Blocks in meteorology are large-scale patterns in the atmospheric pressure field that are nearly stationary, effectively "blocking" or redirecting migratory cyclones. They are also known as blocking highs or blocking anticyclones. These blocks can remain in place for several days or even weeks, causing the areas affected by them to have the same kind of weather for an extended period of time (e.g. precipitation for some areas, clear skies for others). In the Northern Hemisphere, extended blocking occurs most frequently in the spring over the eastern Pacific and Atlantic Oceans. Whilst these events are linked to the occurrence of extreme weather events such as heat waves, particularly the onset and decay of these events is still not well captured in numerical weather forecasts and remains an open area of research. Impact of the polar vortex Polar cyclones are climatological features which hover near the poles year-round. They are weaker during summer and strongest during winter. When the polar vortex is strong, the Westerlies increase in strength. When the polar cyclone is weak, the general flow pattern across mid-latitudes buckles and significant cold outbreaks occur. Extratropical cyclones which occlude and migrate into higher latitudes create cold-core lows within the polar vortex. Volcanic eruptions in the tropics lead to a stronger polar vortex during the winter for as long as two years afterwards. The strength and position of the cyclone shapes the flow pattern across the hemisphere of its influence. An index which is used in the northern hemisphere to gauge its magnitude is the Arctic oscillation. Omega blocks Omega blocks are so-named because the isobars or geopotential height contours with which they are associated in the Northern Hemisphere resemble an Ω, the uppercase Greek letter omega. They typically have a low-high-low pattern, arranged in the west–east direction. Rex blocks Rex blocks (or dipole blocks) consist of a high situated poleward (north in the Northern Hemisphere; south in the Southern Hemisphere) of a low. Very often both the high and the low are closed, meaning that the isobars (or constant geopotential height lines) defining the high–low close to form a circle. Rex blocks are named after meteorologist Daniel F. Rex, who first identified them in 1950. Cut-off highs and lows When an upper-level high- or low-pressure system becomes stuck in place due to a lack of steering currents, it is known as being "cut off". The usual pattern which leads to this