Editing Potential evapotranspiration (section)

== Measurements ==
[[File:Potential_evapotranspiration_Hawaii.gif|thumb|350x350px|Monthly estimated potential evapotranspiration and measured pan evaporation for two locations in [[Hawaii]], Hilo and Pahala]]Potential evapotranspiration is usually measured indirectly, from other climatic factors, but also depends on the surface type, such as free water (for lakes and oceans), the [[soil]] type for bare soil, and also the density and [[Plant diversity|diversity]] of [[vegetation]]. Often a value for the potential evapotranspiration is calculated at a nearby climate station on a reference surface, conventionally on short grass. This value is called the reference evapotranspiration, and can be converted to a potential evapotranspiration by multiplying by a surface coefficient. In agriculture, this is called a crop coefficient. The difference between potential evapotranspiration and actual precipitation is used in [[irrigation scheduling]].

Average annual potential evapotranspiration is often compared to average annual precipitation, the symbol for which is ''P''. The ratio of the two, ''P''/''PET'', is the [[aridity index]]. A [[humid subtropical climate]] is a zone of climate with hot and humid summers, and cold to mild winters. Subarctic regions, between 50°N<ref>{{Cite web |title=Humid subtropical climate (Cfa) {{!}} SKYbrary Aviation Safety |url=https://skybrary.aero/articles/humid-subtropical-climate-cfa-0#:~:text=Description&text=A%20humid%20subtropical%20climate%20is,poleward%20from%20adjacent%20tropical%20climates. |access-date=2023-10-19 |website=skybrary.aero}}</ref> and 70°N latitude, have short, mild summers and freezing winters depending on local climates. Precipitation and evapotranspiration is low (compared to warmer variants), and vegetation is characteristic of the coniferous/taiga forest.