JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
Surface Winds
When we consider winds below 2000 feet AGL, we cannot ignore the role of surface friction in
the analysis of wind direction. Surface friction reduces the speed of the wind, which causes a
reduction in the Coriolis force. This results in a different set of forces that must be balanced: the
PGF, Coriolis force, and friction. When the new balance of forces is reached, the air blows at an
angle across the isobars from high pressure to low pressure. This angle varies as a result of the
type of terrain, but for our purposes, we will assume a 45° angle (Figure 2-7). Another way to
think of this effect is that the Coriolis force still tries to turn the wind to the right, from its initial
intended direction of the PGF, but it does not turn to the right quite as much as with gradient
winds. Thus, surface winds still move clockwise around highs, and counterclockwise around
lows, but since they blow across the isobars at a 45° angle, they also have a component of
motion that moves air out of the high pressure and into the low.
Figure 2-7 — Surface Winds Are Deflected Across Isobars Toward Lower Pressure
Movement of Pressure Systems and Large Scale Wind Patterns
Weather in the Temperate Zone (which includes the U.S.) and farther north, changes almost
constantly with the passage of highs and lows. These migrating systems move from west to east
with the prevailing westerly winds. They are accompanied by wind shifts, and with some
exceptions, large and rapid changes in temperature and broad areas of precipitation. These
systems furnish the most significant means of heat transfer between high and low latitudes.
The Jet Stream
Wind speeds generally increase with height through the troposphere, reaching a maximum near
the tropopause, and often culminating in the jet stream. The jet stream is a narrow band of strong
winds of 50 knots or more that meanders vertically and horizontally around the hemisphere in
wave-like patterns. The jet streams (polar and subtropical) have a profound influence on weather
patterns.
Version 3.2/Dec 08 2-7