A blocking high is a large-scale, long-lasting area of high pressure that blocks or redirects storm systems. It forms part of a nearly stationary atmospheric pattern, preventing migratory cyclones from following their usual path.
Blocking highs can persist for several days or even weeks because the jet stream’s steering flow becomes displaced or weakened. The result is prolonged weather: clear, dry conditions and heat beneath the high, or repeated rain if the circulation redirects lows toward a region.
The term is often confused with “ridge” or “anticyclone.” A ridge is an elongated area of high pressure, while an anticyclone is any high-pressure circulation; a blocking high specifically emphasizes persistence and obstruction of moving weather systems. Blocking patterns are associated with heat waves, droughts, floods, and persistent cold spells.