The forced upward movement (and related forming of clouds) of warmer air at a slightly slanted air mass border (warm front), under which there is cooler air. In this process large cloud fields form (cirrostratus, altostraus and nimbostratus) which usually bring steady rain. Upgliding can be found at the "leading edge" of the low pressure areas, which move from the W to the E.
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M.I.L.K. - Meteorology: Information. Learning. Knowledge.