A low pressure area is created when there is light, warm, moist air. This air rises and as it does it expands and cools. Warm air is capable of holding a lot of moisture in the form of invisible water vapor but cool is not capable of holding much moisture. As the warm air rises and cools, the air is no longer able to hold the moisture as water vapor and it forms clouds that hold the moisture. If there is a lot of moisture in the atmosphere, the cool air cannot hold all of the moisture and this is when rain or snow is formed.
Air is constantly moving around the earth. The air is either warm or cold, light or heavy. These differences put together make up different pressures and cause different effects on the weather. The atmosphere tries to create a state of balance in the air by forming pressure systems and stirring up the wind. These two things together create the weather that effects the earth everyday.
As the light warm air flows upward in the atmosphere it blows counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere, the same direction that the Earth is rotating. This rotating air forms a low pressure systems and is what causes rain and storms depending on other atmospheric indicators in the area where the low pressure system is formed.
The Jet Stream, winds that blow 20,000 to 40,000 feet above the earth’s surface, effect how strong a low pressure system is. Fast moving air causes the warm air from the surface to rise, thus forming a low pressure systems. The faster the jet stream winds are the stronger the low pressure systems and the stronger the potential for bad weather caused by the rising warm moist air.
The basic anatomy of a low pressure is created when winds blow across the surface of the earth and pick up warm, moist air. The warm, moist air is light so it flows upward into the atmosphere, cooling and expanding as it goes. Once the air has reached a temperature where it can no longer hold the moisture, it forms clouds. If there is enough moisture in the air, rain and/or snow will form depending upon the temperatures in the air and on the surface of the earth. The air of the low pressure area flows in the same direction the earth is rotating, creating cyclonic circulation and the potential for storms.