Aerology and Amphibious Warfare
The Invasion of Sicily
CHIEF OF NAVAL OPERATIONS
July 10, 1943
18 January 1944
This is one of a series of pamphlets dealing with the weather aspects of Naval and Amphibious Warfare. The data on which these studies are based are taken from official documents and reports submitted to the Navy Department. The material has been collated and presented in a semi-technical form with particular attention given to the operational aspects of weather.
During the preparation of this study, it was found that weather data submitted by the various commands were occasionally at variance. An effort has been made to reconcile these differences in order to provide an accurate account of the sequence of weather conditions as forecast for the Force Commander, to describe the actual weather conditions observed during the operation, and to give a practical explanation of these weather conditions.
It is hoped that these studies will afford a clear view of the use of weather information during the planning, strategical, and tactical phases of the operation, and that they will form a basis for a better understanding of the application of weather information in future operations.
The primary objective of these analyses is to assist those officers who are charged with the responsibility for the planning and execution of similar operations.
[Signature] J.S. McCain
Vice Admiral, U.S.N.,
Deputy Chief of Naval Operations, (Air).
TOPOGRAPHICAL MAP OF SICILY.
The weather requirements of an amphibious force are extremely complex. A successful campaign must start with a preliminary operation designed to soften the enemy defences. This operation must be followed immediately by the establishment of a force in enemy territory and consolidation of beachheads in the shortest possible time. Any campaign on such a scale must be closely coordinated with the weather in order to be able to proceed with maximum efficiency and a minimum loss in personnel and material.
An ideal weather sequence for a maximum strategic and tactical advantage to the combined forces can be summarized as follows:
- Softening-up Process.
- Favorable bombing and bombardment weather.
- Good visibility over the target area.
- Clear weather or scattered clouds over target for maximum accuracy.
- Upper-wind structure sufficiently favorable to permit maximum bomb loading.
- Satisfactory operating conditions at home base.
- Actual Invasion.
- Air Forces.
- Average or better flying conditions over objective in order that maximum air cover may be given invasion forces.
- Favorable upper winds or an accurate knowledge of upper winds at time of jump in order that troops may land in the immediate vicinity of objective.
- A surface wind of not more than moderate velocity.
- Transport Group.
- Smooth seas in order to facilitate the loading of landing craft and hoisting out process.
- Landing Craft.
- Sufficient surf to prevent grounding on false beaches and sandbars 80 to 100 yards offshore.
- At the same time, landing craft cannot operate without swamping in a surf of over 7 feet.
- A dark night in order to prevent detection.
- Establishment of Beachheads.
- In Transport Area.
- Smooth seas to facilitate loading of supplies.
- Clear weather in order that loading may be accomplished with maximum speed.
- On the Beachhead.
- Surf conditions as in landing operation.
- Clear weather in order that supplies may be handled with maximum speed. (assuming air superiority)
The problem of forecasting the weather for each part of the combined operation presents difficulty, but the choice of a period when the requirements of the over-all operation are met is difficult in the extreme. The weather requirements of each force participating in a landing operation must be carefully considered, and aerological conditions must be chosen which contribute most to the over-all advantage of the force as a whole, rather than for the particular benefit of one particular unit.
An Aerological Survey of the Western Mediterranean was completed in April 1943. The study of weather at Sicily during the months of May, June, and July was given special attention.
During the summer months, general weather conditions in the Western Mediterranean are quite favorable for amphibious operations. In particular, the average weather conditions in the area in Sicily from Licata to Scoglitti are as follows:
1. Cloud Cover: The mean cloud amount during this season is 24 percent, expressed on the basis of a solid overcast as 100 percent.
2. Precipitation: On the average, rain occurs on only from 2 to 7 days during the entire period with a total fall of between one and two inches for the whole season.
3. Winds: The mean wind velocity during this season is 7.3 knots, with a prevailing direction of northwest. Gale winds in this region are rare. Land and sea breezes are well marked along the southern coast of Sicily during this season. The sea breeze is the more prominent. It sets in during the forenoon, increases to a maximum of 15-20 knots at about 1400 local time, and falls off to calm near sunset.
4. Sea and Surf: Owing to the short fetch over adjacent sea areas and to the decreased wind velocities in summer, rough seas and dangerous surf on the beaches of Sicily are relatively infrequent.
In an amphibious operation, the most important weather element is wind force and direction. For this reason a specific study of the topography of Sicily is necessary. Any deformation in wind structure resulting from the influence of a mountain range, or any local effect in the landing areas which may influence the force and direction of the wind, is a primary consideration in the preparation of a weather forecast for the area.
A plateau-like mountain chain in northern Sicily extends in a east-west direction at an elevation of 2000-5000 feet. The orientation of this plateau is such that the Licata-Scoglitti area is in a leeward position when the wind direction lies between 300 and 021 degrees, true.
The effect of the local topography in the immediate vicinity of the Licata -Scoglitti area is also worthy of consideration. Terrain behind the beaches and the exposure of the anchorage is of importance in considering the effect of wind force and direction upon an operation in the area.
At Licata there are two major beaches. One lies east of Torre Falconara and the other at Fondachello.
The beach east of Torre Falconara faces south, is deep and straight, and is about three miles long. There is good anchorage off the beach, but this anchorage is untenable in a strong SE or SW Wind. There are low hills behind the eastern part of the beach and cliffs up to 200 feet high behind the western half.
The beaches at Fondachello also face south and are about 100 yards wide and 3 miles long. The anchorage here is also untenable in a strong SE or SW wind. Behind these beaches are gentle rolling hills which reach a height of 300 feet some 500 yards distant.
The beaches between Gela and Scoglitti face SW, are from 100 to 300 yards wide, and extend for about 13 miles. There is good holding ground offshore, but the anchorage is untenable in a strong SW wind. This beach is backed by a belt of sand dunes about 1,000 yards wide. Behind these dunes is a gentle rolling lowland.
These considerations point out the fact that the Licata-Scoglitti area receives little protection from local topography. The absence of mountainous terrain to the rear of the beaches and the orientation of all of these beaches to the south leaves this whole area more or less exposed to any wind west of 300 degrees or east of 021 degrees and completely exposed to any wind with a southerly component.
Examination of the general topography of Sicily, the local terrain in the vicinity of the landing areas, and the exposure of the anchorages supported the contention that the most favorable sea and surf conditions would prevail during a period when the wind direction was between 300 and 021 degrees. This same examination also pointed out that a southerly wind of any appreciable force, strengthened in the afternoon by the sea breeze, would produce dangerous landing conditions and an untenable anchorage for the force offshore.
Weather Map for 0630 GCT, 8 July 1943.
It is difficult to find a theater of operations in which weather conditions are more favorable for amphibious operation than in the Western Mediterranean during the summer. But any unforeseen departure of weather conditions from the so called "normal" could result in a serious handicap to our forces.
For that reason, an aerological unit was attached to the staff of Commander Naval Forces, Northwest African Waters, early in the preparatory stages of the invasion of Sicily. During the planning stage, the aerological unit prepared an extensive survey of aerological conditions in the projected theater of operations, and issued weather forecasts for the training maneuvers. Aerological and communication personnel were trained in codes, ciphers, weather broadcasts, and conditions peculiar to the theater of operations. The unit also conducted research in the field of swell and surf forecasting. Aerological equipment was prepared for easy mobility in order that a forecasting service might be set up afloat on short notice.
On July 8, the Azores high pressure system was quite strong (1037 millibars) and was centered 250 miles west of Horta. A wave on the front extending from a dying low over Scandinavia was forming over the Balkans. The cold front section of this wave extended across Sardinia and was expected to pass Malta well in advance of D-day. A secondary cold front extending across central France in a NE-SW direction was expected to continue moving to the SE, and to pass Sicily before midnight of the 9th.
Western Europe and the Western Mediterranean were under the influence of a modified polar maritime air mass flowing in from the northwest. A wedge-like intrusion of the Azores High into the Mediterranean was indicated. The increasing circulation around this system and the resulting inflow of fresh polar maritime air was expected to cause the low over the Balkans to deepen and thus further increase the NW gradient.
An analysis of the synoptic situation also indicated that the high pressure wedge forming over northern Spain would continue to advance in an ESE direction, and thus eventually cut off the NW circulation, leaving the Mediterranean in a small high pressure area.
By 0700 on July 9, the Azores high had moved a short distance to the east and the wedge of high pressure had continued its advance to the ESE. The wave over the Balkans had intensified, and the increased NW circulation had maintained the weak secondary cold front which now extended in a SW direction from the center of the low pressure area over the Balkans.
The NW flow of polar maritime air over Western Europe continued and increased in intensity. Force 5 winds were reported over northern Sardinia. It was calculated that the weak secondary cold front between Sardinia and Sicily was moving SE at 20 knots and that it would pass Sicily and the channel to the south in the afternoon of the 9th.
Weather during the approach was typical of Mediterranean summer conditions until the forenoon of July 9 (D minus 1 day). At this time, the steepening pressure gradient caused the northwesterly winds to freshen. By 0800 the wind had increased to 18 knots from the NNW. At 0900 strong
Weather Map for 0700 GCT, 9 July 1943.
An oblique view of the Straits of Sicily, showing the exposure of that area to a NW wind along with the approximate position of the Force Flagship during the afternoon of 9 July. The force and direction of the wind at that time is also shown. This view shows the gentle slope of the terrain immediately behind the beaches from Licata to Scoglitti. It also shows the exposure of the Licata-Scoglitti area to a west or south wind.
The broken line separates the exposed channel from the protected area to the leeward of a NW wind.
This view shows the gentle slope of the terrain immediately behind the beaches from Licata to Scoglitti. It also shows the exposure of the Licata-Scoglitti area to a west or south wind.
Weather Map for 1800 GCT, 9 July 1943.
winds were forecast to continue throughout the day and to slacken after dark.
The wind continued to increase during the day. Between 0900 and 1500 it averaged 25 knots from the NNW.
A. 1500, the H-hour weather forecast was issued:
Mostly clear skies, winds NW 10-15 knots, decreasing, inshore breakers 3-4 feet or less, decreasing, washing down the beach from NW to SE.
From 1600 to 2000 the wind averaged 31 knots from the NW with gusts to 37 knots. The seas rose rapidly to 12 feet. The invasion force was at this time in mid-channel between the northern tip of Tunisia and Sicily under the influence of the full sweep of a northwesterly wind across the Mediterranean.
During this critical period while the invasion force was completely exposed to the NW wind, considerable apprehension was felt in various quarters concerning the success of the operation.
An analysis of the 1800 weather map showed that the wedge of high pressure advancing in a southeasterly direction over the Mediterranean was giving high winds on the eastern and southern edges of the system, and that the continued advance of this ridge would bring with it decreasing winds and fair weather. At 1900 the aerological officer reaffirmed the previous H-hour forecast with confidence and the operation continued as scheduled.
After 2100 the NW wind dropped off sharply to 17 knots and continued to decrease in force. By 0100 on the 10th the wind was NNW, 7 knots, just off Gela.
During this period after 2000, the forces came under the increasing protection of the Sicilian plateau, the effects of the secondary cold front were vanishing and the gradient behind the front was rapidly decreasing.
H-hour weather at the several landing areas was a direct function of surrounding topography, wind direction, and the resulting exposure of the beaches.
Conditions in the Licata-Gela area were recorded as follows:
Sky -- clear
Wind -- NNW 7 knots
Visibility -- 7 miles
Swell -- 2 to 3 feet high
Surf -- 2 to 6 feet high (variously reported)
At Gela, the NW wind was parallel to the beach with the result that the surf swept down the beach. Most of the larger landing craft grounded on the false beaches 30-75 yards from the shoreline.
Weather Map for 0600 GCT, 10 July 1943
Weather conditions in the Scoglitti area were less favorable:
Sky -- scattered cloud
Wind -- NW, 10 -- 16 knots
Visibility -- 12 miles
Swell -- 10 to 12 feet high
Surf -- 2 to 6 feet high (variously reported)
At Scoglitti, the NW wind was normal to the landing beach. Resulting seas created a heavy surf over the outer bars, made boat handling difficult, and made proper beaching tedious. Excessive motion of the transports caused delay in casting loose the landing craft, and several boats were lost in this operation. Debarkation was slow due to the heavy swell and the seasickness of the troops.
Weather conditions during the approach and assault phases of the invasion of Sicily had the following effects on the operation:
(a) High winds and resulting seas on the afternoon of the 9th caused considerable seasickness among the troops, especially those embarked in landing craft.
(b) During the afternoon and early evening of the 9th the landing craft convoys were forced to reduce speed, and station keeping was difficult.
(c) The motion of the transports in the swell off Scoglitti caused considerable difficulty in hoisting out, and resulted in the loss of several landing craft.
(d) This same swell in the transport area slowed the debarkation of troops to some degree.
(e) Assault formations were irregular, and landings in each wave were not simultaneous.
(f) The surf in some cases assisted the landing craft to pass the outer bars.
(g) The action of the surf in washing down the beach caused many boats to broach during the beaching and unloading operations.
On July 10, the Azores high continued to move to the east, and the wedge of high pressure which was over Spain on the 9th continued to advance in a SE direction. Weather over the Western Mediterranean was extremely good - skies were clear, winds were light, and the seas were smooth.
An analysis of the pressure distribution in this high pressure system and the barometric tendencies over Spain indicated that the advancing wedge would continue to move in a SE direction, and that Sicily would be under this wedge on the 11th. The weather resulting could be expected to favor unloading and transport activities in the landing areas.
Weather Map for 1800 GCT, 10 July 1943.
Weather Map for 1300 GCT, 11 July 1943.
Weather in the landing areas on the 10th continued to improve. By noon, the sea breeze effect had turned the surface wind to the west and increased its force to 20 knots and above. As this was purely a local weather phenomenon, the seas did not build up, and although the breakers over the surf did increase slightly, little delay in unloading resulted.
During the 11th and 12th, the Azores high moved east a very short distance and the central pressure continued to fall. The ridge of high pressure over the Mediterranean continued to build up, and the resulting fair weather and slight sea was ideal for the landing of supplies and the consolidation of our positions on the beach.
Several important lessons in the strategic and tactical use of weather were learned in this operation:
(a) The services of a qualified aerological officer must be available during the planning phase of any amphibious operation. A weather summary prepared by an experienced aerologist for the projected theater of operations is essential, and weather forecasts for all practice landings are needed.
(b) If other factors of strategy remain stable, weather in the landing area should be a primary consideration in the selection of a D-day. The weather requirements of each unit of the force must be kept in proper relationship to the weather requirements of the force as a whole, and weather conditions chosen which contribute most to the success of the force as a whole rather than for the particular benefit of some specific unit of the force.
(c) Complete weather information is essential at all times even in a theater of operations which enjoys "generally good" weather. In an area of "generally good" weather, an unseasonable change is likely to come as an unwelcome surprise if it has not been predicted in advance.
(d) Steps should be taken in the planning phase of any operation which will insure the positive dissemination of weather information to all ships concerned at all times. To accomplish this, sufficient priority must be assigned to weather traffic in order that it may be promptly received and distributed.
(e) A rear echelon, especially trained in the specific problems of the operation, should be established at a base near the projected operation in order to insure the prompt transmission of all available weather data to the advance forces.
Experience in Sicily has underlined the following comment, made by the Headquarters of the Commander-in-Chief in a review of The Landing in North Africa:
"The strategic and tactical importance of weather forecasts cannot be overemphasized."