A warm, humid climate with plenty of rainfall is suitable for tea cultivation. It grows well in areas where annual rainfall varies from 1150 to 6000 mm. The effect of rainfall is manifested by its influence on the moisture status of the soil and in inducing vegetative growth. It is, therefore, more important to have evenly distributed rainfall throughout the year rather than the total rainfall. The tea plant seems unable to maintain a favourable water balance in its leaves even when the soil is at field capacity if the saturation deficit of the air exceeds 2.0kPa. Hence, a humid climate is essential for optimum growth of tea plant.
Tea is basically a rain-fed crop. The lower limit of rainfall for successful tea cultivation is 1200mm, if other factors are not limited. However, the distribution of rainfall over the year is as vital as the total annual rainfall. Therefore, the well distributed rainfall is essential for commercial tea cultivation. Tea should not normally be cultivated in areas where the rainfall is below 1150 mm, unless irrigation is available.
Air temperatures above 300 C and below 13 0C are harmful for the growth of tea plants. However, in the plains of North-East India including Assam, the air temperature remains above 300C for a greater part of the harvesting period. In the plains of N.E. India, tea is normally grown under shade tree to keep the temperature below a level where it can not affect the photosynthesis adversely. In some other tropical countries also the air temperature remain above 300C during the plucking period, but shade is not needed there due to high wind velocity,because wind keeps the leaf temperature low. In Assam, due to the shelter effect of the hills on both sides of the tea tract, the wind velocity is low, and hence, shade is essential in the tea gardens.
The mean relative humidity throughout the year should not fall below 60 percent for the better growth of tea. Relative humidity of 80-90 percent is favorable during the growth period of a tea plant. With 50 percent of relative humidity, the shoot growth is inhibited and when it is below 40 percent, the growth is adversely affected.
Day length and temperature influence the growth and dormancy of the tea bush. In the equatorial region, there is no seasonal variation of day length and temperature and devoid of dormancy period. Tea plants enter into the seasonal dormancy (winter dormancy) when the temperature falls below 13o C and the day length is shortened to 11 hours 15 minutes. The length of dormant period increases progressively with the increasing distance from the equator and beyond about 160, the tea bushes show complete dormancy. In N.E. India, we can pluck leaves only for some 8-9 months and in the rest 3-4 months, there is no crop. However, tea cultivated in those countries or areas located on the equator or near the equator (upto160) like South India, Sri Lanka, Kenya, Indonesia, New Guinea, Malawi etc. there is no dormancy and crops are harvested all the year round. On the other hand, in Georgia situated at 420N latitude in the Trans-Caucasian province of Commonwealth of independent state (former USSR) and in Argentina situated at 300S latitude, harvesting is done only for 6 months and 8-9 months respectively. In Assam, plucking is carried out for about 9 months.
Wind velocity is useful in reducing high leaf temperature. The velocity of wind in the Brahmaputra valley in Assam is low due to the shelter effect of hills on either sides, and for that, shade trees are grown in tea plantation to keep down the leaf temperature. However, in those areas where air temperature is high but the wind velocity is also high, shade is not needed.