high and low pressure systems
High and low pressure systems are everywhere. These systems are categorized on what its surrounding areas are. The origins of these systems are caused by rising and falling air. Warm air rises creating low pressure systems (also called anticyclones). Cold air sinks creating a high pressure area (also called depressions). Notice the picture on the left. "In a depression (low pressure), air is rising and blows in an anticlockwise direction around the low (in the northern hemisphere). As it rises and cools, water vapour condenses to form clouds and perhaps precipitation" (Met Office). That's why in a low pressure system we get rough, uneven weather like storms and rain and such. In a high pressure system, the air is usually light causing it to fall (because it is cold; cold air sinks). There are less formations of clouds because there is no warm air to make precipitation to form them. High pressure systems bring pleasant weather because of this.
Picture it this way: Air near low pressure cannot pile into one area; so the warm air is forced up. This air cools. Cold air can hold less room for water vapor so it releases it causing precipitation to form clouds. That's why low pressure systems are associated with stormy weather.
Air near high pressure spreads away from the center. There has to be something that takes its place when it moves, so more sinking air takes its place. Sinking air warms. This means there is more room in the air to store water vapor- leading to less clouds, or fair weather.
High and Low Pressure. Met Office. 3 September 2013. Web. 6 April 2014. <http://www.metoffice.gov.uk/learning/learn-about-the-weather/how-weather-works/highs-and-lows/pressure>.