Napoleon’s Winter

What would New Zealand’s history be like without the First and Second World Wars? Blame the terrible Russian winter and Napoleon’s folly according to historian Adam Zamoyski in his riveting book 1812 Napoleon’s Fatal March on Moscow. The losses suffered in the invasion, particularly to his cavalry, ensured Napoleon’s downfall the following summer. That led to an aggressive Germany unified under a militaristic Prussia while in Russia, the Tsar came to believe he was God’s instrument on Earth. Growing more conservative as he aged, his repression of the generation who defeated Napoleon sowed the seeds of the Russian Revolution.

Napoleon’s army began retreating from Moscow on 19 October in fine weather. Three days later, rain turned the road to mud making travel difficult. On November 6, the first snow fell and soldiers froze to death overnight when their campfires burnt out. Once the snow compacted, it became rock hard and slippery, causing men and horses to fall. Carriages and guns had to be controlled on slopes using ropes held by dozens of soldiers. When they slipped, the guns crashed into vehicles ahead of them, creating chaos.

When horses fell, they often broke their legs. In the space of a few days the army lost tens of thousands of horses to cold and accident. The Polish cavalry came through best as they had shod their horses in Moscow with shoes like crampons. Among the French, only the horses of the Imperial household had followed their example, on the orders of General Caulaincourt, a former ambassador to Russia who had experienced their winters before.

A few of the French acquired small Cossack horses that had broad hooves and a low centre of gravity. These horses also knew how to slide down slopes by sitting down on their hindquarters still with the rider on their back.

Most of the army slept in the open, but the few huts still left standing often became death traps. So many men crowded inside to shelter that some suffocated in the crush. Huts often caught fire, either from their wooden stove overheating or campfires set too close outside the building by soldiers ripping timber off the hut walls.

On the night on November 12 the temperature dropped to minus 24 C and frostbite became widespread among the troops. Not recognising the symptoms or knowing the treatment, many men lost noses, fingers and toes. With such a hard frost, there was no liquid water readily available, so men and horses suffered badly from dehydration.

The churned up road surface froze into sharp ridges that lacerated shoes and feet cutting some to the bone. Stragglers that fell behind were captured by Cossacks, stripped naked and left to die, or sold to peasants who tortured them to death in revenge for their own sufferings.

Men ate horsemeat flavoured with gunpowder for the salt. Horses that died became rock hard in minutes so had to be cut up while still alive – including horses still walking. When the owner looked away, a steak could be slicked from the numbed flank of a horse. The cold froze the blood and the horse would survive a few days longer.

But it was the river crossings that posed the greatest danger. As the traffic bottlenecked, people were knocked over in the crush, trampled underfoot and killed. Russian artillery, drawn on sleds, sometimes caught up and shelled the crowd. The bridge over the Berezina had been burnt so 400 pontooners built two new ones working up to their necks for 15 minutes at a time in freezing water as ice floes washed past. Only eight of these men survived the retreat.

crossing the Berezina

Crossing the Berezina (Source: Wikipedia)

A two day blizzard starting on November 29 was remembered by survivors as the worst time. Some shot themselves. Those whose shoes had disintegrated got such bad frostbite that skin and muscle peeled away from the bones of their feet. Soldiers who feel down had their boots pulled off before they died. Men murdered for fur coats and there were incidents of cannibalism.

When the blizzard passed, clear skies allowed the temperature to plunge. It fell to minus 37 C on December 6. At this temperature liquid water cannot exist in the air and water molecules combine to form ice crystals that sparkle in the clear sky – a phenomenon meteorologists call “diamond dust”.  Men’s breath was as thick as smoke and condensation formed icicles in hair eyelashes and beards which grew thick enough to obstruct vision and breathing.  Eyelashes froze together and had to be pressed between fingers so eyes could be opened. Many got snow blindness, causing tears, which froze.

It was now so cold that men died walking. Blood suddenly streaming from mouth and nose and sometimes eyes and ears, they would stagger a few steps like drunkards before falling. Many now had dementia, some so disorientated that they would walk into fires in bare feet and lie down. One gunner froze to death standing behind his cannon, hand on the breech and facing back towards Russia.

Two fresh divisions were deployed in front of the town of Vilna to stop the Russian advance, but, not hardened to the cold, almost all froze to death over two nights. Nor were the Russians spared the suffering. Of the 97,000 who pursued the retreating French, only 27,000 reached Vilna.

The Russian general Denis Davidov remembered colder winters in previous years, but not with large starving armies wandering around in the open. In all, counting civilians and soldiers on both sides, around a million people died because of Napoleon’s invasion of Russia. In the opinion of general Caulaincourt, had Napoleon left Moscow two weeks earlier it is likely he would have extricated most of his army intact and continued to control much of Europe. But the warm start to autumn deceived Napoleon as he waited in vain for the Tsar to negotiate.

1812 was not the first time an invading army came to grief in a Russian winter. In 1709 the Swedish king Charles XII lost a third of his forces in winter before defeat at Poltava by Peter the Great.

Nor was it the last. Two hard winters during the Second World War contributed to the German Army’s ultimate defeat. A witness later remembered Hitler fuming in December 1941 “These damned meteorologists, also they are talking about Napoleon.”

First Published: New Zealand Geographic July-August 2010

The Big Chill

2:30pm Sunday 14 August 2011

It’s early days in a weather event which is likely to be memorable for its coldness.

Below is a satellite image for midday Sunday. The wind flow over New Zealand is generally from the southwest; the coldest showery air has made its way onto Fiordland, Southland, Otago and south Canterbury.

It’s not raining or snowing over all of southern New Zealand because the wind flow is more or less lined up with the South Island, thereby sheltering some places, and because the precipitation is showery.

Infra-red satellite image for midday Sunday 14 August 2011. Image courtesy Japan Meteorological Agency.

The surface temperature doesn’t necessarily tell the whole story. Below is a graph of the air temperature for the 24 hours from 1:00pm Saturday 13 August to 1:00pm Sunday 14 August. Note how the air temperature at Invercargill Airport and Nugget Point, both fairly open to the southwest, has been on a pretty steady downward trend. At 2:00pm Sunday, the temperature at Nugget Point was 1.0 C and the wind speed was 85 km/h. Brrr.

At Timaru Airport, on the other hand, the temperature rose sharply when the southwest change arrived mid Sunday morning – the reverse of what might be expected. The period overnight Saturday to dawn Sunday at Timaru Airport was one of clear skies and fairly light winds, so the quickly-cooling land surface during this period cooled the air immediately above it and an inversion formed. When the southwest change arrived, the air once again became well-mixed. But this southwest air is expected to also become steadily colder.

Air temperatures at Invercargill Airport, Nugget Point (Otago coast) and Timaru Airport for the period 1:00pm Saturday 13 August 2011 to 1:00pm Sunday 14 August 2011.

4:15pm Sunday 14 August 2011

Here’s another look at why the surface temperature doesn’t necessarily tell the whole story.

In my blog post about the winter storm of early July 2011, I partially explained how showers may form in cold air moving over a warmer sea surface. The (relatively) warm sea heats “blobs” of the air immediately above it; these blobs then ascend because they are less dense than surrounding air. For the ascending process to continue, the surrounding air must remain relatively cooler than the ascending warm blobs. Thus, it is important to have information – that is, observations and forecasts – about the vertical temperature structure of the atmosphere.

Observations of the temperature structure of the atmosphere are primarily made using weather balloons. Below is a graph of the temperature at three levels in the atmosphere above Invercargill, obtained from radiosonde balloon flights. The blue line is the temperature at about 5000ft, the red line is the temperature at about 10000ft, and the green line is the temperature at about 18000ft. These heights are approximate; the height of a given pressure level varies with the air temperature; here, we should probably discuss the idea of the thickness of an atmospheric layer – but I think we’ll do that some other time.

Anyway, the graph shows that the atmosphere above Invercargill has been cooling off steadily since the middle of Saturday 13 August. In depth, it is now very cold.

"Upper air" temperatures obtained from balloon soundings at Invercargill Airport at midnight Fri-12-Aug, midday Sat-13-Aug, midnight Sat-13-Aug and midday Sun-14-Aug.

6:15pm Sunday 14 August 2011

It snowed quite heavily in Wellington City, above about 100 metres, from approximately 4:30pm for at least an hour. This is the heaviest and most widespread snowfall in Wellington City for at least 30 50 years.

Snow in Clifford Road, Johnsonville (altitude approximately 200 metres) early evening, Sunday 14 August 2011. Snow depth 3 to 5 cm.

At midday Sunday 14 August the freezing level around Wellington, obtained from the Paraparaumu radiosonde balloon flight, was just over 1000 metres and falling (it was around 1600 metres at midnight Saturday 13 August). But late on Sunday afternoon, it would still have been well above the level to which snow fell in Wellington. Snow starts melting once it falls below the freezing level – but the melting process draws heat from the surrounding air, which lowers its temperature; thus, the melting snow “drags” the freezing level down with it, at least for a while. How far the freezing level within the area of falling snow is dragged towards the ground depends mostly on the intensity of the snowfall and the vertical variation of temperature and humidity of the air it is falling into.

Wellington radar image, 5:05pm Sunday 14 August 2011.

1:00pm Monday 15 August 2011

So far in this blog, I’ve been talking quite a bit about the temperature throughout the depth of the troposphere (the troposphere is the part of the atmosphere in which weather systems exist). Time, now for a picture. Below is a plot of:

  • Forecast temperature (colours) at the 500 hPa level (roughly 18,000 ft, or about halfway up the troposphere)
  • Forecast wind speed (black lines) at the 250 hPa level (near the top of the troposphere)

… for midday Monday 15 August.

Plot of (a) forecast temperature (colours) at the 500 hPa level (roughly 18,000 ft, or about halfway up the troposphere), and (b) forecast wind speed (black lines) at the 250 hPa level (near the top of the troposphere), for midday Monday 15 August 2011. Data courtesy European Centre for Medium-Range Weather Forecasting.

The colours in this plot are forecast temperature; over most of New Zealand, the temperature at around 18,000 ft was forecast to be -30 C or lower. The important thing to note is that a large mass of Antarctic air covers almost all of New Zealand.

The red arrow on this plot shows the forecast position of the axis of strongest winds, near the top of the troposphere, at midday Monday 15 August. This is the polar jet, on the border between the deep pool of Antarctic air over New Zealand and the warmer mid-latitude air around it.

Incidentally, the forecast temperatures compare very well with the observed temperatures at midday Monday 15 August, as shown in the table below.

Forecast 500 hPa temperature (C) Observed 500 hPa temperature (C)
Whenuapai -29.3 -29.9
Paraparaumu -38.9 -39.1
Invercargill -34.4 -35.1


4:30pm Monday 15 August 2011

Here’s a graph of how the freezing level over New Zealand has changed over the last few days.

As of midday Monday 15 August, the freezing level varied between about 1000 ft at Invercargill to about 2000 ft at Whenuapai. Snow has fallen to sea level in many parts of southern and central New Zealand – that is, to at least 1000 ft below the freezing level.

This is a classic example of the melting effect (see the post made at 6:15pm Sunday 14 August 2011, above). Over the last few days, MetService’s Severe Weather Forecasters have spent a lot of time considering how far below the freezing level snow would fall. This requires a good understanding of cloud physics.

Freezing levels at Whenuapai, Paraparaumu and Invercargill from midnight Friday 12 August 2011 to midday Monday 15 August 2011, derived from radiosonde flights.

9:30am Tuesday 16 August 2011

Here’s a few photos from the Wellington snow of June 1976.

Wellington snow of June 1976: looking approximately southwest from the MetService building.

Wellington snow of June 1976: Karori Road, looking southwest.

Wellington snow, June 1976: MetService building, from Salamanca Road. Look at those cars ...

10:30am Tuesday 16 August 2011

In southerly flows, the West Coast of the South Island is well sheltered by the Southern Alps. Since the southerly took hold on Sunday, the air on the West Coast has been very dry because of the Foehn Effect.

Below is a graph of the dew point temperature (the temperature which air must be cooled to for water vapour to condense into water liquid or water solid) at Hokitika Airport from 10am Sunday 14 August to 10am Tuesday 16 August. On the afternoon and evening of Sunday 14 August, there’s a huge change in dew point (around 13 degrees), down to around -10 C. Since then, the dew point has remained negative, generally fluctuating between about -3 C in the morning and -7 C in the afternoon. Such a low dew point makes the air feel much colder than its temperature would suggest. We take the dew point into account when calculating the “feels like” temperature.

Dew point temperature at Hokitika Airport, 10am Sunday 14 August 2011 to 10am Tuesday 16 August 2011.

2:30pm Tuesday 16 August 2011

This event has been characterised by many places having low daytime (maximum) temperatures.

Location Maximum temperature on
Mon-15-Aug 2011
Lowest daily maximum temperature on record Month / year occurred in Record starts Monday’s max temperature is the lowest since …
Auckland Airport
8.1 8.1 15 August 2011 1966 New record
New Plymouth Airport
8.8 6.0 July 1951 1945 25 July 2011 and 12 July 1951
Napier (Nelson Park)
7.8 3.6 June 1969 1940 June 2006
Paraparaumu Airport
6.1 5.4 August 2004 1972 July 1986
Kelburn, Wellington 5.1 4.5 August 1938 1931 25 July 2011 and 17 July 1995
Christchurch Airport
4.6 1.7 August 1992 1954 July 2007

Monday night / Tuesday morning was very cold in some places, though. Here’s a few notable overnight minima from MetService automatic weather stations.

  Waiouru Automatic Weather Station     -7.7 C (new record for August)  
  Blenheim Airport Automatic Weather Station     -6.2 C (new record)  
  Rotorua Airport Automatic Weather Station     -5.2 C (equals record)  
  Taupo Airport Automatic Weather Station     -5.1 C  

5:00pm Tuesday 16 August 2011

Below is a plot of where the air arriving at an altitude of 500 metres above Auckland at midday Monday 15 August came from. Four days previously, it was over the Antarctic landmass; two days previously, it was still over the Antarctic sea ice. The Antarctic sea ice edge is close to its northern-most extent and is near latitude 60 degrees South. Thus, the air arriving at Auckland passed very quickly over the relatively warm ocean between the Antarctic ice edge and New Zealand. In contrast, the air from the Southern Ocean which arrived over Auckland on Saturday 9 July (see my blog post on the stormy period of early July 2011) had travelled over a much longer stretch of ocean, over a longer period of time, and consequently was warmer and moister.

"Backward trajectories" of the air at 500 metres above mean sea level arriving at Auckland midday Monday 15 August 2011. The red line traces the path of the air over the period from midday Friday 12 August to midday Monday 15 August. Triangles denote the location of the air mass at 12-hourly intervals. Data courtesy NOAA Air Resources Laboratory.

6:00pm Tuesday 16 August 2011

As of 2:00pm Tuesday 16 August 2011, the extent of snowfall in this storm is as shown in the image below.

The scalloped areas delineate where significant snowfall is known (or believed) to have occurred. The dashed areas delineate where lesser amounts of snow have been observed and where little or no snow is believed to by lying. The period is from the beginning of the event on Sunday 14 August 2011 to 2:00pm Tuesday 16 August 2011.

4:00pm Wednesday 17 August 2011

During the next few days, while an anticyclone advances onto the country, the general wind flow will decrease in strength and the depth of cloud along eastern coasts gradually reduce. Near sea level the air over New Zealand remains very cold, and the advancing anticyclone more or less “traps” it in place. Very cold air, clear skies and light winds overnight are a recipe for hard frosts.

Hopefully, the diagram below – known technically as a tephigram – helps illustrate this. It is a plot, in the vertical, of the air temperature and the dew point temperature derived from the radiosonde balloon flight at Invercargill at midday Wednesday 17 August. At Invercargill there is already a large mass of sinking, warming (and drying) air above about 5000 ft (see text in red on diagram). This sinking air presses on the (relatively) colder air beneath it, trapping it near the Earth’s surface. In this particular case, the zone of transition between the two different air masses is known as a subsidence inversion. I’ve marked the subsidence inversion on the diagram; it’s the broad blue horizontal bar near the bottom.

The very cold air trapped below about 5000 ft at Invercargill is much less inclined to move around than the air further up in the atmosphere. As I’ve explained above, this is partly because of the advancing anticyclone. But it’s also partly because cold air is less “runny” than warm air. (Treacle flows much more readily when warm than cold). On the right of the diagram below are the winds in the vertical, as they were above Invercargill at midday Wednesday 17 August. Clearly (see text in green on diagram), the one wind barb shown below 5000 ft indicates quite a different flow from all the winds above 5000 ft: the flow near the surface has become decoupled from that above.

Invercargill tephigram for midday Wednesday 17 August 2011.

In a general sense, this vertical temperature and wind structure is expected to spread over eastern parts of New Zealand during the next few days as the anticyclone moves closer and pressures over the country rise.

Forecast surface pressure field for midnight Wednesday 17 August 2011. Forecast surface pressure field for midnight Thursday 18 August 2011.

3:30pm Friday 19 August 2011

Finally today, cloud over the south of the South Island has cleared enough to reveal the extent of snow cover there.

Below are two visible satellite images. The first is for around 10:00am on the morning of Wednesday 10 August, some days before this extraordinary cold outbreak. The second is for around 10:00am on the morning of Friday 19 August. Nearly all of the white over Canterbury, Otago, Southland and Fiordland is snow. The imagery only shows the extent of the snow, not its depth.

Before: MODIS-Terra visible image for about 10:00am on Wednesday 10 August 2011. Data courtesy NASA/GSFC, Rapid Response.

After: MODIS-Terra visible image for about 10:00am on Friday 19 August 2011. Data courtesy NASA/GSFC, Rapid Response.

The Thunderstorm in History

One of the pleasures of reading history is coming across stories about the weather. Thunderstorms often figure in these. One of the most dramatic examples was recorded in the sixth century AD, by Gregory, Bishop of Tours, in his Historia Francorum (The History of the Franks).

In AD 536 there were three rulers of Frankish kingdoms: Childebert, the king of Paris; his brother Lothar, the king of Soissons; and the brother’s nephew Theudebert, the king of Metz. Childebert and Theudebert joined forces and set out with a large army to attack Lothar, who retreated to a fortified position on a hilltop. Hearing of the imminent battle, Queen Clothild, mother of Childebert and Lothar, went to the tomb of Saint Martin and prayed through the night for divine intervention to prevent her sons fighting.

The next morning, before battle preparations had been completed, a terrific thunderstorm laid waste to the aggressor’s camp. Tents were blown down, gear was scattered and horses driven away by hail and lightning. The hailstones were so large and pelted down with such force that many soldiers, including the two kings, were cut by them, driven to the ground and forced to shelter beneath their shields. Meanwhile, Lothar and his army were untouched by the thunderstorm. Accepting the event as divine chastisement, Childebert and Thuedebert did penance to God begging forgiveness for attacking their own kith and kin, then sued for peace and concord, which Lothar granted. Lothar’s dynasty prospered, leading eventually to the unification of France and the rule of Charlemagne.

The role of weather is also given a prominent place in The Oxford History of the French Revolution by William Doyle. Repeated drought during the 1780s caused soaring grain prices leading to repeated civil disturbances in many parts of France. Then, in July 1788, on the eve of the harvest, widespread hail storms devastated hundreds of square kilometres of crops in the Paris Basin, which was one of the most productive agricultural areas in France. Hailstones were so large they killed men and animals. The inability to gather tax revenue on the destroyed harvest bankrupted the French Government and the price of grain rose to almost 90% of a workers salary.

In order to try to gather tax from the nobility, who were largely exempt, the French King was forced to call the Estates General for the first time in over a hundred years. Once assembled the Estates General moved beyond the King’s control, passing laws he neither wanted nor anticipated. Within the year, the struggle for power escalated into violence, the Bastille was stormed, and the French Revolution was underway.

More intriguing is the story of Martin Luther and the thunderstorm. Having completed a masters degree and a visit home to his parents, Martin Luther was returning to University in Erfurt to study law when, on July 2 1505, near Stotterheim, he was caught in a thunderstorm. Thrown to the ground by a lightning bolt striking near him, he called out to St Anne, promising to become a monk if his life was spared. Two weeks later he abandoned law studies and entered a monastary, starting down a path that eventually changed European history for ever, splitting the church and triggering decades of war.

Told this way, there is a hint of myth about the story. In fact, Martin Luther seems not to have been too keen on a law career and to have been thinking about joining the church anyway, but this was bitterly opposed by his father. Perhaps the weather provided Martin Luther with an alibi. “ Sorry Dad, a thunderstorm made me do it.”

The Year of Slipping Dangerously

With rainfall well above normal last winter, it was a great time for slips: big ones cutting major road and rail links in Kaikoura and other places and thousands of little ones making small mischief on roads and properties all over the country. Houses in Auckland were threatened for days by slow moving slips, while, in Milford Sound, tourists watching waterfalls had to sprint to safety when the sound of the rain took on a deeper rumble and tonnes of rocks and trees crashed down on the spot where they had been standing.

All of this excitement reminded me of one of my favourite New Zealand slip stories. In 1810 a major storm struck the bottom of the South Island. There were no newspapers in New Zealand then but the storm was eventually written up in a Sydney newspaper. Described as a hurricane, the tempest lasted 48 hours, flattening swathes of forest and caused numerous landslides which blocked several streams, resulting in dangerous floods.

On Stewart Island, a gang of eight sealers working in a cove near South Cape were caught by the storm. With difficulty, the men reached the shelter of a cave, where they camped. During the night, while they were asleep, a large slip blocked the mouth of the cave. With no hope of rescue from outside, the men set about digging their way out. Luckily, they had all their camping gear and provisions with them, and a stream broke through the roof of the previously dry cave, providing them with water. Initially the flow of the stream was heavy enough to threaten them with inundation, but it eased off to a trickle once the storm had passed.

The men had only two shovels, one axe and four iron pikes to work with. At first, the more debris they cleared away, the more slid down from above. It took them almost a week to burrow their way out. They were then able to observe that the cave was to one side of the slip. Had it been closer to the main body of the slip, they would never have dug their way free.

Having no timepiece with them, and unable to tell the difference between night and day while they were entombed, the men had lost track of time. When a ship eventually picked them up, they found that they were two days ahead in their reckoning of the date. The storm had blown them into the future!