One of the mysteries of the English language finally explained.
The upper or F stratum of the ionosphere, above the E (or Heaviside) layer; = "F-layer".
1930s; earliest use found in The Fresno Bee. From the name of Edward V. Appleton, British physicist, who discovered the layer + layer.
Appleton layer/ˈaplt(ə)n ˌleɪə/
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