Steady, regular winds that blow in a belt between approximately 30 degrees north and 30 degrees south of the equator. They are caused by the action of the sun on and near the equator in heating the atmosphere and causing it to rise. The heavier air to north and south comes in to fill the vacuum thus caused. In the southern hemisphere the trade winds blow from the south-east and in the northern hemisphere from the north-east. They are called trade winds because of the assistance they gave to merchant ships around the world in the days before steam.