Plentiful seasonal rainwater, well-drained soil and fresh-air fog (away from the ocean, where the fog is salty) are the ideal growing conditions for tea. An overly rainy season can flush out the flavor characteristic of tea and make it weak. Too little moisture can make it weak. Too little moisture can take away key flavor nuances or make it harsher than usual. Ideally tea is grown at an altitude of 3,000-5,000 feet on a 45-degree slope, where the water won't pool and foggy mornings keep the leaves and the roots moist.
From "Champagne of Tea," by John and Kerry Laird (www.pacificbaycoffee.com)
By the way, the above is a description of the Darjeeling area of India—explaining why the black tea grown there is widely considered the "champagne of tea"—but it applies word-for-word to the Aso area, where Mellow Monk tea is grown.