![]() ![]() In the elevated regions of the West, the 850mb level is the surface and the 700-mb level is 1 mile high. This is valid in the eastern U.S., or wherever terrain is near sea level. When measuring the height of the frontal surface (RISE), several approximations are used: 5000 ft = about 1 mile, the 850-mb level is 1 mile high. A front with this slope would be considered steep!!ĭetermination of Frontal Slope. Fast moving fronts may have an almost vertical slope in the boundary layer because of frictional drag. In the planetary boundary layer, frontal slope is much steeper. A slope of 1/20 drawn to scale means that over a distance of 20 inches in the horizontal, the line changes only 1 inch in the vertical. If drawn to scale, even a steep front would have a slope which is hard to detect. The actual slope of a frontal surface is much less pronounced than normally depicted on diagrams. While viewing diagrams of fronts, it is important to realize the drawings are not to scale. A shallow slope suggests the frontal lift would be weak, thus the wind flow allows air to ascend the frontal surface. A steep slope suggests the frontal lift could be strong thus the wind flow forces air to ascend ahead of the frontal surface. The ACTUAL lift depends on the wind flow in the frontal region. Frontal slope is significant because it represents the POTENTIAL lift in a frontal zone. Frontal slope is the ratio of the height of the upper front at some location (RISE) to the distance between that location and the surface front (RUN). The stronger the perpendicular component, the faster the front moves.Īn important characteristic of a frontal surface is its slope. The frontal movement is strongly influenced by the perpendicular wind component in the cold air. This, in turn, corresponds to the density difference across the front. A change in temperature and moisture represents a change in virtual temperature. The dew-point curve will follow the temperature curve and show a moisture increase with height through the transition zone. Above the top of the inversion lies the warmer air mass. The top of the stable layer is the frontal surface. Below the stable layer lies the colder air mass. The base of the stable layer is the cold side of the transition zone. However, a true inversion need not be present. This feature is commonly called a frontal inversion. The stable region could have a negative, isothermal or weak positive lapse rate. The region depicted on a Skew-T sounding which corresponds to the transition zone is called the stable layer. The frontal trough is most distinct at low levels. Troughing in the contours or isobars which is coincident with the frontal surface is known as a frontal trough. Frontal surfaces, mountains and transition zones themselves can serve as boundaries between air masses. The term “front,” by itself, generally refers to a surface front.Ī frontal surface aloft is known as the upper front.Ī boundary is some feature which separates two air masses. The surface front is the intersection of the frontal surface and the ground. ![]()
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