Category Weather & Climate

          The river Nile was the source of life and prosperity in Egypt. The Ancient Egyptians relied on the annual floods of the Nile to irrigate their crops, but studies have shown that the way in which the river floods varies considerably. Working together, historians and climatologists have found links between years of low flooding and periods of instability in Egyptian society. Records show that the famines that followed low floods led to disease and civil unrest — possibly causing the collapse of the Old Kingdom.

          The flooding of the Nile is the result of the yearly monsoon between May and August causing enormous precipitations on the Ethiopian Highlands whose summits reach heights of up to 4550 m (14,928 ft). Most of this rainwater is taken by the Blue Nile and by the Atbarah River into the Nile, while a less important amount flows through the Sobat and the White Nile into the Nile. During this short period, those rivers contribute up to ninety percent of the water of the Nile and most of the sedimentation carried by it, but after the rainy season, dwindle to minor rivers.

          These facts were unknown to the ancient Egyptians who could only observe the rise and fall of the Nile waters. The flooding as such was foreseeable, though its exact dates and levels could only be forecast on a short term basis by transmitting the gauge readings at Aswan to the lower parts of the kingdom where the data had to be converted to the local circumstances. What was not foreseeable, of course, was the extent of flooding and its total discharge.

          The Egyptian year was divided into the three seasons of Akhet (Inundation), Peret (Growth), and Shemu (Harvest). Akhet covered the Egyptian flood cycle. This cycle was so consistent that the Egyptians timed its onset using the heliacal rising of Sirius, the key event used to set their calendar.

          The first indications of the rise of the river could be seen at the first of the cataracts of the Nile (at Aswan) as early as the beginning of June, and a steady increase went on until the middle of July, when the increase of water became very great. The Nile continued to rise until the beginning of September, when the level remained stationary for a period of about three weeks, sometimes a little less. In October it often rose again, and reached its highest level. From this period it began to subside, and usually sank steadily until the month of June when it reached its lowest level, again. Flooding reached Aswan about a week earlier than Cairo, and Luxor 5 – 6 days earlier than Cairo. Typical heights of flood were 45 feet (13.7 metres) at Aswan, 38 feet (11.6 metres) at Luxor (and Thebes) and 25 feet (7.6 metres) at Cairo.

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          1200 years ago, the Mayan civilization thrived in what are now southern Mexico, Belize and Guatemala. The Mayans were brilliant astronomers and mathematicians, and their society was very stable and established. However, at some point during the 9th century, their civilization suffered a sudden and devastating collapse. Archaeologists have struggled to find an explanation for the Mayans’ fate, but recent studies suggest that a massive drought was responsible. Analysis of mud samples from the bottom of Lake Chichancanab in the Yucatan area of Mexico has found that the region’s climate in the 9th century was the driest that it had been for 7000 years.

           The Maya civilisation, which dominated southern Mexico for hundreds of years, appears to have been brought to its knees at least in part by a series of severe, decades-long droughts, scientists say. Conditions were so bad, says Nicholas Evans, a geochemist at the University of Cambridge, UK, that rainfall decreased by 50% on average. During the worst periods, he says, it decreased by up to 70%. The drought was further exacerbated by a 2-to-7% drop in relative humidity, his team found.

          The climate shift coincided with an era called the Terminal Classic Period, between 800 and 1000 CE, when the Maya civilisation was in decline and permanently abandoned many of its cities. The idea that drought may have contributed to this collapse isn’t new. “[It] has been debated for at least 100 years,” says Christopher Baisan, a dendrochronologist, or tree-ring scientist, at the US University of Arizona’s Laboratory of Tree-Ring Research, who was not involved in the new study.

          But just how severely the climate had changed was not clear. All that was really known was that it was drier than at the height of Maya influence. Evans’ team took core samples of sediments in a lake in the central Yucatan peninsula. “These sediments contain muds,” Evans says, “but importantly, they also contain a mineral known as gypsum.”

          Gypsum is a crystal that precipitates out of water when the mineral content grows too large — something that can occur during a drought. It is predominately composed of calcium and sulfate, but it also includes trapped water molecules.

          By examining hydrogen and oxygen isotopes in these molecules and comparing them to water in lake today, Evans says, scientists can chart changes in the lake. From these, he says, it’s possible to deduce variations in rainfall patterns.

          The result isn’t perfect. To begin with, gypsum only forms during periods of drought, when minerals become concentrated enough to precipitate to the bottom. Also, the isotope levels of the trapped water reflect multi-year averages of climate conditions in and around the lake, not an instantaneous measure.

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          Three years after his retreat from Russia, Napoleon faced the allied forces of Britain and Prussia at Waterloo. Again, the weather was to play its part. Very heavy rain in the region made the ground muddy, which delayed Napoleon’s attack. The delay meant that the allies, under the leadership of the Duke of Wellington, were able to send in additional troops and supplies, which ultimately helped them to victory.

          Two months before Napoleon’s historic defeat at Waterloo, a volcanic eruption in Indonesia caused heavy rains in Europe that soon succeeded in bringing him down.

          The defeat of French emperor Napoleon Bonaparte at the Battle of Waterloo in 1815 is widely believed to be due to the inclement weather in England. But a new study suggests that Napoleon’s misfortune with the rain and mud was caused by a massive volcanic eruption in Indonesia two months prior to the battle.

          On the night before Napoleon’s final battle, heavy rains flooded the Waterloo region of Belgium and as a result, the French Emperor elected to delay his troops. Napoleon was worried that the soggy ground would slow down his army.

          While that might have been viewed as a wise choice on Napoleon’s part, the extra time allowed the Prussian Army to join the British-led Allied army and help defeat the French. 25,000 of Napoleon’s men were killed and wounded, and once he returned to Paris, Napoleon abdicated his rule and lived the rest of his life in exile on the remote island of Saint Helena.

          And none of that may have happened if not for one of the largest volcanic eruptions in history. The eruption of Mount Tambora could be heard from up to 1,600 miles away with ash falling as far as 800 miles away from the volcano itself. For two days after the explosion, the 350-mile region that surrounds the mountain was left in pitch darkness.

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          Napoleon Bonaparte was one of the finest military leaders in history. His clever tactics brought a series of victories that allowed him to rule over large parts of Europe over 200 years ago. However, it was the weather that was to prove instrumental in his downfall. He invaded Russia in the summer of 1812 and captured Moscow, following the Russians deeper into the country. By November, a lack of supplies forced Napoleon and his army to retreat, and the extremely harsh winter killed many thousands of troops as they returned to France.

          In the year 1812, the infamous Napoleon assembled the largest army Europe had ever seen, more than 600 000 men strong. His plan was to march into Russia, and his last concern was the approaching winter chills. Napoleon confidently captured Moscow; his soldiers pillaged the city, stealing jewels, furs, and war prizes. However, it was too soon to be celebrating – since Napoleon had failed to consider how very cold Russia can be. As Napoleon’s army marched away with their prizes, temperatures dropped to minus 40 degrees Celsius. Many soldiers died of frostbite and starvation, and in one 24-hour period 50 000 horses died from the cold – leaving men to struggle on foot through the icy environment. Even with their stolen furs to wrap themselves up in – of the 600 000 men who marched into Russia, only 150 000 limped home. This was the beginning of the end for Napoleon’s empire, and heralded the emergence of Russia as a power in Europe.

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          Throughout history, the weather has had a major influence on the outcome of certain events. Adverse weather conditions have helped decide the outcome of battles and military campaigns, while over longer periods of time, climate change is thought to have brought about the end of some civilizations and the beginning of others.

          While searching for some topic of interest to bumble on about in this blog, I remembered an article I read ages ago that left an impression. Maybe the weather is something that most of us at CSAG think about on a daily basis (I hope), but it is interesting to hear how the weather has helped shaped history – and thus the societal world we live in.  As will be discussed shortly, the weather can be a huge deciding point in what happens when, and it is interesting to hear about events that may or may not have happened because of weather conditions (and I’m not talking about a picnic at Kirstenbosch event).

          On the 6th August 1945 it was a fine summer day in Hiroshima. At 7:09am a weather reconnaissance plane passed overhead and radioed back: “Cloud cover less than three-tenths. Advice: bomb primary.” Thus, the sky was clear enough to drop the first nuclear weapon used in war. The lack of cloud cover sealed Hiroshima’s fate, and spared the back-up target. Even more dramatic was the effect of cloud cover on Kokura. On the 8th August 1945, the second nuclear weapon was loaded into a B-29, however the skies were overcast over the primary target, Kokura. Instead, the bomb was released over the backup target: Nagasaki.

          In the 13th century, Kublai Khan, leader of the Mongol Empire, set his sights on the conquest of Japan, but was defeated by not one, but two monsoons. Shinto priests, who believed the storms were the result of prayer, called them kamikaze or “divine wind.”

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          The damage caused to crops by large hailstones has prompted many attempts to prevent hail forming. Techniques similar to those used in cloud seeding have been tried, aiming to turn hailstones into rain, but this does not seem to work. In the early 20th century, people tried using “anti-hail guns”. These would fire huge amounts of debris into the clouds in an attempt to break up the hailstones. They were tried many times, unsuccessfully, in the vineyards of France.

          A Hail cannon is a shock wave generator claimed to disrupt the formation of hailstones in the atmosphere.

          These devices frequently engender conflict between farmers and neighbors when used, because they are repeatedly fired every 1 to 10 seconds while a storm is approaching and until it has passed through the area, yet there is no scientific evidence for their effectiveness.

          In the French wine-growing regions, church-bells were traditionally rung in the face of oncoming storms and later replaced by firing rockets or cannons.

          A mixture of acetylene and oxygen is ignited in the lower chamber of the machine. As the resulting blast passes through the neck and into the cone, it develops into a shock wave. This shock wave then travels at the speed of sound through the cloud formations above, a disturbance which manufacturers claim disrupts the growth phase of hailstones.

          Manufacturers claim that what would otherwise have fallen as hailstones then falls as slush or rain. It is said to be critical that the machine is running during the approach of the storm in order to affect the developing hailstones, although all manufacturers unanimously agree that the area of effect of their device is only 100 to 200 square meters directly above.

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