Sporadic E propagation

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Sporadic E or E_s is an unusual form of radio propagation utilizing characteristics of earth's ionosphere. Whereas most forms of sky-wave propagation use the normal and cyclic ionization properties of the ionosphere to refract (or "bounce") radio signals back toward the earth surface, sporadic E propagation bounces signals off a "cloud" of unusually ionized atmospheric gas in the ionosphere's E region (located at altitudes from about 90 to 160 km above the surface), occasionally allowing for long-distance communication at VHF and UHF frequencies not usually well-suited to such communication.^[1]^[2]

Unsolved problems in physics: How do E layer "clouds" form?

Communication distances of 500–1400 miles can occur utilizing a single E_s cloud. This variability in distance depends on a number of factors, including cloud height and density. MUF also varies widely, but most commonly falls in the 27–110 MHz range, which includes the FM broadcast band (87.5–108 MHz), and the amateur radio 10 and 6 meter bands. Strong events have allowed propagation at frequencies as high as 250 MHz.^{[citation needed]}

Sporadic E can happen almost any time — hence the name — but it does display seasonal patterns. Sporadic E peaks predictably in the summer in both hemispheres. In North America, the peak is most noticeable in mid-to-late June, trailing off though July and into August. Interestingly, a much smaller peak is seen around the winter solstice. Activity usually begins in mid-December in the northern hemisphere, with the days immediately after Christmas being the most active period.^{[citation needed]}

[edit] Further reading

Mid-Latitude Sporadic-E, A Review
Smith, Ernest K. Worldwide Occurrence of Sporadic E. U.S. Department of Commerce, National Bureau of Standards, 1957.