UNIT OF ATMOSPHERIC PRESSURE

Dominion, Volume 32, Issue 153, 24 March 1939, Page 8

UNIT OF ATMOSPHERIC PRESSURE

Use Of Term Millibar

In meteorology, because of the great speed and the long distance covered by aircraft and their dependence on the weather, it has been necessary to develop a world-wide scheme of weather reporting. This has been done by the International Meterological Organization on which every country is represented. Among the points to which the organization has had to give attention is the use of units which will be intelligible to all and also the most convenient. For pressure, it long ago chose the millibar as unit. Practically all broadcasts of weather reports, whether for aviation, shipping or general purposes now give pressure in millibars. That being the case, it is obvious that everyone should become familiar with the use of millibars and that conditions will be much simplified if all other units can be dropped. The New Zealand Meteorological Office is proceeding with this end in view and from April 1, 1939, pressures will be given in the 7.10 p.m. broadcast in millibars and not inches of mercury.

It is obvious that a unit involving inches would not be suitable for international use. It is, in any case, unscientific to express pressure in inches of mercury, a length instead of a force. One never, for example, states the steam pressure in a boiler as so many inches of mercury, but as so many pounds a square inch. The millibar is a unit in the metric system, and is equivalent to 1000 dynes a square centimetre. One million dynes a square centimetre of 1000 millibars is equivalent to a pressure of 29.53 inches of mercury, or approximately the pressure of the atmosphere at sea level. A million dynes is, therefore called a bar, aud one-thousandth of it a convenient working unit, a millibar. The millibar is much more convenient to use with a mercury barometer than the inch. Tables for converting inches to millibars may be obtained on application to the Meteorological Office, Wellington.