CHIEF OF NAVAL OPERATIONS
SUMMARY OF WEATHER CONDITIONS RECORDED BY THE HORNET
The First Raid on Japan
The most dramatic action of the early part of the war was the combined operation which resulted in the bombing of Tokyo, Yokohama, Kobe, and Nagoya. Due to the fact that Army planes, designed to take off from a 5,000-foot runway, had to be flown from a carrier which had advanced far into enemy waters, weather played an extremely important role in the operations.
From start to finish, the weather requirement for the mission were exceedingly stringent. Had the Japanese received information that an American task force was on its way for an attack on their homeland, they could have assembled an overwhelming combination of air and sea power. Aside from the weather conditions required for the actual launching of the aircraft, weather was relied upon to furnish concealment from the time of departure from San Francisco harbor to the time of closest approach to Tokyo and for the trip back to Pearl Harbor.
For convenience, the over-all weather requirements are listed below:
Weather Conditions Desired Reason for Weather Requirement Remarks Departure from San Francisco Low visibility. To prevent observers in and about San Francisco from deducing the mission of a task force accompanying a carrier whose flight deck was loaded with B-25s. It was desirable that the departure of the Hornet be completed under conditions of low visibility. Passage Westward. Heavy cloud cover and/or rain and squalls. To prevent premature detection of the task force by Japanese vessels or aircraft. Bad weather for the trip toward Japan was considered to be highly desirable. Final Fueling at Sea. Low winds, calm sea. The transfer of fuel from the oilers to the other vessels comprising the task force can be completed expeditiously in a relatively smooth sea. An interlude of weather satisfactory for fueling was considered essential for the completion of the mission. Launching of B-25s. A true wind of at least 7 knots. The bombers were not designed for carrier operations. A velocity of 40 knots, or about 10 knots in excess of the usual requirement for carrier aircraft was necessary.
The carrier could create, by virtue of its own speed, a maximum of 33 knots.
Weather Conditions Desired (Cont.) Reason for Weather Requirement (Cont. Remarks The carrier could create, by virtue of its own speed, a maximum of 33 knots. Bombing of Japan. Good visibility over the target. A low-level bombing attack was planned. Essential if the raid were to be effective. Tail winds. Since the planes were to be launched at near maximum range, fuel conservation was vital. Desirable in order that reserve fuel would place bases in China within range. Withdrawal from Japanese Waters. Heavy cloud cover and/or rain and squalls. Once the presence of our force was disclosed, concealment in a bad weather zone would handicap possible retaliation. Desirable but not essential.
The initial plans for this operation were drawn up in COMINCH Headquarters. During the planning stage of an operation, dependent in part on weather conditions, consideration had to be given the limitations to which accurate forecasting is subject. Since a long range weather study as well as specific forecasts were involved, each had to be given separate attention.
The first involved the estimate of weather to be encountered in a specific region during a not-too-specific period of time. (A month is the usual period selected). A tabulation of all weather data available for the target area indicated a high frequency of favorable weather conditions for the scheduled operation during the period, 15 April to 15 May, when a transition period normally exists between the cessation of the northerly monsoon weather and before the southwest monsoon sets in, usually June. Accordingly, COMINCH issued a directive to conduct the operations during this period.
It should be emphasized that the values arrived at in this manner make no attempt to define the weather which will be experienced on any given day or during any particular week. The advantage gained from a study of this kind stems principally from the determination of the mathematical probability that the operation will be completed under satisfactory weather conditions.
The operation also involved synoptic or day-to-day forecasts which attempt to describe completely the weather occurring at a specific time in the near future. The degree of reliability of such forecasts depends largely on the number of weather observations reported in the region covered by the forecast.
For this reason, forecasting for the raid on Japan was extremely difficult. With atmospheric disturbances moving from west to east and with complete lack of reports from the entire area to the west of Midway and the Aleutians, no high degree of reliance could be placed on the analysis of the western portion of the Pacific weather map. Complete weather observations made at one location frequently permit accurate construction of a weather chart by means of inductive reasoning. Forecasting of this kind could be done aboard the Hornet which was equipped for taking its own weather observations. However, no great assurance could be given concerning the reliability of such "spot" forecasting if the forecast period were too long.
With the limitations inherent in the two types of weather analysis understood, consideration could be given the likelihood that the Task Force would meet any or all of the weather requirements listed above. Such a survey follows.
1. Climatic averages for the San Francisco Bay region indicated that dense fog prevails for a period of 18 hours during the month of April. Departure under cover of fog was dependent, then, upon a specific forecast which could be based on wind direction. With light on-shore wind, low visibility might be anticipated.
2. Average weather in the North Pacific during the month of April indicated that the average storm track was Just north of the 40th parallel. By setting the base course near that latitude, the Task Force could hope to enjoy maximum concealment.
3. A study of climatic averages in no way helped in planning the time for final fueling. Whether this could be completed Just prior to the final high speed run into the Japanese waters depended entirely upon the chance that somewhere in the desired vicinity favorable conditions would obtain. It was believed that the recognition and use of a favorable situation could be handled adequately on the basis of a "spot" forecast.
4. The tabulation of average wind velocities in the neighborhood of Japan indicated a statistical average of 13 - 18 knots. There was thus excellent probability that there would be sufficient wind to permit the launching of the B-25s.
5. For several reasons, it was desirable to deliver the bombing attack from a low altitude. In spite of the high percentage of cloudy skies over the target areas, the occurrence of ceilings over 1,000 feet and visibilities greater than 1 1/4 miles over 75 per cent of the time at Kobe, over 85 per cent at Nagoya, and over 90 per cent at Tokyo and Yokohama indicated a high probability that weather conditions would permit this type attack.
THE RAID ON JAPAN
On 2 April at 1000, the Task Force, formed around the Hornet, stood out from San Francisco in a fog which reduced visibility to about 1,000 yards. The fog had been forecast (see map on page 4). A high pressure area just off the West Coast on 1 April indicated that a light circulation of air coming off the ocean would prevail on the following day. At San Francisco, this synoptic situation is almost invariably accompanied by fog. Its occurrence permitted the Task Force to leave the harbor undetected.
THE TRIP WESTWARD
From the point of view of comfort, the weather conditions were bad throughout the voyage. From the operational point of view, conditions were excellent. A course along the 40th parallel was chosen. During April, the mean position of the polar front is just north of this latitude and a certain amount of unfavorable weather and clouds was to be expected. This course still avoided the center of the paths of average storm tracks. The Task Force met rain nearly every day from the time it left San Francisco until it arrived off the Japanese coast, and every day from then until the time it returned to Pearl Harbor.
Heavy seas and high winds coupled with the rain and squalls reduced the danger of being sighted but prevented cruiser aircraft from conducting flight operations. At times, the speed of the force had to be reduced to prevent structural damage to an oiler which accompanied the Task Force.
With her flight deck used for the stowage of B-25s, the Hornet was unable to provide any form of air coverage. To supply this deficiency, another task force was built around the Enterprise and detailed to "proceed from Pearl Harbor for rendezvous with the Hornet and her escorts on the 8th.
Except when bad weather prevented, continuous inner and intermediate air patrols were maintained during the daylight hours. Dawn and dusk search flights were conducted daily to 200 miles, 60 degrees off each bow whenever it was possible for planes to operate.
On the way across, one of the most difficult problems was the choice of the day for fueling. Would it be better to take advantage of a temporary lull in bad weather and fuel early, or would it be better to gamble on the occurrence of another lull closer to Japan in order to have the maximum possible fuel aboard at the beginning of the final high speed approach?
0100 GCT 2 APRIL 1942
On 15 April at 1200 local time, the direction and velocity of the wind indicated that the force would pass through a front in the near future. A well-developed high pressure area was expected to he behind the front within which diminished circulation would provide the light sea requisite for fueling. (See map on page 6). The following day, a rapid rise in pressure to 1024 mb. (30.24 in.) and a secondary cold front passage with northwest winds of force 6, indicated that the Task Force was on the edge of the high. (See maps, pages 7, 8, 9). It would be possible to fuel during the next 24 hours. By midnight, the sky was clear and winds were light and variable. The pressure had risen to 1026.4 mb. (30.31 in.). The Task Force was in the center of a high pressure area.
The fueling of the heavy vessels was undertaken on 17 April when the Task Force was about 1,000 miles east of Tokyo. This operation was barely completed when the wind increased to gale velocity (wind south, 35 knots; sea rough; visibility, 1-2 miles). Another frontal system was approaching. All that evening it rained and the wind velocity remained high. On the next day, 18 April, it was expected that the Task Force, independent of accompanying destroyers which were detached due to fuel considerations, would be able to approach to within 500 miles of Tokyo. The planes were to take off at sundown and launch a night attack on the Japanese cities.
At midnight, the wind shifted from a southerly direction into the northwest, indicating the passage of the second cold front. The weather started to break - the steady rain turning into a scattered showery type weather. The frontal weather before midnight enabled the Task Force to slip through the outer line of Japanese pickets undetected.
ANALYSIS AND FORECAST FOR THE RAID
The weather map for 18 April (page 10) shows how this frontal passage made it possible to issue a reliable forecast for the Japanese area. Observation of all weather conditions before and after this frontal passage supported the hypo-thesis that it marked the outbreak of a fresh polar air mass over the Western Pacific. On this basis, it was logical to assume that Japan would be clear with scattered cumulus clouds. The variations in pressure along the course to be flown by the raiding aircraft were determined and approximate altimeter settings for the various portions of the route were given to the pilots. Tail winds for the last leg of the route were also forecast on the basis of the high pressure area over Japan.
Around 0300, two enemy vessels were spotted by our forces but it seemed unlikely that we were, in turn, spotted by them. Shortly before dawn, fighter patrol and search flights were launched which reported sighting and being sighted by an enemy patrol vessel. Some minutes later, another enemy patrol vessel came within 10,000 yards of our force and was sunk by a cruiser.
THE TAKE OFF
At this time, delay was no longer possible. The chances were great that the report of the presence of our force in enemy waters was relayed to Japan. It was essential that the bombers take off and that the task force retire at full speed. The first B-25 left the flight deck at 0820 at a distance of 650 miles from Tokyo. The sky was nine-tenths covered with stratocumulus at 1,000 feet. Visibility was good and the wind was 24 knots from the west-northwest. Winds aloft were northerly and also 24 knots up to 3,000 feet. The carrier was in maritime polar air, colder than the ocean, with the result that instability showers fell at intervals during the morning. All Army bombers took off without mishap - the last plane leaving the deck at 0921.
The force commenced retirement at 0927 on course 090°T and speed 25 knots. They picked up the same cold front that they had gone through the night before and rode along with it all the way back to Pearl Harbor where they arrived on 25 April.
The sequence of weather met by the pilots following their attack on the Japanese cities can best be explained by reference to the map on page 10. The weather over Japan was as forecast and the winds encountered over southern Japan were tail winds. However, a storm center had developed over the China Sea. The rain and low visibility incident to
0100 GCT 15 APRIL 1942
0100 GCT 16 APRIL 1942
1300 GCT 16 APRIL 1942
0100 GCT 16 APRIL 1942
0100 GCT 18 APRIL 1942
this frontal development forced the pilots to fly "instrument." Due to the lack of adequate navigational facilities, it was impossible for the planes to locate the designated airfields. The combination of inadequate fuel supply and adverse weather resulted in the loss of nearly all the planes taking part in the operation.
That the first raid against Japan was successful is now accepted fact. The result of this carefully planned blow against Japanese morale at a critical period in our history was invaluable.
Despite the fact that the entire operation was masterfully planned and executed, the element of chance involved now appears very large. Had weather postponed fueling operations or adverse conditions prohibited the take-off of the Army bombers at the expedient time, our force would have been exposed to detection in a most unfavorable disposition. Luckily, fortune favored both these operations and, in addition, provided cover for undetected penetration through the Japanese outer patrol.
The removal of this element of chance could have been accomplished to some extent by additional weather reports. To make the greatest use of weather as an element of strategy and tactics, it is essential to collect as complete weather information as possible. for operational forecasting, auxiliary observers, stationed temporarily in advanced positions where they can transmit on-the-spot weather information, can frequently round out the synoptic map in such a manner that more accurate analysis is possible.* Had such observers been used in this operation, a great deal of the risk inherent in the first raid on Japan would have been overcome.
* See Aerology and Amphibious Warfare "The Occupation of Kiska."