Ambient air is blown under the troughs by fans. This will have the effect of withering or drying the green leaf provided that the ambient air is sufficiently dry itself. When we have particularly humid days of 95% humidity and above we can pre-heat the ambient air by using steam from the boiler so that the air that enters the trough a little drier.
There are two stages to the withering: chemical and physical withering. Physical withering is the process of reducing the moisture content of the leaf from 80% to about 68%, a process that takes about 12-18 hours, meaning the green leaf remains in the trough house overnight to complete the withering process. During this time there will also be chemical changes taking place in the leaf, enhancing the quality of the made tea in terms of strength, brightness, colour and briskness.