According to About.com, areas of low pressure within the Earth's atmosphere are caused by unequal heating across the surface and the pressure gradient force. Incoming solar radiation largely concentrates at the equator, resulting in warmer air at the lower latitudes. This warm air has a lower barometric pressure than the cooler, denser air near the poles, and the differences between these types of air create the pressure gradient force.
According to Sciences 360, both high- and low-pressure systems develop from the interactions of an unequally heated environment, such as the difference between a land mass and a nearby body of water. The low-pressure weather system forms at areas of wind divergence in the upper layer of the troposphere. This divergence creates an empty space for the warmer air to rise in to, lowering the surface air pressures as that warm air moves upward.
The warm air carries moisture up with it, creating clouds and ultimately, precipitation. Areas of low pressure are also associated with higher winds, as the air present in the system is moving toward the areas of higher pressure above and away from the lower-pressure system. This constant churning of air between areas of high and low pressure is known as the pressure gradient force.