Prevailing winds

The trade winds (also called trades) are the prevailing pattern of easterly surface winds found in the tropics near the Earth's equator,[4] equatorward of the subtropical ridge. These winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere.[5] The trade winds act as the steering flow for tropical cyclones that form over world's oceans, guiding their path westward.[6] Trade winds also steer African dust westward across the Atlantic Ocean into the Caribbean sea, as well as portions of southeast North America.[7]

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.[8][9] These prevailing winds blow from the west to the east,[10] and steer extra-tropical cyclones in this general direction. The winds are predominantly from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere.[5] 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.[11]

Together with the trade winds, the westerlies enabled a round-trip trade route for sailing ships crossing the Atlantic and Pacific oceans, as the westerlies lead to the development of strong ocean currents in both hemispheres. 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 Hemisphere.[12] The westerlies play an important role in carrying the warm, equatorial waters and winds to the western coasts of continents,[13][14] especially in the southern hemisphere because of its vast oceanic expanse.

The westerlies explain why coastal Western North America tends to be wet, especially from Northern Washington to Alaska, during the winter. Differential heating from the Sun between the land which is quite cool and the ocean which is relatively warm causes areas of low pressure to develop over land. This results in moisture-rich air flowing east from the Pacific Ocean, causing frequent rainstorms and wind on the coast. This moisture continues to flow eastward until orographic lift caused by the Coast Ranges, and the Cascade, Sierra Nevada, Columbia, and Rocky Mountains causes a rain shadow effect which limits further penetration of these systems and associated rainfall eastward. This trend reverses in the summer when strong heating of the land causes high pressure and tends to block moisture-rich air from the Pacific from reaching land. This explains why most of coastal Western North America in the highest latitude experiences dry summers, despite vast rainfall in the winter.[8][9]

The polar easterlies (also known as Polar Hadley cells) are the dry, cold prevailing winds that blow from the high-pressure areas of the polar highs at the North and South Poles towards the low-pressure areas within the westerlies at high latitudes. Like trade winds and unlike the westerlies, these prevailing winds blow from the east to the west, and are often weak and irregular.[15] Due to the low sun angle, cold air builds up and subsides at the pole creating surface high-pressure areas, forcing an outflow of air toward the equator;[16] that outflow is deflected westward by the Coriolis effect.

A: Sea breeze, B: Land breeze

In areas where the wind flow is light, sea breezes and land breezes are important factors in a location's prevailing winds. The sea is warmed by the sun to a greater depth than the land due to its greater specific heat.[17] The sea therefore has a greater capacity for absorbing heat than the land, so the surface of the sea warms up more slowly than the land's surface. As the temperature of the surface of the land rises, the land heats the air above it. The warm air is less dense and so it rises. This rising air over the land lowers the sea level pressure by about 0.2%. The cooler air above the sea, now with higher sea level pressure, flows towards the land into the lower pressure, creating a cooler breeze near the coast.

The strength of the sea breeze is directly proportional to the temperature difference between the land mass and the sea. If an off-shore wind of 8 knots (15 km/h) exists, the sea breeze is not likely to develop. At night, the land cools off more quickly than the ocean due to differences in their specific heat values, which forces the daytime sea breeze to dissipate. If the temperature onshore cools below the temperature offshore, the pressure over the water will be lower than that of the land, establishing a land breeze, as long as an onshore wind is not strong enough to oppose it.[18]

Mountain wave schematic. The wind flows towards a mountain and produces a first oscillation (A). A second wave occurs further away and higher. The lenticular clouds form at the peak of the waves (B).

Over elevated surfaces, heating of the ground exceeds the heating of the surrounding air at the same altitude above sea level, creating an associated thermal low over the terrain and enhancing any lows which would have otherwise existed,[19][20] and changing the wind circulation of the region. In areas where there is rugged topography that significantly interrupts the environmental wind flow, the wind can change direction and accelerate parallel to the wind obstruction. This barrier jet can increase the low level wind by 45%.[21] In mountainous areas, local distortion of the airflow is more severe. Jagged terrain combines to produce unpredictable flow patterns and turbulence, such as rotors. Strong updrafts, downdrafts and eddies develop as the air flows over hills and down valleys. Wind direction changes due to the contour of the land. If there is a pass in the mountain range, winds will rush through the pass with considerable speed due to the Bernoulli principle that describes an inverse relationship between speed and pressure. The airflow can remain turbulent and erratic for some distance downwind into the flatter countryside. These conditions are dangerous to ascending and descending airplanes.[22]

Daytime heating and nighttime cooling of the hilly slopes lead to day to night variations in the airflow, similar to the relationship between sea breeze and land breeze. At night, the sides of the hills cool through radiation of the heat. The air along the hills becomes cooler and denser, blowing down into the valley, drawn by gravity. This is known a mountain breeze. If the slopes are covered with ice and snow, the mountain breeze will blow during the day, carrying the cold dense air into the warmer, barren valleys. The slopes of hills not covered by snow will be warmed during the day. The air that comes in contact with the warmed slopes becomes warmer and less dense and flows uphill. This is known as an anabatic wind or valley breeze.[23]