Question:
I have been reading some accounts of the Hundred Years War and came across an extraordinary meteorological event that took place just outside of Chartres France in 1360. Known as Black Monday, a sudden and violent storm took place on Easter Monday and wreaked havoc on the English army. So much damage was caused that Edward III concluded it was a message from God and hurried to agree peace terms with the French.
My question is – what meteorological conditions would be necessary to cause such storm of thunder, lightning, rain and hail which could kill both men and horses? And have such events ever been recorded since in France or Western Europe?
Answer:
Ah, yes, the infamous Black Monday severe storms – what a fascinating historical weather event!
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Weather records can be found in accounts written by everyone from farmers to military leaders, as severe weather would have greatly impacted life. However, regular, standardized weather measurements weren’t available until the 1850s, so we don’t know every weather event that happened before, unless it had historical significance. And then we are limited to the testimony of the witnesses – modern satellite and observational data aren’t available that far back.
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Here’s a brief analysis of what could have led to this storm and other similar events in Western Europe:
1. Meteorological Conditions Needed
- Cold Front Passage: A sudden, severe storm with hail and intense lightning is often associated with the passage of a strong cold front. When warm, moist air is forced to rise rapidly by an advancing cold air mass, it can trigger severe thunderstorms. The dramatic updrafts within these storms are what lead to intense precipitation, especially large hail. The stronger the updraft, the larger the hail that can form before it falls out of the storm due to its weight.
- Atmospheric Instability: For a storm to reach such destructive levels, the atmosphere must be unstable, with significant temperature and moisture contrast between the surface and the upper atmosphere. This instability can cause the rapid development of cumulonimbus clouds, which are responsible for most severe weather.
- Moisture Source: The location near Chartres, France, is relatively close to the Atlantic Ocean, which provides a source of moisture-laden air. If a strong flow of warm, humid air moved into the region and met with a cold air mass, it could have fueled a violent storm.
- Jet Stream Influence: The position of the jet stream can amplify severe weather, as it provides additional uplift and wind shear. If a powerful jet stream was situated over the region, it could have enhanced the storm’s severity and prolonged its duration.
2. Characteristics of the Storm
- Thunder and Lightning: Significant lightning can occur when powerful updrafts within a cloud separate charges, leading to electrical discharges. If the storm had especially strong updrafts, this could explain the numerous reports of intense lightning.
- Heavy Rain and Hail: Large hail forms when strong updrafts carry water droplets high into the cloud, where temperatures are below freezing. The droplets freeze, collect more water as they fall and are carried back up by the updraft, growing larger until they become too heavy to be supported and fall as hail. For hail large enough to kill both men and horses, the storm’s updrafts must have been particularly powerful – on par with strong storms with baseball to grapefruit-sized hail we sometimes see in the United States plains.
3. Historical Context and Similar Events
- Notable Similar Storms: While storms of this magnitude are rare, severe weather events involving hail and intense thunderstorms have been recorded throughout history in France and Western Europe. Notable instances include:
- The Hailstorm of July 1788: This event devastated large parts of France and contributed to food shortages before the French Revolution. The hail was reportedly large enough to destroy crops and injure livestock and people.
- Highest Mortality due to Hail (1888):  This hail event was said to have killed as many as 246 people with hailstones as large as ‘goose eggs and oranges’ and cricket balls in Moradabad, India, on 30 April, 1888
- Modern Severe Thunderstorms: In modern times, severe thunderstorms with hail larger than golf balls are occasionally recorded in parts of France and Western Europe, usually associated with summer convective weather patterns. The heaviest documented hailstone in the world fell in Bangladesh in 1986 – 2.25 pounds – and the largest measured hailstone fell in either South Dakota or Nebraska – 18.74 inches in circumference vs. 8 inches in diameter, respectively.
- Notable Similar Storms: While storms of this magnitude are rare, severe weather events involving hail and intense thunderstorms have been recorded throughout history in France and Western Europe. Notable instances include:
