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The westerlies, anti-trades, [2] or prevailing westerlies, are prevailing winds from the west toward the east in the middle latitudes between 30 and 60 degrees latitude. They originate from the high-pressure areas in the horse latitudes (about 30 degrees) and trend towards the poles and steer extratropical cyclones in this general manner. [ 3 ]
The term originally derives from the early fourteenth century sense of trade (in late Middle English) still often meaning "path" or "track". [2] The Portuguese recognized the importance of the trade winds (then the volta do mar, meaning in Portuguese "turn of the sea" but also "return from the sea") in navigation in both the north and south Atlantic Ocean as early as the 15th century. [3]
The westerlies can be particularly strong, especially in the southern hemisphere, where there is less land in the middle latitudes to cause the flow pattern to amplify, which slows the winds down. The strongest westerly winds in the middle latitudes are called the Roaring Forties , between 40 and 50 degrees south latitude, within the Southern ...
The westerlies can be particularly strong, especially in the southern hemisphere, where there is less land in the middle latitudes to cause the flow pattern to amplify, which slows the winds down. The strongest westerly winds in the middle latitudes are within a band known as the Roaring Forties , between 40 and 50 degrees latitude south of the ...
The prevailing trade winds are a manifestation of the lower branches of the Hadley circulation, converging air and moisture in the tropics to form the Intertropical Convergence Zone (ITCZ) where the Earth's heaviest rains are located. Shifts in the ITCZ associated with the seasonal variability of the Hadley circulation cause monsoons.
A westerly wind burst is defined by Harrison and Vecchi (1997) as sustained winds of 25 km/h (16 mph) over a period of 5–20 days. [3] However, no concrete definition has been determined, with Tziperman and Yu (2007) defining them as having winds of 14 km/h (8.7 mph) and lasting "at least a few days".
The first is in the area on the east side of upper troughs, which form half of a Rossby wave within the Westerlies (a trough with large wavelength that extends through the troposphere). A second is an area where wind divergence aloft occurs ahead of embedded shortwave troughs, which are of smaller wavelength.
Cold air damming typically happens in the mid-latitudes as this region lies within the Westerlies, an area where frontal intrusions are common.When the Arctic oscillation is negative and pressures are higher over the poles, the flow is more meridional, blowing from the direction of the pole towards the equator, which brings cold air into the mid-latitudes. [1]