occlusion or triple point. Stationary fronts and shearlines A stationary front is a non-moving boundary between two different air masses. They tend to remain in the same area for long periods of time, sometimes undulating in waves. As airmass temperatures equalize, stationary fronts may become smaller in scale, degenerating to a narrow zone where wind direction changes over a short distance, known as a shear line, depicted as a blue line of single alternating dots and dashes. Mesoscale features Mesoscale features are smaller than synoptic scale systems like fronts, but larger than storm-scale systems like thunderstorms. Horizontal dimensions generally range from over ten kilometres to several hundred kilometres. Dry line The dry line is the boundary between dry and moist air masses east of mountain ranges with similar orientation to the Rockies, depicted at the leading edge of the dew point, or moisture, gradient. Near the surface, warm moist air that is denser than warmer, dryer air wedges under the drier air in a manner similar to that of a cold front wedging under warmer air. When the warm moist air wedged under the drier mass heats up, it becomes less dense and rises and sometimes forms thunderstorms. During daylight hours, drier air from aloft drifts down to the surface, causing an apparent movement of the dryline eastward. At night, the boundary reverts to the west as there is no longer any solar heating to help mix the lower atmosphere. If enough moisture converges upon the dryline, it can be the focus of afternoon and evening thunderstorms. A dry line is depicted on United States surface analyses as a brown line with scallops, or bumps, facing into the moist sector. Dry lines are one of the few surface fronts where the special shapes along the drawn boundary do not necessarily reflect the boundary's direction of motion. Outflow boundaries and squall lines Organized areas of thunderstorm activity not only reinforce pre-existing frontal zones, but they can outrun cold fronts. This outrunning occurs in a pattern where the upper level jet splits into two streams. The resultant mesoscale convective system (MCS) forms at the point of the upper level split in the wind pattern at the area of the best low-level inflow. The convection then moves east and equatorward into the warm sector, parallel to low-level thickness lines. When the convection is strong and linear or curved, the MCS is called a