246 



SCIENCE 



[N. S. Vol. XXXVI. No. 921 



FREE AIR DATA IN FORECASTING 



When the exploration of the upper air by 

 means of kites and balloons was first at- 

 tempted about fifteen years ago, many meteor- 

 ologists with more enthusiasm than judgment 

 stated that forecasting would soon be simpli- 

 fied because of the upper air data thus de- 

 rived, and that the weather of a week or a 

 month ahead would be foretold with as great 

 certainty as that for the next 36 hours was 

 then predicted. However, the application of 

 the knowledge gained from soundings in the 

 free air has not been a simple matter. In fact 

 the problem has been a very complex one, and 

 thus far investigations in aerology have been 

 mainly along the lines of pure science. The 

 normal condition of the free air is just now 

 beginning to be understood, and as yet, ab- 

 normal conditions and their relation to sub- 

 sequent weather changes at the ground have 

 not been thoroughly investigated. Various 

 changes aloft, particularly those relating to 

 wind velocity and direction, frequently be- 

 come apparent at the ground six to eight 

 hours later. If this is generally true, weather 

 data from aloft will never be particularly 

 serviceable to the forecaster because of the 

 short time interval involved. At Mount 

 Weather Observatory, one of the best 

 equipped aerological stations in the world, at- 

 tempts have been made, with fair success, to 

 use the data from aloft in connection with the 

 Washington forecasts. Moreover, after the 

 weather maps have been drawn the free air 

 conditions preceding and accompanying the 

 weather at the ground have been studied with 

 a view of determining the forecasting value 

 of the data from aloft. That the hopes ex- 

 pressed fifteen years ago have not yet been 

 realized may be seen from the following state- 

 ment of Professor A. J. Henry,^ Executive 

 Officer of the Observatory: 



While a large number of barometric depressions 

 passed over Mount Weather during the month 

 (March), a careful study of the upper air data on 

 the day previous to the advent of each depression 

 does not show, as yet, that decided changes are in 



'Monthly Weather Review, March, 1912, p. 473. 



progress whereby the course or the intensity of 

 the depression might be inferred. 



PERIODICITY IN PRESSURE VARIATIONS 



Well-marked periods of three and one half 

 years have been observed in the pressure varia- 

 tions at Port Darwin, in the northern part of 

 Australia. From a study of the mean monthly 

 values for 1878 to 1911, Dr. C. Braak° finds 

 that similar changes also occur in India and 

 throughout the Malay Archipelago, and as- 

 sociated with them are variations of tempera- 

 ture with maxima half-way between the 

 maxima of pressure. It has generally been 

 supposed that solar changes account for the 

 temperature variations, which in turn produce 

 variations in pressure. However, when the 

 temperatures and pressures are plotted it ap- 

 pears that a pressure wave resembles the tem- 

 perature wave which follows it rather than the 

 one which precedes it, indicating that the 

 changes in pressure cause the changes in tem- 

 perature, and not conversely. Dr. Braak 

 describes the process somewhat as follows : 

 Through some cause pressure becomes high 

 over the region. The wind thereupon becomes 

 feebler and the air takes less part in the gen- 

 eral circulation than usual. Diminished air 

 circulation means diminished oceanic circula- 

 tion; consequently air and water become rela- 

 tively stagnant, and are hence subject to the 

 continued action of the tropical sun, which 

 gradually increases the temperature, and 

 finally produces a maximum in the tempera- 

 ture curve. The increase goes on until the 

 effect of the high temperature on the pres- 

 sure is to diminish it and eventually to re- 

 place it with a generally low pressure. Condi- 

 tions are now reversed, for low pressure means 

 increased wind velocity, and a considerable 

 interchange of air and water between equa- 

 torial and temperate regions, resulting in a 

 lowering of temperature in the equatorial re- 

 gion. Ultimately this temperature change 

 increases the pressure, until high pressure is 

 once more restored and the cycle begins again. 

 The pressure and temperature changes are 



' Meteorologische Zeitschrift, Vol. 29, pp. 1-7. 



