High light intensity increases the rate of photosynthesis, causing stomata to open so that leaf cells can get enough carbon dioxide. When stomata are fully open, water vapor escapes more easily, increasing the rate of transpiration.
Bright sunshine also warms leaves, increasing the rate of transpiration by raising the temperature.
Humidity is the amount of water vapour in the air. When humidity is low, the air is dry and water evaporates easily from leaves, resulting in a high rate of transpiration.
When humidity is high, water evaporates less easily and transpiration is slower.
An increase in temperature increases the rate of transpiration as it increases the rate at which water evaporates from leaf cells and escapes into the air through stomata.
It also increases the capacity of the air to absorb more water.
Higher wind speeds cause the rate of transpiration to increase. This is because the water vapor that has diffused out of the leaves is moved away quickly by the wind, preventing humidity from building up around the plant.