<P> Wind roses are tools used to determine the direction of the prevailing wind . Knowledge of the prevailing wind allows the development of prevention strategies for wind erosion of agricultural land, such as across the Great Plains . Sand dunes can orient themselves perpendicular to the prevailing wind regime within coastal and desert locations . Insects drift along with the prevailing wind, while birds are able to fly more independently of it . Prevailing winds in mountain locations can lead to significant rainfall gradients within the topography, ranging from wet across windward - facing slopes to desert - like conditions along their lee slopes . Prevailing winds can have differences due to the uneven heating of the Earth . </P> <P> A wind rose is a graphic tool used by meteorologists to give a succinct view of how wind speed and direction are typically distributed at a particular location . Presented in a polar coordinate grid, the wind rose shows the frequency of winds blowing from particular directions . The length of each spoke around the circle is related to the frequency that the wind blows from a particular direction per unit time . Each concentric circle represents a different frequency, emanating from zero at the center to increasing frequencies at the outer circles . A wind rose plot may contain additional information, in that each spoke is broken down into color - coded bands that show wind direction ranges . Wind roses typically use 8 or 16 cardinal directions, such as north (N), NNE, NE, etc., although they may be subdivided into as many as 32 directions . </P> <P> The trade winds (also called trades) are the prevailing pattern of easterly surface winds found in the tropics near the Earth's equator, equatorward of the subtropical ridge . These winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere . The trade winds act as the steering flow for tropical cyclones that form over world's oceans, guiding their path westward . Trade winds also steer African dust westward across the Atlantic Ocean into the Caribbean sea, as well as portions of southeast North America . </P> <P> The westerlies or the prevailing westerlies are the prevailing winds in the middle latitudes (i.e., between 35 and 65 degrees latitude), which blow in areas poleward of the high pressure area known as the subtropical ridge in the horse latitudes . These prevailing winds blow from the west to the east, and steer extra-tropical cyclones in this general manner . The winds are predominantly from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere . They are strongest in the winter when the pressure is lower over the poles, such as when the polar cyclone is strongest, and weakest during the summer when the polar cyclone is weakest and when pressures are higher over the poles . </P>

What is the prevalent wind flow in the tropics