Atmospheric pressure can be defined as the weight exerted by air column on units surface of the earth.
Units of pressure:
1. Height of mercury column measured in inches, cm, and mm
2. Bar, bar is a force equal to 106 dynes/cm2
3. SI (standard International) unit or pressure is Pascal
1. Pascal = force of 1 Newton/m2
= 1 N m2
Standard atmospheric pressure:
The standard atmospheric pressure is given at mean sea level at 450K latitude and at temperature of 2730K
Standard Atmospheric = 29.92 inches or 76 cm or 760 mm
Pressure = 1013.25 mb
= 101.325 kilo Pascal (Kpa)
= 14.7 lbs/inch2
1.014 x 106 dynes/cm2
Any line joining the places of equal atmospheric pressure on the weather map is known as isobar. Where isobars are closely spaced, a rapid of steep change in reassure is indicated. When isobars are widely speeding, a slow change in pressure is indicated. Two isobars are never cross each other. Isobars are plotted on the map to show the distribution of pressure. The isobars are drawn at pressure intervals of 2, 3,4 or 5 mb.
The rate of change of atmospheric pressure per unit horizontal distance between two points at the same elevation is known as pressure gradient or isobaric slope. This change tak3es place and is measured in the direction perpendicular to the isobars preferably from high to low pressures. This exerts a force on air particles and is important in determining the strength of wind. The pressure gradient is expressed in decrease in pressure per unit horizontal distance as mb/100 meters.