Zonal index

Japanese: ゾーナル･インデックス（英語表記）zonal index

An index that indicates the strength of air flow along a line of latitude. It is also called the zonal index or east-west index. Numerically, it is expressed as the average of the east-west wind speed components along a line of latitude in a certain latitude band. Examples of latitude bands that can be considered are 15° to 35°, 35° to 55°, and 55° to 75°. Wind can be approximated by geostrophic winds, so when calculating the zonal index for a latitude band of 35° to 55°, for example, the difference in mean air pressure or the difference in mean isobar height on the 35° and 55° latitude lines is taken and converted to wind speed by multiplying it by an appropriate coefficient.

Source: Heibonsha World Encyclopedia, 2nd Edition Information

Japanese:

緯度線に沿う空気の流れの強さを表す示数。帯状示数，東西示数ともよぶ。数値的には，ある緯度帯の東西方向の風速成分の緯度線に沿っての平均によって表す。緯度帯としては，例えば15゜～35゜，35゜～55゜，55゜～75゜などを考える。風は地衡風で近似できるので，例えば35゜～55゜の緯度帯のゾーナル･インデックスを計算する場合，35゜と55゜のそれぞれの緯度線上の平均気圧の差，あるいは平均等圧面高度の差をとり，それに適当な係数を掛けて風速に直す。

出典　株式会社平凡社世界大百科事典第２版について　情報

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