Concerning weather: what does the "pressure" mean? Example- 30.04 in/1017hPa.?

Answer 1

From Wikipedia:

The bar and millibar are defined as:

1 bar = 100 000 pascals (Pa) = 1 000 000 dynes per square centimetre
1 mbar = 0.001 bar = 100 Pa = 1 000 dyn/cm²
(A pascal is one newton per square meter.)


[edit] Origin
The word bar has its origin in the Greek word βάρος (baros), meaning weight. Its official symbol is "bar"; the earlier "b" is now deprecated, but still often seen especially as "mb" rather than the proper "mbar" for millibars.

The bar and millibar were introduced by Sir Napier Shaw in 1909 and internationally adopted in 1929.

Atmospheric air pressure is often given in millibars where "standard" sea level pressure is defined as 1013.25 mbar (hPa), equal to (1.01325 bar). Despite millibars not being an SI unit, they are still used locally in meteorology to describe atmospheric pressure.

The article goes further to explain that 1 mbar = 1 hPa, or 1 millibar = 1 hectoPascal.

In terms of inches Hg, 30.00 has long been accepted as the standard, or long-term average pressure at sea level. 30.04 in. is slightly above standard pressure, as 1017hPa is slightly above 1013.25. In weather terms this would indicate clear or clearing skies, with winds shifting from counter-clockwise to clockwise.

The popularity of the millibar (now hPa) unit seems to stem from the coincidence that 1 bar is almost equal to one atmosphere.

Answer 2

the "pressure" refers to barometric pressure. It is basically the measurement of rising or sinking air. It is measured by a barometer, which works sort of like a thermometer. A very rudimentary barometer would be a plate with mercury on it with an upright tube placed in the center with the open end down. THe pressure of the air pushes the mercury on the plate causing it to travel up the tube. you then get your pressure by measuring how far the mercury goes up the tube. Hope that helps and isn't too confusing