The trade winds are caused by a combination of the temperature gradient between the equator and the poles and the rotation of the Earth. These factors act together to form a global circulation in which warm, humid air rises at the equator, travels towards to the subtropics aloft where it subsides, and then returns to the equator along the surface of the Earth. This entire circulation is deflected westward by the Coriolis force, causing the easterly trade winds. The trade winds start just poleward of the intertropical convergence zone and end near the respective subtropical ridges of the northern and southern hemisphere, at a latitude of approximately 30°.
The trade winds blow throughout the year and average between 15 and 25 kts; however, they are the strongest and most consistent during winter when temperature gradients are the strongest. Despite being some of the most consistent surface level winds on the planet, the trade winds can still be impacted by the tails of deep cyclones in the midlatitudes. These systems can weaken and in some cases reverse the trade wind circulation. Strong El Ninõ events can also reverse the trade wind circulation in the Pacific. In the summer, weaker trade winds and high sea surface temperature make the trade wind zone the main development region for hurricanes and tropical storms.
In addition to consistent winds, trade wind regions are characterized by low level cumulus clouds that intensify and strengthen into fairly frequent squalls towards the western edges of ocean basin. While these squalls can produce intense rain and strong winds, they do not usually produce lightning during the winter months because of a feature known as the trade wind inversion. The trade winds are part of a large scale circulation in which the surface winds and winds aloft travel in different directions. The warm, humid trade winds blow at the surface to a height of approximately 2-3 km. Above this, warmer and drier air flows in the opposite direction. This warm and dry air provides a stable cap on top of the trade winds, known as an inversion, preventing mixing and impeding deep convection and preventing the vertical development of cumulonimbus and lightning. The height trade wind inversion is lower in the east and gets higher the further west you go. This is why squalls are more common the further west you travel in the trade winds. When the trade winds weaken during summer, the inversion also weakens, which is why thunderstorms are more common in the summer.
Another characteristic of trade wind weather is more frequent squalls overnight. In fact, rain squalls peak in the early morning, just before dawn. This is because the tops of the clouds cool overnight faster than the surface, creating an enhanced vertical temperature gradient and unstable conditions which promote convection.
In the North Atlantic, the trade wind circulation forms the southern section of a synoptic feature known as the Azores High. The Azores High marks the location of the subtropical ridge in the northern Atlantic and shifts north and south according to the season. By November, hurricane development in the trade wind zone becomes unlikely and the Azores High begins to shift south, creating a strong temperature gradient and the establishment of steady trade winds. This is a popular time for east to west Atlantic crossings. In late December and into January, there is a phenomenon known as the Christmas trade winds when the trade wind circulation is further strengthened. The Christmas trades are attributed to a lag between the southerly movement of the Azores High and the southerly movement of the Intertropical convergence zone. This lag squashes the Azores High (look for an elliptical shape on synoptic weather charts) compressing the temperature gradient driving the trade winds, giving them extra intensity.
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