Weather Theory

Wind and Pressure Representation on Surface Weather Maps S o f s i f m o o e f u r e r r p h f e e o t i a a h a g r c c c e t h e a h h n l w a o l s t n o e l c t d y a c a a , t a t t l i h i t i o o o i t e o w n n r d n . s m e . p T p o a F r h i f e p o c i s s s t f r s s r p u m t o y t r r n h o o e p t e v , r e s e i a w d a o n i e n i f n d n d i f w n d o s f p e r u o a m r a r n r e f t m a d h a s t s c e a i p u e r o t r r i n o m e e w s n o a i s s n n a y p u d b s r s a s o e t u e l u l r a a m o t f n t a f w s d t r c , h o s e p b e n p a r u t s e s i n t l u s , a o m s a r l t f u r y s a o e r s c e a r t i o e e s s s 10 1 2 0 8 24 I I s s o o b b a a r r s s Clo p s re e s ly s u s r p e a g c r e a d d i i e s n o t b a a n rs d m st e ro a n n g a w s i t n e d e s p . and weather depiction charts, see Chapter 13, Aviation 1020 Weather Services. Wind conditions are reported by an arrow attached to the s r i a a n e t r a n p e t t o r i h d e o r e e s t n h e s d w c n l i r t o e r i s c b e s a c t t e h t t d w i i e o o b i n n n h y d e c f a t r i m h d o r e c m e o l a d e f n i . w r s t e [ h h F c t e i h t c i i a g a o h t r u n r t r t o h h e f w e r e o 1 , w m w 2 w i - i n 1 n w i d t 8 d h h ] i i s i t c s h T h b e h b l t o e l a h o w r e s w r y i t o n a i w n g b ti g l o f o p . n r w o o W i m c , n i i t t r n h i t c n h u d l g e e s s W m g i d e l r i a g e a d h n ly 1 i t e a 0 s w n p 1 s i t n a h 6 a d c a n s e l d l . d o w r i e s l o p a b r t e i a v s r e s s ly ure L 1012 northwest toward the southeast. The speed of the wind is 1008 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. Figure 12-19. Isobars reveal the pressure gradient of an area of high- or low-pressure areas. The pressure for each station is recorded on the weather chart and is shown in mb. Isobars are lines drawn on the chart to wind direction is modified by the friction and wind speed depict lines of equal pressure. These lines result in a pattern decreases due to friction with the surface. At levels 2,000 to that reveals the pressure gradient or change in pressure over 3,000 feet above the surface, however, the speed is greater distance. [Figure 12-19] Isobars are similar to contour lines and the direction becomes more parallel to the isobars. on a topographic map that indicate terrain altitudes and slope steepness. For example, isobars that are closely spaced Generally, the wind 2,000 feet above ground level (AGL) is indicate a steep pressure gradient and strong winds prevail. 20° to 40° to the right of surface winds, and the wind speed is Shallow gradients, on the other hand, are represented by greater. The change of wind direction is greatest over rough isobars that are spaced far apart and are indicative of light terrain and least over flat surfaces, such as open water. In the winds. Isobars help identify low- and high-pressure systems, absence of winds aloft information, this rule of thumb allows as well as the location of ridges and troughs. A high is an for a rough estimate of the wind conditions a few thousand area of high pressure surrounded by lower pressure; a low feet above the surface. is an area of low pressure surrounded by higher pressure. A ridge is an elongated area of high pressure, and a trough is Atmospheric Stability an elongated area of low pressure. The stability of the atmosphere depends on its ability to resist vertical motion. A stable atmosphere makes vertical Isobars furnish valuable information about winds in the first movement difficult, and small vertical disturbances dampen few thousand feet above the surface. Close to the ground, out and disappear. In an unstable atmosphere, small vertical air movements tend to become larger, resulting in turbulent airflow Calm NW/5 kts SW/20 kts 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 E/35 kts N/50 kts W/105 kts 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. Figure 12-18. Depiction of winds on a surface weather chart. 12-12