什么是 Jet stream; subtropical and polar-front jets？

A jet stream is a narrow band of strong wind, normally a horizontal wind high in the troposphere, whose position changes from day to day. The subtropical jet blows from the west near the poleward edge of the Hadley cell, usually between $20{}^{\circ}$ and $30{}^{\circ}$ of latitude at heights of $12\text{ to }14\,\mathrm{km}$. The polar-front jet blows from the west above the boundary between polar and subtropical air, between about $50{}^{\circ}$ and $60{}^{\circ}$; it is the fast core of the westerlies, steered by and steering the depressions and anticyclones of the mid-latitudes.