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Air Pressure
Surface air pressure measurements are based on the weight of the atmosphere above a location. Therefore, the air is denser -- or heavier -- above areas of high pressure. This happens in two ways. One is when a mass of air is cooled from below by the sea. This often occurs in sub-tropical zones, such as off the west coast of California, where ocean temperatures are low relative to land temperatures. The second way is when the air is cooled from above. This tends to happen during winter when the sun is unable to sufficiently heat up the cold land air masses.
Formation
When an air mass cools, it shrinks and this allows surrounding air to fill up the space. This increases the mass of atmosphere above a location, and a home barometer will show raised pressure. This air descends slowly, and as it does it warms the air around it and prevents clouds forming, which is why seeing an "H" on the weather map indicates good weather.
Anticyclones
Wind flows from areas of high pressure to low pressure, caused by the difference in air density. Another term for high pressure weather systems is anticyclone, which refers to winds circulating around the center of a high pressure system. Winds formed within high pressure areas flow outwards in a clockwise direction in the northern hemisphere and counter-clockwise in the southern hemisphere. Anticyclones clear the skies, and the air is usually drier and cooler.
Weather Conditions
If the weather forecast predicts high pressure where you live, you can expect higher temperatures because there will be no clouds to reflect the sunlight away from the Earth. However, at night the lack of cloud cover means that temperatures will be low by contrast with the daytime heat. In urban areas, the light surface winds can lead to a buildup of particles, creating a heat haze. Also, if humidity in the area rises toward the 100 percent mark during the night, fog is a possibility in the morning.