Two ways of categorizing convergence boundaries are natural and man-made convergence boundaries. This writing will focus on man-made convergence boundaries. Man-made boundaries help create convergence that can influence where convection first forms. Examples of man-made boundaries include farming-vegetation boundaries, urban and rural boundaries and land use boundaries. Humans have dramatically changed the vegetation type is certain areas. Some of this is due to agriculture, farming and ranching. The introduction of different vegetations will change the character of the surface. Some crops give off more moisture than others while some vegetation absorbs solar radiation more than others. These moisture and thermal differences that change over distance due to differing vegetation type can create a subtle convergence boundary. Urban and rural areas vary in the amount of thermal absorption and moisture emission. During the day, urban areas are often warmer and less humid while rural areas are cooler and more humid. The temperature difference can be especially remarkable. When winds are not too strong, mesoscale air masses can develop over the urban and rural areas. The differing thermal and moisture characteristics between these mesoscale air masses can cause convergence to occur which can be the focal point for thunderstorms. The extra heat over an urban area can set off a burst point of convection that first forms near the urban area and then moves downstream with the wind. Land use boundaries are created by the division of farm land from forest land. The differing moisture and thermal characteristics between trees and agricultural crops can create a subtle convergence boundary. It needs to be stressed that these boundaries will tend to be subtle (not more obvious like cold fronts). These subtle boundaries can make all the difference in a borderline situation between storms forming or not forming and where the storms first develop. |