle, recorded a maximum temperature of 38°C (100.4°F) at about 09:00 UTC (19:00 local standard time) on 20 June 2020. Following that measurement, the WMO created an ad hoc international panel to analyze and verify Arctic extreme temperatures, specifically the potentially record setting Verkhoyansk measurement.

The first work of any such evaluation committee normally falls to the local representative, who is typically affiliated with the country’s meteorological service. That person is charged with obtaining the relevant raw observations from the weather station that recorded the candidate event and from surrounding stations, as well as metadata like photographs of the installation and instrument calibration records.

The local representative for the Verkhoyansk station (WMO# 24266 at 67.56°N, 133.40°E and an elevation of 138 meters) obtained the raw data and requested metadata, including photographs of the station (one such photo appears at the beginning of this article). The metadata supplied by the Russian Federation indicated that the Verkhoyansk equipment, siting, and logistics had been certified by the local authority, the Yakutia Department of Roshydromet (the Republic of Yakutia’s office for Russia’s Federal Service for Hydrometeorology and Environmental Monitoring), in April 2020, prior to the 38°C observation in June of that year. In addition, members of the evaluation committee examined the synoptic background for the 20 June 2020 extreme temperature event at Verkhoyansk using data from the European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA5) [Hersbach et al., 2020].

Local meteorological data for 18–23 June provided a continuity check for the observed surface temperature extreme reported at 09:00 UTC on 20 June. And meteorograms (records of meteorological conditions at a given location) for various atmospheric pressure levels were produced for that time period for the region around and including Verkhoyansk.

What was clearly evident from this information was the strength of the upper-level ridge—an area of high atmospheric pressure often associated with warm and dry conditions in the summer—over the region at the time. Furthermore, the ERA5 model reanalysis for the region indicated maximum temperatures at an atmospheric pressure of 1,000 hectopascals (corresponding roughly to Earth’s surface) on 20 June of more than 33°C, comparable to an observed temperature of 36°C at Verkhoyansk near 09:00 UTC and a daily maximum value of 38°C (Figure 1).

Fig. 1. This meteorogram documents the 20 June 2020 extreme temperature event (09:00 UTC) at Verkhoyansk, Russia, from the European Centre for Medium-Range Weather Forecasts interim model reanalysis (ERA5). The 1,000-millibar (1,000-hectopascal) meteorogram covers 18 to 23 June 2020 for the area 70°N to 65°N and 130°E to 135°E. Temperature (°C) is shown in red, and specific humidity (grams per kilogram) is shown in green. Credit: Arizona State University

Because “Arctic Circle highest temperature” (the highest temperature recorded above the Arctic Circle) was a new climate category for the WMO archive, our investigation included looking into temperature records from the other Arctic countries: Canada, Finland, Greenland (an autonomous part of Denmark), Iceland, Norway, Sweden, and the United States. For instance, we contacted national climate data experts from Environment Canada, the country’s national weather agency, for assistance with Canadian records. Those experts rigorously checked all station records from north of the Arctic Circle for past observations that eclipsed the 38°C temperature at Verkhoyansk but ultimately concluded that none did. In addition, the recognized state record for Alaska and the national records from all other Arctic countries included no temperature measurements of 38°C or above at any locations above the Arctic Circle.

Consequently, the WMO committee that was investigating the temperature observation recommended unanimously that the 38°C reading recorded on 20 June 2020 at Verkhoyansk be accepted as the “highest recorded temperature at or north of 66.5° (the Arctic Circle).” The rapporteur accepted that recommendation, and the record is now listed in the official WMO World Weather and Climate Extremes Archive.

It is possible—indeed, likely—that current records in the WMO archive, including that for the highest temperature above the Arctic Circle, will be eclipsed in the future

Climate Change Will Drive New Records

As with all weather extremes, the extremes of temperature, precipitation, pressure, wind, and other phenomena evaluated by the WMO represent snapshots from the regional and global climate. It is possible—indeed, likely—that current records in the WMO archive, including that for the highest temperature above the Arctic Circle, will be eclipsed in the future. When such observations of new potential extremes are made, new WMO evaluation committees will be formed to adjudicate and verify them.

As in the case of Verkhoyansk, this process of documentation and verification of record-setting temperatures and other meteorological variables is important for understanding how climate change will affect Earth’s sensitive environmental regimes, including those in which we humans live. And only through continual monitoring of these regimes, from the Arctic to the Antarctic and everything in between, can we ensure that we have the information needed to carry out this evaluation process and assess climate change at regional and global scales.