Page:EB1922 - Volume 31.djvu/980

930

TABLE II. Mean Pressure. (Millebars)

4->

JB M

'S a EJ2

Petrograd

Scotland

Manches- ter

c "u

B

m

TJ C.

"ltd ft/5

u

en


 * c

a

P-,

1 J3

C &

C/)

1 3

>

_rt 1


 * l

B 3

Sw

Toronto

&

a

3

cr W

20

55-o

55-o

55-2

54-8

54-9

56-0

54-7

55-o

54-8

54-9

53

19

64-0

64-2

64-6

64-0

64-1

65-6

64-0

64-4

64-0

64-1

63

1 8

74-5

74-8

75-4

74-8

75-o

76-6

74-8

75-2

75-o

75-o

75

17

87-0

87-3

88-0

87-4

87-5

89-6

87-6

88-0

87-6

87-8

90

16

IOI

102

103

103

1 02

105

1 02

103

103

102

107

15

118

118

I2O

I2O

I2O

123

120

121

121

1 2O

120

128

H

138

138

I4O

140

140

143

141

142

142

140

142

'52

13

161

161

164

164

I6 4

167

165

165

165

164

167

178

12

187

187

192

192

192

195

193

193

194

192

195

209

II

218

219

224

225

224

228

226

226

227

225

228

244

IO

255

256

261

262

261

266

263

263

264

262

266

283

9

297

299

32

305

303

309

307

306

307

305

309

327

8

346

348

352

354

352

357

355

354

356

353

358

376

7

400

402

407

408

407

412

410

409

412

408

413

430

6

461

464

468

470

469

473

472

471

474

470

475

491

5

529

532

537

538

538

541

54

539

542

538

543

558

4

606

608

613

614

615

617

616

615

618

614

618

632

3

692

694

698

699

699

701

700

700

703

699

73

713

2

787

787

793

795

795

796

794

795

797

794

798

803

I

896

894

898

900

900

900

900

900

901

899

903

903

TABLE III. Density, grammes per cubic metre.

Height km.

England, S.E.

Europe

Canada

Equator

20

87

87

88

96

19

1 02

102

1 02

"3

18

119

119

121

135

17

139

139

144

162

16

162

162

169

191

15

191

191

I 9 8

225

H

223

223

233

261

13

261

26l

268

294

12

305

307

3H <

331

II

355

358

365

374

10

409

411

415

419

9

463

467

470

469

8

524

528

528

522

7

589

590

592

58i

6

658

66 1

662

645

5

735

735

733

7H

4

819

819

8i5

789

3

909

913

90S

871

2

1014

1017

ion

968

I

1128

1128

"34

1067

O

J253

1258

1258

1174

thus renders the connexions between different pairs of events comparable with each other. The velocity of the wind and the steepness of the barometric gradient may be taken as an ex- ample. The actual connexion is obvious from the daily weather charts; on some it is well marked, on others badly, but the fact that there is a connexion is quite apparent from even two or three charts. The correlation coefficient is about -70.

The application of the method of correlation to forecasting can hardly be looked upon as very successful. Two highly correlated events are required happening with a definite time interval between them. A correlation coefficient may be high accidentally if it be founded on too small a number of instances, but genuinely high coefficients between meteorological events occurring with more than a few days' interval betweem them are hard to find. The most suc- cessful instance is perhaps the forecast of the monsoon rain of India by G. T. Walker from the correlation between it and sundry other events occurring in the spring of the same year or earlier. In this case the correlation coefficients on which the forecast is based have values of about -50; if values of -80 or -90 could be obtained very much greater success would be secured. There are a few coefficients of from -70 to -80 between the rainfall at various periods and the subsequent yield of sundry crops. Thus in the eastern counties of England if April and May be wet it is a practical cer- tainty that there will be a large hay crop, and if the autumn be dry there will almost certainly be a large crop of wheat the next year. Mr. R. H. Hooker has calculated a most interesting set of figures relating to the correlation between the weather and the crops. Similar work has been done for the potato crop in America by J. Warren Smith, and many correlation coefficients relating to agricul- tural matters are available from Sweden and elsewhere.

The case is different where correlation is resorted to for the pur- pose of elucidating some physical process in the atmosphere ; here a

small coefficient is just as likely to give information as a large one. But the interpretation of the meaning of the coefficient is often difficult, and in many cases the value obtained is quite different from that which most meteorologists would have expected.

G. T. Walker in addition to his statistical work on the monsoon rain has published several sets of correlation coefficients, and amongst them a set of 100 showing the correlation between the sunspot number and the temperature at 100 stations well dis- tributed over the earth's surface. The correlation is negative and small but it is large enough to be significant and to prove that during the 40 or so odd years considered the temperature of the earth as a whole was lower at the time of the sunspot maxima than at the time of the minima. It is commonly supposed that the sun is giving out most energy when its surface is most disturbed, and this idea has been confirmed by direct observation of the radiant heat. A per- fectly satisfactory explanation is at present wanting. Another case is the low correlation between the direction of the wind and the temperature of the lower air strata (see a paper by Capt. C. K. M. Douglas, Q. J. Met. Soc., Jan. 1921, vol. xlvii., No. ,197), a most unexpected result. Walker also correlated between sunspots and rainfall, and found the coefficient too small to be significant. How- ever, in none of these cases has the work been wasted since important conclusions have been established.

For high correlation coefficients one must take data relating fo the upper air. The relation between pressure and temperature is so remarkable and has such a close relationship to the theory of cyclones and anticyclones that it will be treated separately. The correlation coefficients between the thickness of the troposphere, a height commonly denoted by H, the surface pressure, the tempera- ture of the stratosphere and other variables often exceed -70, and the generally high values show quite plainly that there is an ordered sequence in the processes going on above, which is strikingly absent from the surface phenomena.

Cyclones and Anticyclones. In the meteorological literature of the past no subject has been so much discussed or has had so much attention directed to it as the causes of cyclones and anti- cyclones. When it became possible to obtain observations of temperatures and humidity, and in clear weather of wind direc- tion, from the upper air it was confidently hoped a solution would be found a hope as yet unfulfilled. But the mass of informa- tion collected from Europe, more particularly from the northern and western parts where cyclones are frequent, has given a large amount of detailed information and we have a clear conception of what happens as a cyclone passes over. It is true that we have no simultaneous sets of observations, so that we cannot draw a chart of any one particular cyclone, but we have numerous observations showing the departures from the mean correspond- ing to any observed surface pressure and to any special section of the cyclone.

The facts that stand out are that in a cyclone the troposphere is cold and the stratosphere warm, in an anticyclone the reverse is the case; in a cyclone the tropopause is low, in an anticyclone high. Thus as an area of low pressure passes across the map the following changes occur in the various air strata above. The deficiency of pressure is about the same from the surface up to some 10 km., above which it falls off rapidly until the normal value for the height