During the past decade the interest of the naval service, as well as that of the world at large, in the matter of weather phenomena has been very rapidly increasing. In every newspaper we read of droughts, floods, tornadoes, hurricanes, dust storms, and tidal waves. Weather has always been with us but, until fairly recently, the general tendency of the layman has been to consider the vagaries of the weather as acts of God and to take refuge from them only in prayer. Weather phenomena may still be considered as acts of God but the realization is dawning that “they cast their shadows before” and that while they cannot be prevented they may, at least to some extent, be anticipated and prepared for. It is only the unknown that strikes terror to the heart of the brave soldier and the easiest way to remove the unknown is to understand it.
The rapid growth of aviation has been chiefly responsible for the rapid growth of interest in, and increased study of, weather phenomena. However such study, and the results of such study, have been proving increasingly valuable in gunnery, tactics, strategy, and in fact in every branch of the naval service. Forecasting is as yet far from perfect, but forecasting improves only as the understanding of the phenomena involved improves, and as the demand for correct forecasts increases. It is for this reason that the Navy Department has added questions on the Polar Front Theory to officers’ promotion examinations. To be prepared for high command an officer should know and understand his weather just as he knows and understands his engineering, his gunnery, and his navigation. In the crisis that comes with battle, it may be more important.
The writer recently accompanied Major E. H. Bowie, of the U. S. Weather Bureau, on a tour of the weather observatories in the Orient. While the nations of the Far East have not as yet achieved the international co-operation which is necessary to an effective weather reporting system such as is now in use in Europe, they have made remarkable progress in their individual efforts to solve their weather problems. The Japanese Weather Bureau can probably be numbered among the most complete and effective organizations of its kind in the world. Its forecasters and observers are thoroughly trained and their characteristic attention to detail is producing excellent results. For example, they forecast and track individual thunderstorms and, on the basis of such Weather Bureau advices, power is shifted from threatened transmission lines to duplicate lines which are not threatened by the storm. The relatively young Republic of China is now developing a weather service that should within a few years be outstanding. China has been for years without a national weather service. The fact that one is included among the first items in their present program of modernization proves that its importance has not been underestimated. There is no doubt that Bjerknes, Bergeron, and the Polar Front have penetrated to the inner recesses of China and Japan.
During our voyage from California to Guam, the Philippines, China, and Japan in the U.S.S. Ramapo, we had the doubtful pleasure of coming into more or less intimate contact with five separate typhoons. During this period we were drawing complete weather maps of the Pacific Ocean in addition to our local observations. From these observations, together with the reports received from neighboring stations, some ideas were formed as to the structure and cause of these typhoons; ideas not strictly in accordance with the generally accepted ideas on the subject. Of course the observation of five, or even a dozen, typhoons from very few stations is not sufficient to prove beyond contradiction their cause and their structure. However, it is believed that the ideas are worth setting down for verification or refutation by other observers. Acknowledgment is made to Major Bowie for his suggestions and assistance. The ideas expressed herein are our joint ideas, arrived at through discussion of each individual problem.
In practically all modern works on the subject of meteorology, cyclones are divided into two general classes: tropical cyclones and extra-tropical cyclones. Tropical cyclones include all rotating storms formed in the tropical regions. They are known as “hurricanes” in the Atlantic, as “typhoons” in the Pacific, and as “cyclones” in the Indian Ocean. All of these are similar in characteristics and are generally admitted to possess the same causes and to follow the same laws of development and movement. The extratropical cyclones are those which are formed in upper and middle latitudes. They have been assumed to have different causes and to follow different laws of movement than the tropical type.
Until the last decade, tropical cyclones were generally believed to be purely convectional phenomena. It was believed that air flowed southward to the region of the doldrums or the vicinity of the thermal equator where it became stagnant and heated to such an extent that the hot air would rise vertically, precipitating a part of its high-water vapor content and flowing off horizontally at high levels. The rising air would be further heated by the heat released by condensation thereby increasing the rate of ascent. The ascending air would cause a lower pressure at the center of the ascending current. The earth’s rotation would set up a whirling movement in the air which tended to flow into the low-pressure area. Thus a tropical storm was set in motion which drifted westerly in the equatorial current and recurved to the eastward as it moved out of the equatorial current into middle latitudes. This theory was satisfactory in some respects but it failed to explain many of the attendant phenomena and it offered practically no means of predicting the formation and movement of the cyclones other than experience and past performances.
In the meantime the Norwegian school of meteorology, under the leadership of Bjerknes and Bergeron, developed the Polar Front Theory and showed its application to extra-tropical cyclones. The Polar Front Theory may be briefly stated as follows:
The atmosphere receives more heat from the sun in the equatorial regions than it radiates. It radiates more heat in the Polar regions than it receives. Therefore in order to maintain a temperature and pressure balance, cold air from the polar regions must flow southward and warm air from the equatorial regions must flow northward. When air tends to flow from areas of high pressure to areas of low pressure in order to bring the atmosphere into equilibrium, the air stream is deflected to the right by the rotation of the earth. This force causes the air to flow nearly parallel to the isobars instead of across them. This combination of forces makes it necessary for the exchange of air to take place by means of air masses, i-e., globules, or bubbles of whirling cold air breaking away from the areas where greatest cooling occurs and moving irregularly southward. Naturally these masses of air, depending on place of origin, season, age, path of movement, etc., will have different characteristics as to temperature and humidity, and therefore density.
From the observations taken at a number of stations, lines can be drawn separating these air masses of different characteristics. These lines of discontinuity are called “fronts.” If a colder air mass is displacing a warmer in a given area, the front is a “cold front.” If warmer air is replacing colder air that is moving off, the line of discontinuity is a “warm front.” When a warm and a relatively cold air mass are flowing side by side in opposite directions, a whirl or cyclone may develop between them in much the same way that a whirlpool develops between two oppositely flowing streams of water. These cyclones will, for physical reasons, move along the front or discontinuity in the direction of the free air isobars of the warm sector. The discontinuity on the cold side will be wedge-shaped, underrunning the warmer air while the advancing warm air will flow over the cool air ahead of the cold front. These interactions cause what is called the warm front and cold front precipitation.
The Polar Front Theory has now been accepted by leading meteorologists of all nations as the correct explanation of weather phenomena in connection with extra-tropical cyclones. More recently, researchers have been finding evidence of a tropical front or line of discontinuity in the general vicinity of the thermal equator separating air masses of the northern and southern hemispheres. Still more evidence has been found that all tropical storms originate along this front. Here, however, the northern and southern air masses do not have the distinct differences in temperature and humidity that may be observed in connection with the Polar Front. On account of the distribution of land and water over the surface of the globe, both northern and southern air masses generally arrive at the Equatorial Front after a long journey over water. Both masses are warm as compared to the polar masses and both are humid in the surface layers. The principal difference in temperature and humidity, if any, must be found at some distance above the surface. Unfortunately very few observations of upper air temperature and humidity are made along the Equatorial Front.
Several researchers have attempted, but without success, to apply the principles of the Polar Front to the tropical cyclones forming on the Equatorial Front. The difficulties are principally the following (see Fig. 1); typhoons of the Western Pacific are used for illustration (but the arguments apply equally to West Indian hurricanes and Indian Ocean cyclones) and the warm sector is assumed to be the south:
(1) By the physical laws involved in the Polar Front Theory, the storm should move eastward along the isobars of the warm sector. Actually, it invariably moves to the westward. (2) Since the warm sector is to the south, then as you ascend through the less dense air in that area you must eventually come to a point where the pressure is higher to the south than at a corresponding level to the north in the colder air. This pressure distribution would require a wind flow from west to east at higher altitudes. Actual observations show an upper-air wind-flow from east to west. (3) The area of maximum rainfall as observed and charted by Cline is in the right front of the cyclone, whereas, by the Polar Front Theory, it should be ahead of the warm front as shown in Fig. 1, or in the right rear. (4) The cold front actually backs into the so-called cold air and the warm front into the warm air which is, by physical laws, an absurdity. Since these difficulties are apparently insuperable, the researcher’s tendency has been to give up and to conclude that the tropical cyclone is a different breed of cats.”
Having discussed these problems in connection with several typhoons observed on our weather maps, and having passed through the edges of three typhoons and near the center of one, we came to the conclusion that the explanation was so simple that, like the “Purloined Letter,” it has been overlooked because of its very simplicity. In short, the warm sector must be on the north side.
This conclusion was brought about not only by theoretical discussion, but also by actual observation. As the Ramapo approached Guam, on August 28, 1936, we Were in a typical warm sector—temperature 88 degrees, humidity 90 per cent, sky partly cloudy with cumulus clouds, wind northeast force three. Before arrival we passed through the warm front which lay just to the north of the island. The temperature dropped to 80, and wind became light westerly. On the following day, the disturbance which had been forming to the northeast of Guam overtook us and passed about 100 miles north of us, moving west. Our winds increased to fresh west to southwest, and the temperature dropped to 76 degrees. In the meantime the Army transport Meigs, which was behind us and well to the north of the Equatorial Front, was reporting ENE winds and a temperature of 86 degrees.
Two days later a second disturbance approached, following in the track of the first. Our surface winds veered from southwest to westerly. Just before the clouds moved in, a pilot balloon sounding was obtained. Our upper winds had shifted from southwest to northeast. The situation was later analyzed as shown in Fig. 2.
Later during the voyage while en route from Chefoo, China, to Yokohama, October 1-4, 1936, we had opportunity to observe at close range a typhoon in the process of recurving. Complete reports were received from both ship and land stations through the Japanese Central Meteorological Office at Tokyo, at 6-hour intervals. The analysis of the successive positions of this typhoon is shown in Fig. 3. The Ramapo passed through the warm front from the warm sector at 0930 on October 3. On passing from the warm into the cold sector, the temperature dropped from 80 to 71 in the space of 30 minutes and humidity dropped from 95 to 80 per cent in the same period.
The assumption that a tropical storm has a warm sector and a cold sector and that the warm sector is on the north side is, after all, the only logical solution of the problem. If the Polar Front Theory holds good for extra-tropical cyclones, there is no reason why it should not also be applicable to tropical cyclones. The laws of physics are not altered by a mere matter of geographical location. Tropical storms form most frequently in the Northern Hemisphere when the thermal equator is farthest north, but the thermal equator and the Equatorial Front are not necessarily identical. The season when they are most frequent occurs during the winter of the Southern Hemisphere. The outbreaks of cold air from the south polar region move northward and surge against the Equatorial Front in the Northern Hemisphere. The region north of the Equatorial Front is at the height of its hot season. The greater proportion of land area in the Northern Hemisphere adds to this heat imbalance. It is therefore only to be expected that the air north of the Equatorial Front will be warmer than that to the south. The storms are found to form in the western parts of the oceans. This is adequately explained by the long trajectory required to give the northern warm winds their high humidity, and by the presence of winter cold in the Australian, South American, and African continents to give the southern winds their greatest cooling. The slowing down and recurving of the storms are the logical development of the application of the Polar Front principles. A glance at practically any September weather map will indicate the gradual warming of the trades as they progress across the Pacific. Honolulu will have a temperature of the order of 78°F., Midway 78° (though farther north), Wake 86°, and Guam 88°F. (except when it is south of the Equatorial Front). While the temperature and humidity of the air north of the Equatorial Front is not much greater at she surface than that of the air from the touth side, it is believed that representative soundings in this area will show that the difference increases with altitude.
Of course there is one marked difference between tropical and extra-tropical cyclones. That is that the principal source of the energy which intensifies the tropical storm is the condensation of water vapor, whereas the principal source of energy for the extra-tropical cyclone is the difference of temperature in the two adjacent air masses. This, however, is not a fundamental difference. Both sources of energy are present in the tropical and in the extratropical cyclones. There is only a difference in degree. It is hoped that further observations and research will be made on this question in the near future. However, until it can be shown that our analyses are in error, we believe that the tropical cyclone forms and moves in accordance with exactly the same laws that govern the formation and movement of extra-tropical cyclones.