Spring 2004 Issue

PROPAGATION

The Wind-Shear Theory

 By Tomas Hood, NW7US

The E-layer (E region) of the ionosphere is where sporadic-E propagation occurs, while tropospheric propagation modes occur at a much lower altitude, below 20 miles. (Courtesy of the Horizontal E-region eXperiment [HEX])

As we move from April to May, and into June, long-distance VHF and sometimes UHF propagation opens up by a mostly summer-time phenomenon called sporadic-E. Sporadic-E (Es) is the term given to the mode of propagation where clouds of highly dense ionization develop in the E-layer of the ionosphere. These clouds might be very small, but regardless of their size, they seem to drift and move about, making the propagation off these clouds short and unpredictable. It is well documented that Es occurs most often in the summer, with a secondary peak in the winter. These peaks are centered very close to the solstices. The winter peak can be characterized as being five to eight times less than the summer Es peak.
Scientists are still pursuing the multiple causes of sporadic-E. As far back as 1959, ten distinct types of sporadic-E and at least nine different theories of causation were offered. The classification of distinct types has been retained, but since the 1960s the wind-shear theory has become one of the most accepted theories.

Wind shear occurs when the wind blows at different directions and speeds as you increase in height. Simply, the wind-shear theory holds that gaseous ions in the E-layer are accumulated and concentrated into small, thin, patchy sheets by the combined actions of high-altitude winds and the Earth’s magnetic field. The resulting clouds may attain the required ion density to serve as a reflecting medium for VHF radio waves. Although most research has confirmed a close association between wind shear and sporadic-E, not all aspects of the sporadic-E phenomenon can be explained, including its diurnal and seasonal variations.

During periods of intense and widespread sporadic-E ionization, two-hop openings considerably beyond 1400 miles should be possible on 6 meters. Short-skip openings between about 1200 and 1400 miles may also be possible on 2 meters.

If wind shear is one of the most pronounced causes of sporadic-E, what is the trend of our global weather patterns for 2004 in the Northern Hemisphere? Since warmer ocean waters cause less wind shear, we would have to watch the Pacific and Atlantic Ocean temperature trends from December 2003 through the summer of this year.

The period between 1995 and 2003 has been the most active for Atlantic hurricanes in the historical record. Since 1995, seven of the nine hurricane seasons have been above normal (the exceptions being the El Niño years of 1997 and 2002). In the years when El Niño dominated (1997 and 2002), the presence of El Niño suppressed hurricane activity in the Atlantic. The sporadic-E summer season of 2002 was intense. There seems to be a clear correlation between the increased wind shear (resulting in less hurricane and severe storm activity) and Es activity.
 

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