Local Winds

CHAPTER TWO

AVIATION WEATHER

These winds average about 100 to 150 knots but may reach speeds in excess of 250 knots

(Figure 2-9). Since the jet stream is stronger in some places than in others, it rarely encircles the

entire hemisphere as a continuous river of wind. More frequently, it is found in segments from

1000 to 3000 miles in length, 100 to 400 miles in width, and 3000 to 7000 feet in depth.

The average height of jet stream winds is about 30,000 feet MSL, but they can be above or below

this level depending on the latitude and the season. During the winter, the position of the jet

stream is further south, the core rises to higher altitudes, and its speed is faster than in the

summer.

Figure 2-9 Jet Stream

The existence of jet streams at operational altitudes requires additional aircrew flight planning

consideration. The greater headwind component for westbound aircraft will increase fuel

consumption and may require additional alternate landing fields along the route. Wind shear

associated with the jet stream may also cause turbulence, forcing the aircrew to change altitude

or course.

Local Winds

The term "local," in the case of wind systems, applies to areas whose sizes range from tens of

miles across, to long, geographically thin areas. The local wind systems created by mountains,

valleys, and water masses are superimposed on the general wind systems and may cause

significant changes in the weather.

2-10 Atmospheric Mechanics of Winds, Clouds and Moisture, and Atmospheric Stability