Trade Winds
Sources: Trade Winds , Wikipedia and various internet sites.
Trade winds are permanent winds blowing from east to west towards the equator between horse latitudes. Horse latitudes represent the region at about 30 degrees north and south of the equator. Thus, they occur only near the equatorial region. Trade winds are also known as tropical easterlies.
Trade winds are very powerful prevailing winds of predictable nature. They blow from the north-east in the Northern Hemisphere and from the south-east in the Southern Hemisphere. Trade winds maintain a consistent force and direction. One can also understand these winds as air currents blowing very close to the surface of the Earth.
Across centuries, the trade winds have helped the sailors to navigate the world oceans and establish communication. These winds have played an instrumental role in facilitating trade and commerce between countries across continents. Even to date, world shipping relies heavily on the trade winds.

There are two very important factors responsible for the trade winds: Hadley Cell atmospheric circulation and the Coriolis effect. While Hadley Cell is the reason behind the formation of the trade winds, it is the Coriolis Effect that determines the direction of the trade winds.
Hadley Cell. Earth's atmospheric temperature and pressure are not the same at all places. This variance results in the creation of circulation especially between two different latitudes. This circulation is called the Hadley Cell. The heating of the earth at the thermal equator leads to large amounts of convection along the Intertropical Convergence Zone ("ITCZ"). This air mass rises and then diverges, moving away from the equator in both northerly and southerly directions. As the air moves towards the mid-latitudes on both sides of the equator, it cools and sinks. This creates a ridge of high pressure near the 30th parallel in both hemispheres. At the surface level, the sinking air diverges again with some returning to the equator, creating the Hadley cell.
Coriolis Effect. It is a tendency of wind to flow from high pressure to low pressure. Accordingly, the winds blow from subtropical high-pressure belts on either side of the hemisphere towards the equatorial low-pressure belt as a part of the Hadley Cell air circulation process. The strong trade winds, however, experience a deflection from its straight part while moving towards the equator from the sub-tropics. This deflection is due to the Coriolis Effect which is caused by the rotation of the Earth on its axis. As a result, the prevailing winds end up moving in a curved path towards the equator.

Horse Latitudes
The horse latitudes are the latitudes about 30 degrees north and south of the Equator. They are characterized by sunny skies, calm winds, and very little precipitation. They are also known as subtropical ridges or highs. It is a high-pressure area at the divergence of trade winds and the westerlies.
A likely and documented explanation is that the term is derived from the "dead horse" ritual of seamen. In this practice, the seaman paraded a straw-stuffed effigy of a horse around the deck before throwing it overboard. Seamen were paid partly in advance before a long voyage, and they frequently spent their pay all at once, resulting in a period of time without income. If they got advances from the ship's paymaster, they would incur debt. This period was called the "dead horse" time, and it usually lasted a month or two. The seaman's ceremony was to celebrate having worked off the "dead horse" debt. As west-bound shipping from Europe usually reached the subtropics at about the time the "dead horse" was worked off, the latitude became associated with the ceremony.
An alternative theory, of sufficient popularity to serve as an example of folk etymology, is that the term horse latitudes originates from when the Spanish transported horses by ship to their colonies in the West Indies and Americas. Ships often became becalmed in mid-ocean in this latitude, thus severely prolonging the voyage; the resulting water shortages made it impossible for the crew to keep the horses alive, and they would throw the dead or dying animals overboard.
A third explanation, which simultaneously explains both the northern and southern horse latitudes and does not depend on the length of the voyage or the port of departure, is based on maritime terminology: a ship was said to be 'horsed' when, although there was insufficient wind for sail, the vessel could make good progress by latching on to a strong current. The maritime use of 'horsed' described a ship that was being carried along by an ocean current or tide in the manner of a rider on horseback. The term had been in use since the end of the seventeenth century.