Radio Resources

Predicting Hot Tropo Days

by Gordon West, WB6NOA
 

Police and fire dispatchers serving rural areas outside of Houston, Texas, knew they were going to have a tough shift ahead. “The sun is just barely up, and the temperature is 90 degrees, and the air smells like stale smoke,” said one of the dispatchers for three VHF high-band law enforcement services. “Tampa and Key West, Florida will be stronger than our local units,” adds another dispatcher, referring to the UHF 460-MHz law net frequencies.

Sure enough, their predictions were right on target: distant VHF and UHF signals coincidentally using the same CTCSS tones were overriding their local squelch and sometimes drowning out their own mobile units just miles away. The condition is called tropospheric ducting.

Tropospheric Ducting

This weather-related radio phenomena occurs every July through September in almost every part of the country. Ham radio operators on VHF and UHF anticipate these conditions with the excitement of working well beyond line-of-sight, yet public safety dispatchers hope the condition goes away and things get back to normal as soon as possible. But unlike ionospheric VHF skip, tropospheric ducting of far away VHF and UHF stations may last for days.
Ducting of VHF, UHF, and microwaves occurs in our lower atmosphere, called the “weather region.” The long-range propagation of normally short-range VHF/UHF signals has nothing to do with the ionosphere, but everything to do with local weather conditions. When a high-pressure system settles in over a widespread region, tropospheric ducting may span short-range signals over hundreds and sometimes more than 1,000 miles distant.

The trigger to a VHF/UHF and microwave tropospheric duct is layers of unmixed air that may stratify over a large region, usually in the presence of a high-pressure system. High-pressure cells are most common July through October and are continuously fed by warm, moist air flowing up from the equator. The concentration of this equatorial air stalls-out just north of 30 degrees latitude, and it begins to rotate clockwise, consistent with high-pressure cell rotation. As more air gets pulled into this region of high pressure, the concentration of air becomes heavier than the air below, and it slowly begins to spiral clockwise down toward the earth’s surface.

This sinking air is called subsidence, and it may drop thousands of feet until it “bottoms out” at about 1,000 feet above the earth. As more air sinks on top of the stratified layer, dropping air from the high-pressure area, the air gets compressed. When you compress air, it gets warm, and the warm stratified air may begin to build over a widespread region. With little surface wind to stir up these radically different air masses—moist air along the surface of the earth, capped by a stratified layer of warm air which has been compressed to drive out much of its moisture content, and then cool air above—a thin, long tropospheric duct begins to form.

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