There are five main conditions:

Light Intensity - When light intensity increases the stomata open and allow more rapid evaporation.

Humidity - When the atmosphere is saturated with water vapour, little more can be absorbed from the plants, and transpiration will be reduced. In a dry atmosphere, transpiration will be rapid.

Temperature - A high temperature increses the capacity of the air for water vapour; hence transpiration increases. When the leaf itself becomes warm, evaporation from it occurs more rapidly. Direct sunlight without a warm atmosphere will have this effect, since the leaf absorbs radient energy and its temperature rises.

Air Movements - In still air, the region around a transpiring leaf will become saturated with water vapour so that no more can be absorbed from the leaf; in consequence transpiration is much reduced. In moving air, the water vapour will be swept away from the leaf as fast as it diffuses out, so that transpiration continues rapidly.

Leaf Surface Area - The larger the surface area the faster the rate of transpiration.

Also, the general local and global conditions of the Carbon-Nitrogen cycle and the general health of said plant.

Also, interestingly for global warming, the CO2 concentration affects transpiration. High levels of CO2 supress opening and closing of stomata (since the plants needs to 'breathe' less, it takes fewer breaths). This means lower water loss from the stomatal opening and so less transpiration. What this means for modelling of global climate change is, as usual, unknown.

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