Sea level pressure is a measurement that is done in millibars or hectopascals. It is also known as atmospheric pressure. It is the average pressure of the air over a given area.
Along 30° N and 30° S are found the high-pressure areas known as the subtropical highs. Further pole wards along 60° N and 60° S, the low-pressure belts are termed as the sub polar lows.
At sea level the mean pressure is about 1,000 mb (100 kPa), varying by less than 5 percent from this value at any given location or time.
Mean sea-level pressure values for the mid-winter months in the Northern Hemisphere are summarized in this first diagram, and mean sea-level pressure values for the mid-summer months are illustrated in the next diagram.
Since charts of atmospheric pressure often represent average values over several days, pressure features that are relatively consistent day after day emerge, while more transient, short-lived features are removed.
Those that remain are known as semipermanent pressure centres and are the source regions for major, relatively uniform bodies of air known as air masses.
Warm, moist maritime tropical (mT) air forms over tropical and subtropical ocean waters in association with the high-pressure regions prominent there.
Cool, moist maritime polar (mP) air, on the other hand, forms over the colder subpolar ocean waters just south and east of the large, winter oceanic low-pressure regions.
Over the continents, cold dry continental polar (cP) air and extremely cold dry continental arctic (cA) air forms in the high-pressure regions that are especially pronounced in winter, while hot dry continental tropical (cT) air forms over hot desert like continental domains in summer in association with low-pressure areas, which are sometimes called heat lows.
