the middle latitudes to be closer to vegetation zoning and genetic climate systems. It was considered a more true or "real world" reflection of the global climate. The Trewartha climate classification changes were seen as most effective on the large landmasses in Asia and North America, where many areas fall into a single group (C) in the Köppen–Geiger system. For example, under the standard Köppen system, Washington and Oregon are classed into the same climate zone (Csb) as parts of Southern California, even though the two regions have strikingly different weather and vegetation. Another example was classifying cities like London or Chicago in the same climate group (C) as Brisbane or New Orleans, despite great differences in seasonal temperatures and native plant life. Scheme Trewartha's modifications to the 1899 Köppen climate system sought to reclass the middle latitudes into three groups: C (subtropical)—8 or more months have a mean temperature of or higher; D temperate—4 to 7 months have a mean temperature of 10 °C or higher; and E boreal climate—1 to 3 months have a mean temperature of 10 °C or higher. Otherwise, the tropical climates and polar climates remained the same as the original Köppen climate classification. Thornthwaite Devised by the American climatologist and geographer C. W. Thornthwaite, this climate classification method monitors the soil water budget using evapotranspiration. It monitors the portion of total precipitation used to nourish vegetation over a certain area. It uses indices such as a humidity index and an aridity index to determine an area's moisture regime based upon its average temperature, average rainfall, and average vegetation type. The lower the value of the index in any given area, the drier the area is. The moisture classification includes climatic classes with descriptors such as hyperhumid, humid, subhumid, subarid, semi-arid (values of −20 to −40), and arid (values below −40). Humid regions experience more precipitation than evaporation each year, while arid regions experience greater evaporation than precipitation on an annual basis. A total of 33 percent of the Earth's landmass is considered either arid or semi-arid, including southwest North America, southwest South America, most of northern and a small part of southern Africa, southwest and portions of eastern Asia, as well as much of Australia. Studies suggest that precipitation effectiveness (PE) within the Thornthwaite moisture index is overestimated in the summer and underestimated in the winter. This index can be effectively used to determine