When warm air and light air rise from the equatorial low pressure zone, cool and heavy air flows towards the equatorial zone from the Cancer and Capricorn high pressure zones. According to Ferrell's formula, this wind blows from the northeast of the northern hemisphere and from the southeast of the southern hemisphere. In ancient times merchant ships sailed in the direction of this wind, so they were called ion winds or trade winds. In the northern hemisphere it is known as northeast ion wind and in south hemisphere it is known as south-east ion wind. Northeast ion winds blow at a speed of about 16 kmph and southeast ion winds at about 22.54 kmph. When the north-east and south-east ion winds are near the equator, they become warmer and lighter with extreme heat. Then the horizontal flow of air in the equator stops and a calm zone up to 5 ° latitude is formed north-south on both sides of the equator. This ring is called the equatorial calm ring ( Doldrum) .