High-Pressure Weather Systems

Large, sluggish, high-pressure systems (or anticyclones) create the most dramatic and widespread tropospheric ducts due to sub- sidence. Subsidence inversions in high-pressure systems are cre- ated by air that is sinking. As air descends, it is compressed and heated. Layers of warmer air—temperature inversions—often form between 500 and 3000 m (1500-10,000 ft) altitude, as shown in Fig 21.26. Ducts usually intensify during the evening and early morning hours, when surface temperatures drop and suppress the tendency for daytime ground-warmed air to rise. In the Northern

Fig 21.25—Temperature and dewpoint profile of an early- morning radiation inversion. Fog may form near the ground. The midday surface temperature would be at least 30°C.

Fig 21.26—Temperature and humidity profile across an elevated duct at 1000 m altitude. Such inversions typically form in summertime high-pressure systems. Note the air is very dry in the inversion.

Hemisphere, the longest and strongest radio paths usually lie to the south of high-pressure centers. See Fig 21.27.

Sluggish high-pressure systems likely to con- tain strong temperature inversions are common in late summer over the eastern half of the US. They generally move southeastward out of Canada and linger for days over the Midwest, providing many hours of extended propagation. The southeastern part of the country and the lower Midwest experience the most high-pres- sure openings; the upper Midwest and East Coast somewhat less frequently; the western mountain regions rarely.

Semipermanent high-pressure systems, which are nearly constant climatic features in certain parts of the world, sustain the longest and most exciting ducting paths. The Eastern Pacific High, which migrates northward off the coast of Cali- fornia during the summer, has been responsible for the longest ducting paths reported to date. Countless contacts in the 4000-km (2500 mi) range have been made from 144 MHz through

5.6 GHz between California and Hawaii. The Bermuda High is a nearly permanent feature of the Caribbean area, but during the summer it moves north and often covers the southeastern US. It has supported contacts in excess of 2800 km (1700 mi) from Florida and the Caroli- nas to the West Indies, but its full potential has not been exploited. Other semipermanent highs lie in the Indian Ocean, the western Pacific and off the coast of western Africa.