If a relation between the number densation nuclei and estimating the of fog particles, water content of fog probable amount of supersaturation and visibility were known, we could from the diagram, substantially im- after measuring the number of con- prove forecasting of fog. 6. TRANSFORMATION OF THE DIAGRAM Equations la, 2a and 3a may be written as follows: / e ew x T-Tw = 0.622 (4) Cp \p-e p-ew J Lw Te - Tw = 0.622 (-2- — ^. (5) \p-0 p-ew J T-T a =-- 0.622 (-1 ^y (6) cP \p-e p-0 J Consider e and ew small in compari- ACKNOWLEDGMENTS son with p, then again: in a T, e - The present note is inspired by the diagram the points (T,e), (Tw, ew) lectures of Dr. Sverre Petterssen on and (Te,0) lie on a straight line weather analysis and forecasting, held with a slope to the T-axis at the Oslo University in spring 1939. I am greatly indebted to Dr. Hessel- arc tan (- - Xp). 0.622 Lw berg, director of the Norwegian Me- After construction of the b, e -lines teorological Service and Dr. Petterssen and the relative humidity-curves the for their permission in joining this different elements can be found in the course. same way as explained in paragraph Dr. Postma and Mr. van Zutphen 3. The slope of the 6, e- lines in a gave very valuable assistance in cal- T, e- diagram is dependent on the culation of the diagram. REFERENCES pressure. The relations between tem- Von Bezold, 1905, Wissenschaftliche Luftfahr- perature, wet-bulb temperature, equiv- ten, vol. 3, Vieweg & Sohn. Braunschweig. Brunt, 1934, Physical and Dynamical Mete- alent temperature, water vapour pres- orology. Cambridge University Press. Brunt, 1935, Quarterly Journal Royal Mete- sure and dew point at different pres- orological Society, vol. 61, 1935, pp. 213- sures can not be obtained from a T, e 216. Normand, 1921, Memoirs of the Indian Mete- — diagram with only one set of 6, e orological Department, vol. 13, part I. Petterssen, 1939, Geofysiske Publikasjoneri Vol. - lines. XII, No. 10. Certain Statistical Relationships Bearings on the Preparation of Five-Day Weather Forecasts in the United States* H. C. WlLLETT Massachusetts Institute of Technology, Cambridge HE FOLLOWING REMARKS are in- M. I. T. This project is being sup- T tended to indicate in a prelimi- ported in part by the Bankhead-Jones nary manner some of the leads Fund through the Weather Bureau. which we have tried to follow, and the It has been realized since the daily tentative conclusions reached in our analysis of the Northern Hemisphere attempt to formulate a basis for the map was started at M. I. T. in the preparation of five-day weather fore- fall of 1936, that variations of the casts at M. I. T. This constitutes the general circulation pattern, as defined immediate primary aim of the synop- by the intensity and position of the tic portion of the long-range weather- •Read at the Washington Meeting, April, forecast project being carried on at 1939 Unauthenticated | Downloaded 10/02/21 04:11 AM UTC principal centers of action of the trough of low pressure over the con- northern hemisphere, largely deter- tinent, which in turn shifts its posi- mine the persistent anomalies of the tion eastward or westward, depending meteorological elements which are ob- upon the position and extent of the served on this hemisphere. At the Bermuda High. In other portions of same time, little if any practical basis the country the correlation between for the forecasting of such changes in the orientation of the isobars and the the general circulation has been recog- temperature is less pronounced. This nized up to the present. probably follows from the fact that in During the past winter, in connec- other portions of the country the cir- tion with the preparation of five-day culation aloft may be either cyclonic forecasts for certain definite regions or anticyclonic, a difference which has in the United States, an attempt has important thermal implications. In been made (1) to initiate a statistical the northeast it is nearly always cy- check as to the extent to which ano- clonic in winter. During the summer malies of temperature and precipita- this ceases to be the case, so that it tion for the five-day forecast periods seems doubtful if this relation would may be correlated with those features hold as well during the warm season. of the general circulation which might Precipitation fails to show much be expected to affect the individual correlation with the general pressure forecast regions, and (2) to find some distribution. This fact suggests that basis for (forecasting important lapse rates and frontal action are changes in the general circulation. more important than warmth or mois- The record is still much too short to ture content of the atmosphere in pro- draw final conclusions, but some ducing anomalies of this element. In- facts of interest seem rather well es- dications are that lapse rates, whether tablished. Temperature as an ele- dependent upon the production of ment correlates more closely with the unstable air masses or upon the turn- mean state of the general circulation, ing of the wind with elevation, must as given by the mean pressure distri- be given careful study in conjunction bution, than does precipitation. If in with the general circulation pattern. the northeastern United States, the A general synoptic investigation of 3-km mean-pressure map is used in- this problem is under way, and some stead of the surface pressure, sur- preliminary empirical statistical re- prisingly high correlation is shown. sults will be discussed by Mr. Namias A correlation of +0.75 was shown in the paper he is giving before the for the New York area between the American Geophysical Union. (Cf. temperature anomaly for each five- Trans. A. G. U., 1939, Pt. Hi;. day period from October 23, 1938, to The really important problem to be April 13, 1939, and the northward or solved, however, if long-range weather southward transport cf air at Lat. forecasting is going to make progress, 40 °N and Long. 80 °W as given by is that of arriving at some ra- the 3-km mean-pressure map for the tional basis of anticipating important same period. Evidently, then, a fore- changes in the general circulation cast of the general orientation of the pattern. There seems to be little doubt isobars at this level will serve as the that if the essential character of the basis of the temperature forecast. general circulation pattern can be Now this orientation of the isobars forecast, that for most regions it will aloft depends upon the location of the be possible to develop a reasonably characteristic upper-level winter sound basis for anticipating impor- Unauthenticated | Downloaded 10/02/21 04:11 AM UTC tant temperature and precipitation synoptic fact is that in general a anomalies. The first lead in this di- slowing down of the general zonal rection which was checked up on sta- circulation of the northern hemisphere tistically was that expressed in the should be accompanied by a tendency frequent assumption that intense cy- of the centers of action to move clogenesis in one region (deepening of westward, whereas a general Speeding a cyclonic center of action) will be up should go with a tendency for followed by a corresponding anticy- them to move eastward. As they are clogenesis in another region. This displaced from their normal loca- was checked, for the Aleutian Low tion, they should tend to weaken, and and the Icelandic Low, with the re- a new center to be established near gion to the eastward of the Aleutian the point where they are normally Low, to the westward of the Icelandic. maintained. The tendency for such The former case showed a quite pro- movements to occur has been found nounced positive correlation, but with statistically to be quite pronounced. almost no time lag. The coefficient The forecasting significance of this amounted to 0.56 for cases in which relationship lies to some extent in the the Aleutian Low reached an intensity fact that there is some lag tendency lower than 29.10 inches, for concur- between the changes in the intensity rent changes. In the case of the Ice- of the general circulation and the sub- landic Low only small negative cor- sequent movement of the centers of relation was obtained, which seems to action. However, this lag effect show conclusively thai only to the amounts to less than one five day eastward of the rapidly deepening period, so that it is not directly of center is active anticyclogenesis to great assistance in focecasting. But be anticipated, and that without lag. the more important fact, from the When, however, deepening of the forecasting angle, is that there is a Aleutian Low in a specific region was rather pronounced regularity shown correlated with anticyciogenesis at a in the variations of the intensity of fixed point to the eastward, negligibly the general zonal circulation, such small coefficients were obtained. Thus that changes in this intensity, at least a correct forecast of the deepening of in winter, may be rather well anti- the Aleutian Low could scarcely be cipated. It is hoped that further used to forecast anticyclogenesis in a synoptic investigation of these changes definite locality over North America. will furnish a much better basis for The most successful lead in fore- anticipating the future general circu- casting significant changes in the lation pattern than we have had up to general circulation pattern has been the present.