Weather Extremes

Extreme Weather: The Weather of the Summit of Mt. Everest

By: weatherhistorian, 6:04 AM GMT on May 21, 2011

Extreme Weather: The Weather of the Summit of Mt. Everest

On May 11th, Nepali climber Apa Sherpa (a resident of Salt Lake City, Utah) made his 21st successful ascent of Mt. Everest, breaking his own record for the most such by any mountaineer. There is a small window of time used by most climbers for the ascent of the world’s tallest mountain (for now with an official elevation of 29,028’/8848 meters—but measured at 29,035’/8850 meters by the National Geographic Society in 1999). That window of opportunity is the month of May when the jet stream normally shifts to the north of the Himalayan Range and relaxes its icy blast on the summit. In fact, all but three of Apa’s successful climbs of the mountain were accomplished during May.

Mr. Apa Sherpa has reached the summit of Mt. Everest every single year since 1990 except for 1996, 1997, and 2001, a total of twenty-one times. This must be one of the most amazing human feats ever achieved.

Mt. Everest, the tallest mountain in the world at an official 29,028’/8848 meters (it is rising about 2mm each year and the National Geographic Society claims the actual height was 29,035’/8850m as of 1999). The retreating Khumba Glacier, in the foreground, has been a focus of concern so far as the climate change debate is concerned. Photo from Agence France Presse.

Mount Everest literally scrapes the top of the troposphere and for those who wish to climb to such heights the weather, of course, is the single most important factor regardless of the equipment or physical condition of an expedition. Every May hundreds of climbers line up to attempt such an ascent during the special ‘window of opportunity’ that the weather provides at this time.

Summit Wind

The average atmospheric pressure for the summit of the peak is 346mb, not far from the standard 300mb elevation atmospheric chart used to track the position and speed of the jet stream on the planet. During the winter months of November through March the jet stream normally will be flowing in the vicinity of the Himalayas if not directly over them (around 28°N latitude). February has reported the highest summit wind speeds, estimated at 175mph (78 meters per second) as was the case on February 6, 2004. This is believed to be approximately the maximum wind speed to be expected on the summit. Between Oct. 20 and the end of February the summit experiences an average of three out of four days with hurricane force (75mph) or stronger winds. Obviously, it is impossible for someone to stand on the summit in such a wind, so any Everest winter expedition has to be extremely lucky to make it to the top.

As the jet stream drifts to the north in March and April the winds diminish and by the end of May there are even times of absolute calm on the peak (although rare!). As a rule of thumb, climbers plan to summit as long as the wind is blowing under 35mph (15 meters per second). Looking at the GFS 300mb chart below for 12Z May 19th we see the wind was blowing at about 20knts (25mph) at the summit level at that time and so perhaps making for a good climbing opportunity (as long as the skies were clear). The summit winds normally stay relatively light throughout the summer months before ratcheting up again in late October as the jet stream slips southward again.

The 300mb GFS northern hemisphere map for 12Z on May 19, 2011. Looks like a good climbing day so far as the wind at summit level is concerned, although a jet impulse to the south may be cause for concern. This a northern hemisphere view with the North Pole at the center of the map and looking south (in all directions) from that perspective.

Summit Precipitation

The actual summit of Mt. Everest probably receives very little precipitation being above the usual condensation/precipitable elevation, and what little that does fall (as snow of course) is usually blown clear of the summit. The Base Camp for climbing expeditions is located at an elevation of 17,600’/5400m and receives an average annual precipitation of 18” (450mm).

This map shows the normal climbing route used from the Everest Base Camp to the mountain summit.

Most of the precipitation (at least 80%) falls during the summer monsoon season of June-September. May is usually still dry and thus the other major factor (aside from wind) in making this the most popular month for climbing in the region. The Everest Base Camp lies to the northwest of Mt. Everest itself and is in something of a rain shadow since it is the southerly monsoon flow that provides the bulk of the precipitation that falls in the Himalayas. The area to the southeast of Mt. Everest probably receives as much as 100” (2500mm) of precipitation in favored valleys and ridges below 10,000’ (3,000m).

Maps displaying the topography and annual average precipitation of Nepal. Mt. Everest is located in the very northeast of the country on the border of Tibet.

Summit Temperatures

There is no weather station on the summit of Mt. Everest. The highest meteorological observatory in the area is situated on the South Col at an altitude of 16,568’/5,050m but climbers often carry thermometers on expeditions and we have a general idea of the temperature trends. At no time does the temperature ever rise above freezing. The warmest months, July and August, seem to average around -2°F-0°F (-16°C to -18°C) during the night and perhaps a few degrees above this during the day. I would speculate that the warmest temperature to ever be reached on the summit to be in the 10-15°F (range -10°C to -12°C) on still and sunny days. The coldest months are December-February, with January night temperatures averaging -33°F (-36°C) and the daytime just a couple of degrees warmer if at all. The coldest temperature actually measured on the summit was in February 2003 with a -41.8°F (-41°C) reading. There have been some published reports speculating that the absolute minimum temperature may fall as low as -76°F (-60°C). This seems reasonable when Siberian air spills over the Tibetan Plateau and Himalayan Range. During the peak climbing months of May and October the summit temperature averages -13° to -17°F (-25° to -27°C). The lowest wind chill temperatures are irrelevant since there is little effect beyond wind speeds of 40mph, which are ubiquitous on the summit, and the temperature is almost always below zero Fahrenheit (i.e. the wind chill factor in high wind is ‘off the scale’ well below -100°F).

The highest weather observatory in the Mt. Everest region is the so-called Pyramid site located on the mountain’s South Col at an elevation of 16,568’/5,050m and its real-time data may be found here Pyramid Site. Apparently a new research project is underway as I write this to establish a weather observatory at an even higher altitude by this team.

Obviously, climbing Mt. Everest is no small feat even for the most experienced climbers (except perhaps for the amazing Mr. Apa Sherpa!). Although some 100 manage to do it every year recently, there are also over 120 unrecovered corpses still littering the mountain’s slopes.

The staggering beauty of the Himalayas is Nepal’s greatest attraction, not just for serious climbers but mountain trekkers as well. Photo by Serge Holmstice.

Extreme Weather

Updated: 4:14 AM GMT on May 22, 2011


World’s Worst Regional River Floods

By: weatherhistorian, 8:19 PM GMT on May 14, 2011

World’s Worst Regional River Floods

With the lower and middle Mississippi River having approached and, in some locations surpassed, its greatest flood stage on record I thought it timely to compare this spring’s event with those in the past from around the world. Aside from drought-induced famines no weather-related natural disaster has exacted the toll that massive regional floods have on the earth’s population.

Huang He (Yellow) and Yangtze Rivers, China

The deadliest river in the world, the Huang He (Yellow River) of China, has taken the lives of as many as seven million people over the course of the past 150 years. This included the single deadliest flood of all time that occurred in 1931 and drowned between one and four million in China’s Shandong and Henan Provinces (many of these fatalities were also the result of an ensuing famine). The same year the Yangtze River also flooded, killing another 145,000. In 1887, the Huang He caused the deaths of 900,000 to two million during another of the river’s catastrophic flood events.

A riverboat rests high and dry along the shore of the Yangtze River following a flood in the 1930s. The Yangtze has sometimes risen up to 100 feet above its normal stage.

The cause of these floods is silt washed into the river from loess deposits in the river headwaters (see this link for an explanation of loess deposits) that have elevated the river above the surrounding flood plain and so dikes have been built to contain the river’s course, thus when the dikes are overwhelmed the ensuing flood quickly submerges the thousand of square miles of neighboring flood plains. In the past decade the Three Gorges Dam Project has so far been successful in mitigating the river’s flooding.

A dam on the Huang He (Yellow River) releases a massive wall of water in order to flush silt out of the river’s lower reaches. The silt-causing loess deposits are the reason for the river’s eponymous name.

Floods have changed the course of the Huang He many times as this map below illustrates.

Rivers of the Gangetic Plain of India and Bangladesh

Given the Ganges River and its tributaries flood on a regular basis and that this part of the world is prone to mass casualties in the event of a natural disaster, it is interesting that no truly catastrophic flood is on record for the region. The deadliest such flood in modern history was that of September 1988, when an estimated 2,000 - 5,000 died in West Bengal State of India and in Bangladesh following four months of torrential monsoon rains.

The Indus River of Pakistan occasionally floods as well, but never in as spectacular fashion as it did last summer (August 2010) when up to 20 million people were displaced and over 1,500 died.

Rivers of Europe

The most flood-prone River in Europe is the River Arno in Italy. It has produced a catastrophic flood about once every hundred years for the past millennia. The last such was in November 1966 when Florence was inundated and 149 lives were lost. The flood was best known for the terrible destruction wrought to Florence’s cultural heritage including severe damage to the world-famous Uffizi Gallery, the primary art museum of Florence.

The Arno River rages out of control through the city of Florence, Italy during the flood of 1966 Photo by Balthazar Korab.

The deadliest river flood in European history was that of the River Neva when it overflowed its banks in and around St. Petersburg, Russia on November 19, 1824. Over 10,000 were downed. Virtually every dwelling in the city was flooded to the top of its first story including the Winter Palace of the czar and his family.


The Nile River has flooded virtually every year since time immemorial. Since the flooding is anticipated there has never been a catastrophic (in terms of human casualties) flood along its length. The possible exception to this was in August 1988 when Khartoum, Sudan was flooded destroying a refugee shantytown that had been erected along the banks of the Nile. Over 100 drowned according to press reports.

Mozambique and South Africa have had several devastating river floods in recent years as a result of tropical storms. The worst such was in 2000 when two tropical storms that made landfall between February and April that year unleashed torrents of rain and many rivers went into flood. Over 800 died as a result.

Flooding in Mozambique in 2000 trapped hundreds on this bridge leading to the town of Xai-xai. Photo from AP.

South America

The Amazon River, in spite of its vast size and drainage basin, rarely floods in an unexpected way. This is because its two principle tributaries, the Rio Negro and the Madeira River lie on opposite sides of the equator and experience different rainy seasons, so their contribution to the Amazon flowage tend to cancel one another out. Also, the Amazon has a series of large, shallow lakes along its way that act as a natural flood control mechanism. All of South America’s worst flood disasters have been the result of flash floods in mountainous regions (as happened in Brazil earlier this year and has been on-going in Columbia since last November).

North America

Obviously, the Mississippi River is the most flood-prone river on the continent and second in the world only to the rivers of China. So far (as of this writing), the highest flood stages on record this past week have not yet resulted in catastrophic flood damage or loss of lives. This, of course, is a result of the amazing system of levees and flood-control projects along its length. The last time the river flooded (along with the Missouri River) in a major way was in 1993. The Great Flood of 1993 resulted in $26 billion in damage (in current dollars—actual amount at time was $18 billion) which qualifies this as the costliest river flood in U.S. history.

The Missouri River floods a highway interchange near the Jefferson City, Missouri Airport on July 30, 1993. Photo from the Missouri Highway and Transportation Department.

The flood of 1927, however, killed and displaced far more people than the flood of 1993 even though the crests reached were not as high as those attained in 1993 or now. The death toll from the 1927 flood has variously been reported at between 246 and 313 and that of 1993 at 48. In 1927 some 750,000 were displaced versus 55,000 in 1993.

Flood refugees crowd onto an evacuation barge on the Sunflower River in western Mississippi during the 1927 flood. Photo from Library of Congress.

Deadliest Regional River Flood in U.S. History
The deadliest regional river flood in U.S. history was that of the Ohio River in March 1913. Four days of torrential rain (up to 11.16” in Bellefontaine) sent most of Ohio’s rivers into flood. The Miami River in Dayton crested some 34 feet above flood stage inundating the city and killing 123. All told 467 lives were lost in the region.

A map and table produced by USGS illustrate the rivers of risk in the United States and some of the historical floods of the 20th century.

Noah’s Flood

Obviously, great river floods have occurred in times prior to the modern records I list above. The historical event of the so-called ‘biblical flood of Noah’ has its basis in fact. Archeological evidence shows that at some time around 2400 BC the Tigris-Euphrates Valley was entirely submerged and silt some 10 feet deep was deposited in a very short amount of time.


Updated: 6:10 PM GMT on May 17, 2011


April 2011 Global Weather Extremes Summary

By: weatherhistorian, 7:24 PM GMT on May 06, 2011

April 2011 Global Weather Extremes Summary

Extreme weather highlights for this past April include the almost unbelievable tornado outbreak of April 25-28 in the Deep South of the U.S.A., wild fires in Texas, record warmth and drought in portions of Western Europe, and more flooding in Columbia.

Below are some of the month’s highlights.


Of course, the tornado outbreak of April 25-28 was the defining extreme weather event of the past month anywhere in the world. As of this writing the death toll is still undetermined ranging somewhere between 318 (latest NOAA estimate) to 350 (according to some media outlets). In any case, it was one of the most catastrophic natural disasters in U.S. history and perhaps the 3rd deadliest tornado outbreak of such. At least three of the tornadoes were rated as EF-5s, the first so rated since 2008 and only the 2nd time since 1900 that three or more F-5 or EF-5 tornadoes have ocurred during a single outbreak, the other time being on April 3-4, 1974 when six were recorded.

This is F-5 tornado damage in the Hackelberry, Alabama area following the tornado outbreak of April 27th. Note that there is just the bare slab of what was once a structure and the scouring marks in the surrounding fields. Photo a still from an aerial survey made by an ABC affiliate.

Another tornado outbreak killed 26 in North Carolina and Virginia on April 17th. More than another dozen were killed by tornadoes in other parts of the country during the month, with the total number of tornadoes counted being the most ever for any April on record.

Meanwhile, a severe drought combined with excessive heat and winds fueled massive wildfires in Texas. So far 1.5 million acres have been consumed destroying dozens of homes and killing at least one firefighter. Temperatures peaked at 111°F in Laredo on April 26th (although Richard Berler informs me the actual temperature was 110°F since the airport site thermometer is over exposed). Never the less, this was just shy of the 113°F April state record for Texas set at Catarina on April 20, 1984.

In Mexico the temperature peaked at 48.8°C (119.8°F) at Matlapa, San Luis Potosi on April 27th. This was the warmest temperature measured in the world during the month of April.

Ironically, the month closed with a fierce blizzard in North Dakota on April 30th. 14” of snow fell at Trotters and winds gusting to 78mph blew down 100 power poles between Berthold and Stanley. The 7.9” of snow that fell at Williston brought the seasonal snowfall to an all-time record amount of 107.2” (previous record was 94.7” in 1894-1895).

NOTE: Record flooding of the Mississippi River will likely be a lead story for my May summary.

The coldest temperature in the Northern Hemisphere during April was -57.3°C (-71.1°F) at Summit Station on Greenland on April 21st, an unusual value for so late in the season.


Columbia continued to be plagued by deadly floods and mudslides during April with over 20 dying in the city of Manizales during the week of April 10-17. Torrential rains have been pounding the country since last November, a result of the strong La Nina condition that prevailed this past winter.


April was the warmest such on record for many parts of Western Europe including London where the temperature averaged 4.0°C (7.2°F) above normal and peaked at 27.2°C (81.0°F) on April 23rd. A reading of 27.8°C (82.0°F) at Wisley was the 2nd warmest April temperature ever measured in the British Isles (the record remains 29.4°C/84.9°F at Camden Square, London on April 16, 1949). It as also the warmest April on record nationwide for the United Kingdom with a departure from normal of +3.7°C (+6.7°F). The warmest April temperature ever measured in all of Europe occurred at Orihuela, Spain on April 9th with a 39.0°C (102.2°F) reading. Other national April records included readings of 33.9°C (93.0°F) in France and 32.0°C (89.6°F) in Switzerland. In Switzerland wildfires broke out in several locations burning thousands of acres of forest. Drought has reached critical levels in both Switzerland and the Low Countries, especially Holland where outdoor burning and smoking have been banned. In the United Kingdom the March-April period has been the 4th driest on record since 1766 (the driest such period was in 1938).

A forest wildfire consumes a mountainside near Visp, Switzerland on April 27th. Photo by Jean-Christophe Bott.


I am unaware of any significant extreme weather events in Africa during the month (see note about Mexico above concerning the hottest temperature in the Northern Hemisphere). The warmest temperature in the Southern Hemisphere for the month occurred at Vredenal, South Africa on April 10th with a 41.3°C (106.3°F) reading measured.


Heavy early monsoon thunderstorms resulted in the deaths of six by lightning in Nepal on April 12-13. The deaths occurred at three different locations in Sindhuli, Makwanpur, and Udaypur Districts.
In early April the Burmese media reported that 700 fishermen remained missing following the severe storms that affected the Andaman Sea on March 14-17. It is unclear at this time if they have yet been accounted for.

Tropical Storm Errol lashed East Timor on April 16-17 resulting in torrential rains and gusty winds. It was a minimal tropical storm and quickly dissipated, but it is quite unusual for tropical storms to strike Timor which lies just 10° south of the equator. There were no reported injuries or significant damage.


Wet and cool weather continued over much of Australia during April although not nearly as much as March. Extreme temperatures ranged from -5.1°C (22.8°F) at Perisher Valley, New South Wales on April 23rd to 40.0°C (104.0°F) at Roebourne, Western Australia on April 14th. The wettest day was recorded at Mt. Hart, Western Australia when 206mm (8.11”) was measured.

While portions of northern Western Australia experienced their wettest April on record, most of the country received close to normal precipitation with the national average running just 18% above normal. Map courtesy of the Australian Bureau of Meteorology.


The coldest temperature in the Southern Hemisphere and the world occurred at Dome Fuji on April 26th with a -70.9°C (-95.6°F) reading.

KUDOS Thanks to Maximiliano Herrera for temperature data, Richard Berler for Laredo information, and Paul Simons for U.K. data.

Extreme Weather

Updated: 1:18 AM GMT on May 07, 2011


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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Weather Extremes

About weatherhistorian

Christopher C. Burt is the author of 'Extreme Weather; A Guide and Record Book'. He studied meteorology at the Univ. of Wisconsin-Madison.