A: “High pressure” and “low pressure” correspond to geographical locations where the surface pressure of the atmosphere is higher or lower than average. Although air is much less dense than water, it still has mass, and surface pressure is simply caused by the weight of the atmosphere pressing down on the Earth. The average value of atmospheric pressure at sea level is 1013 millibars (the conventional meteorological unit) or 14.7 pounds per square inch in English units. The amount of mass in the atmosphere is not uniformly distributed around the globe, and areas with less mass of overlying air have low surface pressure, and areas with more mass have high surface pressure.
At middle latitudes, where the United States is located, areas of high pressure and low pressure typically travel from west to east and are associated with changes in weather. In regions of low pressure, air generally moves upward and cools, leading to condensation of water vapor, clouds, and precipitation. In regions of high pressure, air generally moves downward and warms, leading to drying and clear skies. One exception to this is San Diego, where the marine layer brings in low-level clouds even though the air higher up is dry and moving down. High pressure prevails in San Diego during most of the year, which is why it so seldom rains here. Only in winter do low pressure systems occasionally propagate into our neighborhood.
Areas of low pressure in the tropics typically travel from east to west and are also associated with upward moving air and precipitation. Sometimes these low pressure systems intensify and become hurricanes.
— Joel Norris, Climate Research Division, Scripps Institution of Oceanography