1. The obstructing, on a large scale, of the normal west-to-east progress of migratory cyclones and anticyclones. A blocking situation is attended by pronounced meridional flow in the upper levels, often comprising one or more closed anticyclonic circulations at high latitudes and cyclonic circulations at low latitudes (cut-off highs and cut-off lows). This anomalous circulation pattern (the “block”) typically remains nearly stationary or moves slowly westward, and persists for a week or more. Prolonged blocking in the Northern Hemisphere occurs most frequently in the spring over the eastern North Atlantic and eastern North Pacific regions. Compare blocking high. 2. The retardation of stable, low-level, forced upslope flow on the windward side of a mountain or mountain barrier; Smith (1979) asserts that “this windward-side slowing is due to the difficulty that the heavy (cold) surface air has in running upslope. ”

The stable flow is characterized by having a Froude number much less than 1. In cases where a gapless mountain range is long enough to be a complete barrier to the flow, very stable air may be totally blocked or “dammed” (see damming), and the near-surface flow may be diverted back down the slope. If the blocking condition persists for more than a significant portion of a day, a barrier jet can form parallel to the mountain range. “In. . . Flow near an isolated mountain or a ridge with ends or gaps, absolute blocking of flow is not possible. The layer of dense air may pile up slightly ahead of the mountain, but this can be relieved by airflow around the mountain or through gaps in the ridge” (see also gap wind). In stable flow (Froude number less than 1) over an isolated peak, the flow in the lower portions is partially blocked and diverts around the peak, whereas in the upper portion the flow can pass upward over the peak. The boundary between the lower and upper regions has been called the dividing streamline.