CO129-453 - Acting Governor Claud Severn - 1919 [1-3] — Page 453

COPY.

REPORT ON DR. WALKER'S NOTE ON

HYGROMETRIC METHODS.

2:237 1

432

RES

IR 26 APR 19

4

I am aware that the methods of measuring humidity

have been frequently investigated by physicists of first

class repute both in Europe and America.

I am also aware that the use of Blanford's tables

for deducting the humidity from readings of sling thermome-

-tera ie open to critcism. It was to remedy this defect

that the correspondence was opened.

As regards my statement that the Hong Kong practice

is known to be more accurate than the Indian method, in his

note of 17th. November, 1917, Dr. Walker states that "a

sufficiently sensitive thermometer swung round one end, in

the shade for two minutes, will give a true reading of the

air temperature; and that for a wet bulb thermometer the

result is alightly more accurate than that given in the

shed under existing conditions".

The situation appears to me to be this:-

The psychrometric factor A varies considerably with

the wind velocity until the latter reaches a certain value.

For unaspirated thermometers therefore a different formula

ie required for each velocity below this value. The labour

of such a procedure would, however, be prohibitive, and up

to the date of the St. Petersburg Conference the Interna-

-tional Meteorological Committee were unable to make

definite recommendations as regards hygrometric methods.

In 1906 however Hr. J.R. Sutton, of Kimberley,

suggested a solution which I recommend in my report of 14th.

April, 1918; namely, that the readings of unaapirated wet

bulb thermometers should be corrected for wind velocity.

I recommend the use of the sling thermometer for

first class Observatories because I have always considered

this