ISOBAROMETRIC (1's6-b1iir'o-met'rik) LINES (from Gk. Ivor, isos, equal Eng. baro metric, from Gk. gdpos, baros, weight + Idrpov, metron, measure), or ISOBARS. Lines joining the places at which the atmospheric pressure re duced to a common level is the same. Ordinarily the weather maps show the isobars for sea-level, and by recent decisions of the International Meteorological Congress the barometric indica tions must also be corrected for the influence of the variations of gravity so that atmospheric pressures may all be expressed in uniform abso lute measures. Formerly meteorologists avoided the troubles and uncertainties of the reduction to sea-level by using the departures of individual barometric readings from the normal reading of that instrument. These isabnormal lines were then plausibly converted into isobarometric lines by assuming that the normal pressure at sea level was everywhere the same, namely, 760 mil limeters, or 30 inches, and adding the departures from the normal algebraically to this assumed normal. The resulting pressures were assumed to represent closely the result that would have been given by reduction to sea-level. In 1S57 Ferrel showed that the normal barometric pres sure at sea-level could not possibly be uniform, but must be lower in the polar and higher in the tropical regions. In 1S6S Buchan showed that the pressures are higher over the continents in the winter and over the oceans in the summer. Since these dates the importance of correct iso bars based upon a correct method of reduction to sea-level has been more and more thoroughly appreciated.
The term `isobarometrie was applied by Kaemtz, about 1S30, to lines representing the oscillations of the barometer or its range from maximum to minimum during any month or year, but this usage is now obsolete. These oscil lations are greatest in regions subject to the passage of severe and numerous storms. They are greater in the northern portion of the Atlan tic Ocean than in the southern portion or in cor responding latitudes on land.
The distance between two neighboring isobars is greater in proportion as the winds are less, and vice versa, the distance is small when the winds are strongest. The rate of change of pres sure in a unit of horizontal distance is called the gradient of pressure. This barometric gradient may be as large as 5 millimeters or 0.25 inch of barometric pressure per degree of a great circle when gales of wind prevail, and much greater in the narrow region around a storm centre within which hurricane winds prevail. The barometric gradients attending storm minds must not be considered as producing the winds. On the con trary, the winds have a great influence in pro ducing the gradients, and much steeper ones would occur if the resistances to the motion of the wind did not prevent. :Maps of isobars and gradients will be found in connection with the article on METEOROLOGY.