How can we obtain accurate weather data about vast spaces in the middle of the Atlantic where storms can die out, maintain themselves, or intensify, without revealing their behavior to forecasters on either side of the ocean? That was the question asked and answered by officers of a USAAF weather reconnaissance squadron during the past two years. What they learned during the course of their investigation makes it fairly certain that scheduled operations along North Atlantic air routes particularly flights from east to west may never again be drastically curtailed during winter months.
The method used was simplicity itself. They equipped aircraft with special meteorological and navigational instruments, put a trained forecaster-observer aboard as one of the crew and sent them out over the North Atlantic routes.
From that calm appraisal of the weather problem grew a carefully-planned weather reconnaissance project which has proved that year-round operations in the North Atlantic are practical. Military pilots traveling the short northern routes between the United States and Europe are now putting the new flight technique developed during the weather hops through its first exhaustive tests.
Here is the first complete account of the history-making project, as told by Lieut Robert A Doremus of the Eighth Weather Squadron and Lieut Charles D Brewster of the First (formerly the 30th) Weather Reconnaissance Squadron, who are attached to the North Atlantic Division of the Army Air Transport Command:
Early in 1943 the 30th Weather Reconnaissance Squadron was organized for the specific purpose of safeguarding movements of military aircraft over the northern routes of the Atlantic. The squadron was equipped with a number of Mitchells stripped of their armament and provided with bomb bay fuel tanks and a few special meteorological instruments. The bombardier's "greenhouse" made a perfect observation post for the meteorologist, and the speed, range and service ceiling of these airplanes were admirably suited to weather reconnaissance over these medium distances.
The routes extended in jumps of from 700 to 800 miles, from Presque Isle, ME, through Labrador, Greenland and Iceland to far northern Scotland. The squadron operated continuously throughout the summer and fall of 1943 and flew a total of 1,100 hours in all kinds of weather.
These planes were essentially pathfinders for ferrying movements of military aircraft. They took off well in advance of a projected flight and radioed back frequent reports on weather along the route. From such information ground forecasters could construct a cross-section chart of the atmosphere, making a copy for each pilot about to fly the same route.
As a further guide for the control officers responsible for planning such flights, a reconnaissance plane nearing the end of its trip would radio a coded message summarizing the state of the weather in terms of the kinds of planes which could safely fly in it. For example, "pursuit craft in formation" (requiring extremely favorable condition) or "individual experienced pilot" (requiring much less favorable conditions). By such means the routes were used to their capacity without unnecessarily endangering the lives of the crews though none of the flights were picnics since risk could never be entirely avoided.
The AAF project was not, of course, the very first weather reconnaissance work on the North Atlantic. To provide forecasters with information necessary to plan the operation of fighter aircraft in the Battle of Britain and of bombardment craft in the growing attacks on the continent, the British established reconnaissance flights westward over the ocean. Each day planes took off from bases on the coast, flying westward, southwestward and northward, measuring pressure, winds and temperature and charting the cloud systems and storms and returning to their stations. The reports from such flights could be used on the daily weather maps just like land reports. And they measurably increased the accuracy of forecasts for Britain and the continent at a time when accuracy was of enormous importance. Similar flights later were based on Gibraltar, Iceland and Nova Scotia. Some of the original routes are now being flown by American personnel in American planes.
Meanwhile, the 30th Weather Reconnaissance Squadron was preparing to expand its operation with additional Mitchells and a flight of Flying Fortresses and also to shift the emphasis in the type of work done. This change was progressively away from reconnaissance (designed for the clearance of transoceanic air travel over one prescribed path) toward scheduled daily flights along routes that covered the Atlantic's most doubtful weather areas. The information from such flights (patterned after the British method) is used not so much to clear specific movements of tactical aircraft as to supplement the data used in drawing weather maps and analyzing current weather systems. Well-drawn maps and accurate analysis enable the meteorologist to forecast weather for several routes rather than for just one.
These synoptic flights were arranged to produce information for both the 10.000-foot chart and the surface map drawn by forecasters. Time of flights is chosen to coincide with established map dates. Aircraft fly out from their bases along predetermined routes, sending reports at prearranged positions every 100 miles; the trip out is made near 10,000 feet and the trip back near the surface, or vice versa.
For the most accurate reports, it is necessary to fly as close as possible to the level for which information is desired. One flight, for example, makes its surface observation from an altitude of 500 feet above the waters of the North Atlantic it must fly 800 miles at this low level.
Toward the end of 1943 route reconnaissance around the northern rim of the Atlantic had almost ceased for the winter. Of course, British reconnaissance flights covered the weather immediately west of the British Isles but there were vast spaces in the middle of the ocean where storms could maintain themselves without revealing their behavior. Yet it was still necessary to fly heavy bombers from Newfoundland and Labrador to Britain all during the winter.
It was also necessary that at least a few cargo aircraft be kept in operation over similar routes. Their only other route was through the Caribbean and the relatively circuitous paths of the South Atlantic. The projected synoptic flights of the 30th Weather Reconnaissance could not reach the center of the ocean. How could this gap in the weather data be bridged?
The solution was to use the cargo planes themselves. Most of the load capacity of a Skymaster, for instance, could be devoted to fuel, so that its range became enormous enough to span the North Atlantic in either direction in any but the very strongest headwinds. Their service ceilings of 25,000 feet would enable them to climb over most storms. These aircraft were designed for long-distance flying and trained crews were ready to operate them.
A plan for experimental winter operation of Skymasters was therefore put into operation. There were several purposes: first, to prove the feasibility of operating transport aircraft over existing North Atlantic routes in the winter; second, to prove that such operation would be practical with regard to the useful cargo carried and the time involved; third, to answer questions concerning operational procedures, flight techniques, instrument development, meteorological research, and special equipment that might be required to effect a safe and reliable operation; fourth, to collect useful meteorological data to be used immediately by the Air Transport Command for the movement of tactical aircraft over the North Atlantic route during the winter months, as well as to gather weather data suitable for later analysis. The aim was to meet both immediate and long-range needs.
Three standard Douglas Skymasters were assigned to the project four-engined craft whose wing fuel tanks were supplemented by extra tanks within the fuselage that doubled fuel capacity. The planes were operated by American Airlines, Transcontinental & Western Air and the Air Transport Command. The airline planes were based respectively at LaGuardia Field and Washington, DC, and were operated by airline personnel. The ATC plane was based at Presque Isle, ME, and flown by Army Air Forces personnel. To each plane were assigned two Air Force weather officers of the Eighth Weather Squadron, especially trained to observe weather from the air, the two men making alternate round-trip flights. The eastern terminus of each hop was Prestwick, Scotland.
The two airline planes generally flew from their bases to Newfoundland, then directly across the ocean, aided by prevailing westerly winds. They returned directly if wind and weather permitted, or by way of Iceland (sometimes Greenland) or, if necessary, along the North African-US route. The Wing plane more commonly flew by way of Labrador and Iceland, with occasional stops in southern Greenland. Thus, all North Atlantic routes were covered in achieving the four-fold purpose of the missions.
To achieve other objectives originally set up development of techniques and instruments and the gathering of meteorological data these three aircraft were provided with special equipment. They contained, in addition to the usual pressure altimeter, a radio altimeter measuring absolute height above terrain. Each had a new and secret radio aid to navigation, making possible the determination of a plane's position to the nearest mile, day or night, without sight of the stars or earth surface. There were aerographs whose three pens recorded on a drum rotated by clockwork the barometric pressure, temperature, and humidity of the air outside the plane. An especially accurate pair of mercury-bulb thermometers secured wet and dry bulb temperature readings.
This and similar equipment subsequently installed on planes of the 30th Weather Reconnaissance Squadron compiled the most accurate information ever collected on winter weather over the North Atlantic.
The aerograph was chiefly useful in providing a continuous record of temperature and humidity. Its record of pressure indicates whether changes in temperature and humidity were due to changes in altitude of the airplane or to actual differences in the air mass through which the plane was flying.
It was originally planned to use two thermometers, one dry bulb and one wet bulb, to determine humidity at any point. The aerograph uses the expansion and contraction of human hairs to measure moisture. Such a hair hygrograph, besides being a notoriously sluggish and fickle instrument, requires, when exposed to a 200 mph airstream, certain corrections which are but imperfectly understood. Unfortunately, the wet bulb also requires corrections and failure to find out just how much of a change occurs led to the abandonment of the wet bulb readings and the development by Signal Corps of a new psychrometer, wherein the air stream is slowed down relative to the plane to a speed at which no appreciable heating occurs at the thermometer bulbs. Very complicated corrections still must be made, but they can now be exactly determined. It is now possible to measure relative humidity from an airplane with an error of only one or two percent.
More important even than temperatures to meteorological forecasting are the winds aloft. Wind fixes were made possible by special radio navigation equipment which afforded a ready means of determining the wind directly when such standard methods as double drift observations were not possible. It was found that not only winds but barometric pressures could be ascertained through the combined use of pressure and radio altimeters.
A pressure altimeter is merely a sensitive aneroid barometer provided with a dial which translates pressure into altitude according to an assumed relationship. It has a zero-setting device to take into account variations in the sea level pressure. For any given setting, it is always possible to convert an altitude reading in feet into pressure millibars at flight level. And flight level is determined by the radio altimeter which, over the ocean, measures the exact height of the airplane above the water surface.
From the pressure at flight level the meteorologist can easily calculate the pressure at the standard levels (such as 5,000 and 10,000 feet) for which he draws his upper air charts. A line of such hourly observations from a plane flying across the Atlantic fills in some very empty spaces on his map.
Still another use is made of the readings from pressure and radio altimeters. A plane flying at a constant pressure altitude is flying along what the meteorologist calls an "isobaric surface" an imaginary surface in the atmosphere over which barometric pressure is everywhere constant. If the airplane flying at this constant pressure altitude measures its absolute height from time to time, it is also measuring the variations in height of that isobaric surface, somewhat as a surveying party measures the variations in height of the earth's surface.
From all this data the meteorologist can determine the force of the wind perpendicular to the true heading at the plane's flight level and its subsequent drift, or angle at which the wind is blowing him off course. This is of considerable value to a plane not equipped with the new radio system of navigation equipment much less common than a radio altimeter for the method can be used when neither sky nor sea is visible from the plane.
Accompanying all such instrument data were the direct visual observations made by the trained forecaster-observers. The extent and kind of clouds, the amount and duration of icing, the severity of frontal systems all these were carefully recorded on cross-sectional charts of the atmosphere, reported to the forecasters on each side of the ocean and subjected to later research and statistical analysis.
A few figures will show the amount of material available for such analysis and indicate the scope of operations. From the first flight on January 7, 1944, to the end of March, these airplanes made 40 round trips between North America and the United Kingdom with a few shorter trips to Greenland and Iceland. Total flying time amounted to 945.5 hours. The weather officers drew 113 atmospheric cross-sections (one for each flight between any two terminals) and passed through 88 fronts. The highest altitude flown was 23,500 feet; the average was near 10,000. Length of continuous flight ranged from slightly more than two hours to 17.5 hours, the latter on a direct flight from Prestwick, Scotland to Presque Isle, ME.
This record speaks for only one three-month period in one winter. Yet analysis of the reports yields very significant indications of North Atlantic weather in general. In most kinds of fronts, for example, it was found that there is an optimum flight level where there is a better than 50 percent chance that no clouds will be found.
Icing was reported in various forms 112 times. But it proved less of a hardship to winter flying than might be supposed, simply because it was generally possible to fly at altitudes where icing was nonexistent or not dangerous, even though moderate to severe icing was might exist at other levels on the same route. Terminal conditions and adverse winds were the most common causes of delays and rerouting, yet there were only 10 such cases out of the 69 flights flown during January and February.
It was also shown that storms could sometimes actually be used to advantage. Winds in the northern hemisphere blow counter-clockwise around the low pressure center associated with all severe storms. If such a storm lay directly on the projected flight course, it was sometimes found possible to fly a special course to the south, going east or to the north, going west detouring the storm center but taking advantage of its winds. Distance for an airplane is measured in hours of flying time, not in miles of ground distance. To fly a roundabout course with tailwinds often means shorter flight time than a direct course without tailwinds. Detouring a storm center also means less turbulence or bumpy flying, which causes fatigue to the crew and discomfort to the passengers. The weather officer's actual routine in the air is not simple. He must first of all keep a close watch on the amount, kind and height of clouds, and draw them accurately on his atmospheric cross-section. Not always easy in the daytime, it is a particularly difficult task at night. It is possible, however, even in the absence of the moon, to detect the presence of clouds below the plane and to judge their altitude if there are breaks so that the speed with which the dark spaces seem to move may be used as a gauge of height. Clouds above the plane are apparent from the disappearance of stars.
While he is not keeping watch on the clouds, precipitation and icing, the observer makes wet and dry bulb temperature measurements. He must also keep watch on the traces made by the aerograph pens, looking particularly for temperature changes marking fronts. At least every half hour he makes simultaneous readings of the pressure and radio altimeter. This is complicated by the tendency of the airplanes in flight to change altitude slightly but continually, so that both altimeters must be read at the same time, and by the fact that the radio altimeter, although a remarkably accurate instrument, is quite difficult to read exactly.
The observer must, once an hour, set down his observations in a number code for a radio message. This coded form in turn must be enciphered so that the enemy may not intercept and profit by it.
The ciphered messages are given to the plane's radio operator, who transmits them to one of the regular stations of the Army Airways Communications System. Taken at once to the weather office, the message is there deciphered. If it is of particular importance (as in locating a severe front), it is relayed by radio-teletype within an hour to all the important weather stations on the North Atlantic routes. At the end of the flight the observer gives all his messages to the weather station at his terminal of arrival, and a complete set is relayed by radio-teletype throughout the region. Forecasters all over the North Atlantic thus have available a complete line of reports from the point of departure to the terminal of arrival a comprehensive record of winds, pressures, clouds and weather along the route. Combined with daily flights of the 30th Weather Reconnaissance Service, these route reports give an unparalleled coverage of North Atlantic weather.
This article was originally published in the March, 1945, issue of Flying magazine, vol 36, no 3, pp 30-32, 140, 144.
The PDF of this article includes photos of a weather front over Iceland, a B-17 over Greenland, weathermen, a weather briefing, and a flight crew.
Photos credited to ATC.