Wind and Pressure Representation on Surface  Weather Maps  Atmospheric Stability  Surface weather maps provide information about fronts, areas  of high and low pressure, and surface winds and pressures  for each station. This type of weather map allows pilots to  see the locations of fronts and pressure systems, but more  importantly, it depicts the wind and pressure at the surface  for each location. For more information on surface analysis  and weather depiction charts, see Chapter 13, Aviation  Weather Services.  Wind conditions are reported by an arrow attached to the  station location circle.  [Figure 12-18]  The station circle  represents the head of the arrow, with the arrow pointing  in the direction from which the wind is blowing. Winds  are described by the direction from which they blow, thus  a northwest wind means that the wind is blowing from the  northwest toward the southeast. The speed of the wind is  depicted by barbs or pennants placed on the wind line. Each  barb represents a speed of ten knots, while half a barb is equal  to five knots, and a pennant is equal to 50 knots.  The pressure for each station is recorded on the weather chart  and is shown in mb. Isobars are lines drawn on the chart to  depict lines of equal pressure. These lines result in a pattern  that reveals the pressure gradient or change in pressure over  distance.  [Figure 12-19]  Isobars are similar to contour lines  on a topographic map that indicate terrain altitudes and  slope steepness. For example, isobars that are closely spaced  indicate a steep pressure gradient and strong winds prevail.  Shallow gradients, on the other hand, are represented by  isobars that are spaced far apart and are indicative of light  winds. Isobars help identify low- and high-pressure systems,  as well as the location of ridges and troughs. A high is an  area of high pressure surrounded by lower pressure; a low  is an area of low pressure surrounded by higher pressure. A  ridge is an elongated area of high pressure, and a trough is  an elongated area of low pressure.  Isobars furnish valuable information about winds in the first  few thousand feet above the surface. Close to the ground,  wind direction is modified by the friction and wind speed  decreases due to friction with the surface. At levels 2,000 to  3,000 feet above the surface, however, the speed is greater  and the direction becomes more parallel to the isobars.  Generally, the wind 2,000 feet above ground level (AGL) is  20° to 40° to the right of surface winds, and the wind speed is  greater. The change of wind direction is greatest over rough  terrain and least over flat surfaces, such as open water. In the  absence of winds aloft information, this rule of thumb allows  for a rough estimate of the wind conditions a few thousand  feet above the surface.  The stability of the atmosphere depends on its ability to  resist vertical motion. A stable atmosphere makes vertical  movement difficult, and small vertical disturbances dampen  out and disappear. In an unstable atmosphere, small vertical air  movements tend to become larger, resulting in turbulent airflow  and convective activity. Instability can lead to significant  turbulence, extensive vertical clouds, and severe weather.  Rising air expands and cools due to the decrease in air  pressure as altitude increases. The opposite is true of  descending air; as atmospheric pressure increases, the  temperature of descending air increases as it is compressed.  Adiabatic heating and adiabatic cooling are terms used to  describe this temperature change.  NW / 5 kts  SW / 20 kts  E / 35 kts  N / 50 kts  W / 105 kts  Calm  Figure 12-18.  Depiction of winds on a surface weather chart.  Figure 12-19.  Isobars reveal the pressure gradient of an area of  high- or low-pressure areas.  1028  1024  1020  1016  1012  1008  I s o b a r s  I s o b a r s  C $_{l}$o s e $_{l}$y s p a c e d $_{i}$s o b a r s m e a n a s t e e p  p $_{r}$e s s u r e g r a d $_{i}$e n t n d s t r o n g w i n d s .  W i d e l y s p a c e d i s o b a r s  m e a n a s h a l o w p r e s s u r e  g r a d i e n t a n d r e l a t i v e l y  l i g h t w i n d s .  12-12 