It can often be noticed that storms develop in patterns. These patterns can be random, along a linear boundary or in a clump. Random tends to occur more when there is not a dominate front or convergence boundary that groups the storms together. Air mass thunderstorms such as within a warm and humid air mass with no fronts around will often have a random appearance with lines and clumps of storms not being the dominate tendency. Clumping of storms can occur when the outflow from one storm leads to the development of storms in its vicinity. A group of storms will often exist longer than any individual storm since new development replenishes the storms that dissipate. Many times storms will form in a line. This is known as linear forcing. Linear forcing occurs due to lifting being optimized in the vicinity of a low level convergence boundary. Examples of these types of boundaries include cold fronts, warm fronts, dry lines, sea breezes and land surface boundaries such as the boundaries between different vegetation types. This common forcing mechanism results in storms forming along the edge of the boundary and then propagating downstream with the upper level wind flow and/or movement of the low level convergence boundary.