Thunderstorms!

The last week in January 2010 will be remembered by many over the central North Island for the frequent thunderstorms that developed in the afternoon, often lasting well into the evening. Conditions changed little during the week with a slack pressure gradient over the North Island allowing afternoon sea breezes to combine with abundant low level moisture, triggering heavy showers and thunderstorms inland. Many of these storms were slow moving, prompting a number of Severe Thunderstorm Warnings as radar detected torrential rain and hail in some cells.

To illustrate the impressive number of lightning strikes that resulted, and just how widespread the thunderstorms were, here’s the North Island strike maps (courtesy MetService and Transpower) covering the afternoon and evening periods (all about 12 hours) from Monday 25th through to Thursday 28th January.

Monday 25th – more than 9,900 strikes:

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Tuesday 26th – more than 32,200 strikes:

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Wednesday 27th – more than 18,900 strikes:

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Thursday 28th – more than 17,100 strikes:

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The colours of the lightning strikes give an indication of when the strikes occurred, with each colour on these maps representing two hours worth of lightning. The times are pink and blue for early to mid-afternoon (about midday to 4pm), green and brown for late afternoon and early evening (about 4pm to 8pm) and orange and red for late evening and night (about 8pm to midnight). Armed with this knowledge you can now deduce where the lightning started in the afternoons and where the last strikes were in the evening. For example, on Wednesday some of the first strikes occurred on a line from South Taranaki to Gisborne with the last strikes of the day occurring in Waikato near Hamilton.

In addition to the colours for each lightning strike, you’ll also note different symbols on the maps. These tell us something about the nature of each strike including the charge. Cloud-to-ground strikes are plotted as a ‘plus’ or a ‘circle’ for positive and negative strikes respectively, while cloud-to-cloud strikes are plotted as a ‘square’.

While it’s not unusual to get afternoon and evening thunderstorms in summer, this week certainly stands out as a particularly active period with a large number of strikes on four consecutive days. And as I write this on Friday afternoon we are again expecting more thunderstorms inland over the North Island today (but perhaps not as many strikes as the above examples).

Lastly, I’ll leave you with a nice example of a large thunderstorm with anvil seen over the Wairarapa on Wednesday 27 January. I took this  panorama from the roof of the MetService building at about 7:25pm:

Click for full size image.

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.”

Thunderstorms in Wellington

Cumulonimbus (CB, thunderstorm) cloud is not a common part of the Wellington cloudscape, and certainly is not in response to diurnal heating as happens in other regions like Waikato and Bay of Plenty. When Wellington gets thunder and lightning it is because of the development of cumulonimbus other than diurnal heating. CB will develop in a cold unstable air mass, and can be triggered by low level convergence usually caused by the wind flow interacting with the land forms. This is why they can occur at any time of the day in Wellington, and it is what happened on Wednesday morning (6 May 2009) when the southern suburbs were rudely awoken by crashing thunder and the sky lit up with spectacular lightning.

It started at 5:45am when a small trough arrived in Cook Strait. The convergence was between the westerlies ahead, and the southerlies behind the trough being squeezed into the harbour entrance. That was the trigger and away it went! Half an hour later it was over and the trough moved away to the northeast.

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The map is from our Lightning Detection Network and shows the lightning strikes between 5:45 and 6:10am as red dots concentrated over the southern suburbs and south of Island Bay and the harbour entrance.

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The radar scan at 6am shows the position of the trough line (by its rain echoes) near Wellington and extending out to the east.

I was in Upper Hutt at this time, just getting out of bed. I heard nothing. While I was waiting for the train at 6:40am I could see the top of a CB to the south. “Hmmm. Interesting. Palliser Bay perhaps?”. When I got into the office here, the place was buzzing with excitement, and tales of lightning, hail, and deafening thunder.

Here is a great video posted by Matt Kovesdi. Thanks, Matt.
There is a growing collection of New Zealand weather videos on the video section of our website.