Wednesday, May 26, 2010

The Forecast for D-Day

During the early months of 1944, the allied forces, under Supreme Commander General Dwight D. Eisenhower, had begun to get the upper hand in the battle with Nazi

Germany, and plans were put in progress to invade occupied Europe using a massive sea-borne army launched from England.

Above - The Supreme Commander of the Allies, 5-Star General Dwight D. Eisenhower. (Image - Wikipedia Commons - click to enlarge)

This assault force would have to cross the English Channel before landing at several beachheads along the French coast near Normandy. The entire military operation was given the code name “Overlord” and the first day of the invasion was to be called “D-Day”.

The timing of the operation was paramount – for military reasons it had to be a time of full moon and spring tide which immediately defined several windows of opportunity that could be accurately predicted from astronomical and tide tables.

However, the other variable, weather, was not so easily predicted. If rough conditions developed whilst the invading armada was at sea, an absolute disaster could have followed. In particular, the landing barges, crammed with troops, were vulnerable to capsize in even moderate seas, and soldiers in full combat gear and carrying weapons, would have little chance. Eisenhower was acutely aware that in such circumstances, his massive army of some 200,000 troops could be decimated before even reaching the battlefield.

Predicting the weather on possible invasion dates therefore became of paramount importance. The period 4, 5 and 6th June 1944 were periods of full moon and spring tides, and therefore became the focus for the weather forecasting team.

Predicting the weather in 1944 was much more difficult than today. There were no meteorological satellites, no computer simulations, and no organised network of automatic weather stations, as we see today. In addition, there was no international exchange of weather information because it was a time of warfare and weather conditions were classified as “secret”.

Meteorologists of the day relied mostly on “analogue” forecasting methodology. This consisted of

(a) Preparing today's weather map by delineating all the “highs”, “lows” and cold fronts.

(b) Manually searching the archives to find another similar map from the past.

(c) Looking at what weather followed in the historical example

(d) Assume the same would happen tomorrow.

The main problem with analogue forecasting was that it took little account of what was happening in the upper levels of the atmosphere. It was common to have two weather maps that were very similar but with different patterns in the upper winds, and this resulted in quite different weather to follow.

The forecasting team assembled comprised the top meteorologists from several countries. Group Captain James Martin Stagg, Superintendant of the Kew Observatory before the war was the head meteorologist, and he led the daily weather briefings that were held in the run-up to the invasion day.

The Allies Head of Meteorology, Group Captain James. M. Stagg

(Image from Wikipedia Commons - click to enlarge)

The American team included the well-known weathermen Benny Holzman and Irving Crick. The British had Charles Douglas, who was well known for his photographic memory of past weather events, and there was the brilliant Norwegian Sverre Pettersen, one of the foremost synoptic meteorologists of the day. In addition, experts from the Royal Navy were added to the team.

As the 4th June approached, Stagg advised Eisenhower that there was a strong low-pressure cell located over the North Sea and that gale force westerly winds were likely over the invasion area. Stagg considered that the 4th and 5th would be too rough to attempt a crossing. After considerable consultation between the senior military officers and the meteorological team it was decided not to launch on either day.

The weather map for 9am on June 6, 1944. The invasion area was under the influence of a weak ridge of high pressure, but with a deep low to the north.

The red arrows indicate the direction from which the wind was blowing. (Click to enlarge)

But Stagg and his weathermen then gradually reached agreement that the 6th may be suitable. The invasion area would lie between the strong “low” to the north and a weak high-pressure ridge that extended across Spain and southern France. The meteorological team considered that the influence of the high would be just strong enough to produce a temporary moderation in the weather. But it would be a close call – if the low moved even slightly southwards conditions across the channel could quickly collapse into gales and rough seas.

Eisenhower was faced with making one of the biggest decisions in history – stay or go in a marginal situation. His force was ready – a postponement would mean thousands of men would have to disembark, return to their land based units and wait for the next window of opportunity, increasing the chance of German readiness. To go would place his army at the mercy of a razor edge weather situation. He gave the order to go and 200,000 men went into action.

General Morgan, Eisenhower’s Chief of Staff, remarked to Stagg at the time

"Good luck Stagg: may all your depressions be nice little ones: but remember, we'll string you up from the nearest lamp post if you don't read the omens aright."

History reveals that the decision was the right one. The Germans had considered that the weather was too rough to attempt an invasion and were not on full alert and the allied forces were able to successfully cross the channel and establish beachheads. D Day was a resounding military victory and the Germans were put into a retreat that would ultimately lead to the collapse of the Third Reich in 1945.

American troops storm ashore from a landing craft, June 6th 1944

(Image - Wikipedia Commons - click to enlarge)

Interestingly the weather during the next window of opportunity, some two weeks later, was a disaster, with one of the worst summer storms in decades raging across the Channel. Eisenhower later confided to Stagg “I thank the Gods of War that we went when we did.”

General Dwight D. Eisenhower went on to become one of the most successful American Generals in history and ended the war a national hero. He became the 34th President of the United States and finally passed away, a revered figure, in 1969.

James Martin Stagg returned to civilian life after the war and became Director of the United Kingdom’s Meteorological Office. He was knighted in 1954 and as Sir J.M. Stagg was elected as President of the Royal Meteorological Society in 1959. He lived on until 1975 and goes down in history as one of the key meteorologists of the 20th century.

Reference: The Forecast for Overlord, J. M. Stagg, Ian Allen, 1971.

Saturday, May 1, 2010

Indigenous Weather Knowledge

One of the oldest ways of forecasting the weather comes from the indigenous peoples around the world, including the Australian Aborigines. These techniques evolved over many thousands of years and involved observing linkages between the weather and the behaviour of various plant and animal species.

Initially dismissed by western science, it has only recently been realised that there is an underlying scientific basis underpinning much of the indigenous weather knowledge (IWK), and a renewed interest has resulted.

There are many examples of IWK, both from Australia and around the world, and only a few are given here – the first three from Australia and the other two from external sources.

(a) The gidgee tree: This is a type of acacia native to inland Australia, and the belief is that when you can smell the gidgee tree, rain is on the way. It turns out that when the air humidity rises, the tree exudes a pungent smelling sap. And rising humidity is often associated with the onset of rain. This knowledge comes to us through peoples from various inland areas of NSW and Queensland.

(b) Flying foxes: In the Northern Territory, these large fruit bats move from inland areas to the riverbanks when the dry season is imminent and back the other way when the wet season is about to start. This behaviour is used by the local people to predict the changes of season.

A flying fox colony in northern Australia. (Image: Justin Welbergen, Wikipedia commons - click to enlarge)

(c) Brolga: The brolga is a large bird in the crane family. Its breeding is determined largely by rainfall and in the tropics takes place soon after the end of the wet season, from February to May. When breeding behaviour is observed early in the year it is taken as a sign that the dry season is imminent.

The mating behaviour of the brolga is used to forecast the onset of the dry season in tropical Australia.
(Image: Wikipedia commons - click to enlarge)

(d) Mango tree: A belief common to several Pacific Island peoples concerns the flowering of the mango tree – if this happens earlier then normal it is said to imply an increased number of tropical cyclones in the upcoming season. The modern explanation is that the mango tree will flower early if temperatures are warmer than normal and this occurs across island areas when the surrounding sea surface temperatures are warmer than normal. And warmer ocean temperatures promote increased tropical cyclone formation.
These conditions are characterised by what we today call the "La Nina".

The mango tree in flower - the timing of the blossoms is related to the intensity of the upcoming tropical cyclone season.
(Image: Wikipedia commons - click to enlarge)

(d) South American Sea Temperatures: For thousands of years, native Peruvian fisherman trolled for anchovies, a cold water fish that thrives off the coast of South America. However they observed that during some years warm water develops along the coastline, killing the anchovies by the hundreds of thousands. The local peoples monitored the ocean temperatures by immersion and “feel”, and when these warmer waters were detected, they planted increased crops of sweet potatoes in anticipation of reduced anchovy catches. Today we know this warm water development off the coast of South America as "El Nino".

Indigenous weather knowledge is a fascinating topic and because of its great antiquity forms a valuable goldmine of information that can be well integrated into today's scientific knowledge.

The Bureau of Meteorology web site contains some fascinating information on this subject at:

For some other information on the more usual ways of defining the seasons go to

Reference: The Complete Book of Australian Weather, Richard Whitaker
Allen and Unwin, 2010