Somewhere Over There
Bernard Hall of San Diego, California, asks:
"This past summer I routinely heard U.S. Coast Guard VHF transmissions made from hundreds of miles away. Is there something peculiar about my VHF installation, or is the reception the result of something else?"
Gordon West replies:
It’s clear that your installation was done very well, but chances are that the extraordinary VHF reception you are getting results from a large high-pressure system sitting over your boat. What happens is that a band of warm air becomes stratified about 600 feet above the surface of the water; the weather people call this a tropospheric duct inversion. The refractive index of air in the layer of warm air between the inversion above and the cool air at the water’s surface can sustain a VHF transmission with almost no reduction in signal strength for hundreds of miles.
Because a sailboat’s masthead antenna is well above the cooler surface air and in the inversion layer, there is a smooth path for VHF transmissions and receptions. In summer high-pressure conditions it is not uncommon to hear WX broadcasts coming from distant locations. Boston might hear Miami, Tampa might hear Galveston, and Hawaii might get Los Angeles. A good installation and high-pressure create the potential for long-range VHF transmissions—but it’s strictly a summer phenomenon.