Evapotranspiration is the combined processes moving water from the earth's surface into the
atmosphere.
Transpiration is the movement of water through a plant and out of its leaves and other aerial parts into the atmosphere. This movement is driven by solar energy. In the tallest trees, such as
Sequoia sempervirens, the water rises well over 100 metres from root-tip to canopy leaves. Such trees also exploit evaporation to keep the surface cool. Water vapour from evapotranspiration mixed with air moves upwards to the
point of saturation and then, helped by the emissions of
cloud condensation nuclei, forms clouds. Each gram molecule (
mole) of condensing water will bring about a marked 1200-fold plus reduction in volume.The simultaneous release of
latent heat will drive air from below to fill the partial vacuum. The energy required for the surrounding air to move in is readily calculated from the small (one-fifteenth of latent heat) reduction in temperature. A small amount of that water transpired is used for growth and
metabolism.
Photosynthesis takes place in the cells of plants and other organisms such as
algae, that contain
chlorophyll. This process uses the radiant energy from the sun to split water molecules into hydrogen and oxygen that when combined with the carbon sourced from carbon dioxide, produces sugars. Photosynthesis is therefore the basis of almost all food production and produces oxygen as a byproduct. Leaves have many functions. In addition to receiving water from the roots and creating the raw materials for photosynthesis, they also have a large internal surface area to enable the exchange of gases. Their
stomata control the flow of water vapour out of the leaf and air into the leaf. In many plants, this is achieved in a structure thin enough to be semi-translucent, to enable some light to pass through to neighbouring leaves. The water that becomes raw material for sugar production, also cools the leaf and supports its structure through the pressure of
turgidity. In 2022, attempts to mass-produce artificial leaves to replicate this process and create hydrogen were still in the development stage. All organic matter, living and dead, originated as sugars. Part of the process of creating those sugars was splitting the water molecule into its component parts. Vegetation has a huge influence on climate, enacted through photosynthesis and transpiration.Botanists have calculated that there are about 600 square inches [3,871 cm2] of surface inside a leaf for every cubic inch [16.38 cm3] of its bulk and that a large elm tree has in all some 15 million leaves within an area, if spread out whole, of nearly 10 acres [4.05 ha] or, if unfolded into the sum total of air-breathing light-absorbing surfaces of all the internal chloroplasts something like 25 square miles [64.75 square kilometres]. (p. 29). An individual tree can transpire hundreds of litres of water per day. Transpiring 100 litres is equivalent to a cooling power of 70 kWh. The city of Melbourne "plans to plant 3000 trees in Melbourne every year to increase the resilience of the urban forest and to cool our city by 4°C." Increasing tree cover and evapotranspiration provides a localised mitigation solution. On a larger scale, The Mau Forest complex in Western Kenya was deforested from 5,200 km2 in 1986 to 3,400 km2 in 2009. Satellite images revealed temperature increases with deforested areas being 20 °C hotter or more. There were about six trillion trees on the planet, but human activity has destroyed roughly half. This temperature reduction was achieved in four years. We can anticipate a larger reduction as the vegetation cover increases. == The movement of water vapour and thermal energy in the atmosphere ==