DISCUSSIONS 109

observing, partly maintained by a rotation of observers. Any effects due to variations in sea-surface temperature, which may be analogous to orographic effects over land would not affect the validity of the method because whatever the cause of the precipitation it should not matter to the observer who merely records what he sees falling. But if an observer is continually observing under, say, tropical conditions, he may well be so used to what we would call ' heavy ' precipitation that he might classify it as ' moderate.' In these circumstances the parameters would have to be re- evaluated. A rotation of observers therefore seems necessary in order to achieve uniform standards.

With regard to the measurement of precipitation on board ship, I understand that this long-standing problem is being reconsidered yet again by several organizations. I was interested in getting a fairly accurate immediate climatological picture.

I had not considered Dr. Frith's interesting point about the observer at sea positioning himself in the least exposed position and therefore ' seeing ' less precipitation. This may be true, but it may also be true that at land stations during periods of precipitation the observer's present-weather assessment is based on a view through a window after he has dashed out and back to the screen as quickly as possible. I do not think either habit would materially affect the observa- tion, but the effect of the ship itself on the airflow is difficult to assess.

Dr. Murgatroyd has asked for the relative contributions due to each of the x , y and z para- meters. In view of the arbitrary classifications I think little physical information can be gained from these. However, typical values of the rainfall in cm due to each parameter are given below for twelve months for ocean weather ships A, C and I

X Y A 72 42 C 79 14 I 57 28

z 11 2 4

These cannot be regarded as the relative contributions of light, moderate and heavy rain because various combinations of x , y and z are used elsewhere, e.g., in the shower category. The relative contributions of ' convective ' rain and ' dynamical' rain could be obtained if the 80 and 90 groups could be considered as representing ' convective ' rain, but it is probable that cloud types would also have to be considered for a true classification into these categories.

Because of the open network of weather ships over the North Atlantic Ocean, it is difficult to verify Dr. Stagg's suggestion that there may be a broad band of maximum precipitation from south of Newfoundland to south of Iceland. For each month the ships A, B and M had the highest precipitation estimates. It may be that the actual maximum is south of A and M (Kline, 1957, suggests this), but in the west the main depression tracks are located near B and well north of Newfoundland for most months.

REFERENCE Kline, W. H. 1957 ' Principal tracks and mean frequencies of cyclones and anti-

cyclones in the Northern Hemisphere,' US. Weather Bureau, Research Paper, No. 40, Washington, D.C.

551.501.7 : 551.547.5 : 551.509.313

Some experiments in the objective analysis of contour charts

By G. A. CORBY

(Read 21 June 1961. See Q.I., 87, P. 34)

Dr. J. M. STAGG (President) : As I understand this, a simple quadric surface can give a good account of the whole area of the contour chart except the centres of depressions, and Mr. Corby can make even these conform by elaborating the analysis. But wouldn't an alternative way be for the human analyst to provide the machine with his assessments of lowest points in the contour surface : with all the bits of information he has which cannot be fed into the machine

110 DISCUSSIONS

he could do that quickly and fairly accurately. The machine would probably be grateful for this help and it doesn’t seem to me to be undignified for a machine to accept human assistance in one of its difficult jobs.

Dr. C. D. WALSHAW : In connection with Dr. Stagg’s remark, I may mention that our com- puter at Cambridge, EDSAC 2, has a cathode-ray tube output which is frequently very useful in allowing the operator to watch the course of a calculation and to take appropriate action if necessary. An experienced analyst might sit at such a tube and press a button when he thought the depressions were getting too deep.

Mr. G. A. CORBY (in reply) : The objective analysis process described makes use of a forecast map which is given a lower weight than the observations. Essentially the forecast is used as a preliminary guess at the analysis and this guess is modified to accommodate the observations. In that sense the forecaster’s estimate of the chart, including the positions and depths of the depressions, is allowed to play a part in the analysis. I do not see how this estimate by the forecaster could take account of the recent data being analysed by the computer because at the appropriate stage in an operational analysis system there would not have been sufficient time for the plotting and scrutiny of the data.

For similar reasons monitoring the analysis on the basis of a cathode-ray tube display would not be practicable; unless the analysis was computed a few hours later, the forecaster would have no opportunity to see the data in convenient form and exercise his judgment. Furthermore, I would regard the need for such a procedure as a serious shortcoming in the objective analysis scheme.