TropoMap Tropospheric-Ducting Forecast Map

Why I built it. I'm a long-time DXer and the developer behind the OTAMap apps. I wanted a live tropospheric-ducting overlay in my apps, so I asked to use an existing tropo data source — and was politely turned down. Fair enough; it's someone else's work. So I wrote my own. This map runs on a ducting index I compute from the NOAA GFS weather model — temperature, humidity and geopotential-height profiles turned into an atmospheric refractivity (M-unit) gradient. It now drives the live tropo overlay in my OTAMap DVR and OTAMap Live TV apps, and this free web version.

If it's been helpful to you, I'd genuinely love to hear about it.

My apps: OTAMap on the App Store  ·  all my apps

What is tropospheric ducting?

Normally VHF/UHF signals are limited to roughly line-of-sight. When the lower atmosphere forms an inversion (warm-over-cool air, sharp humidity gradients — common with high pressure, calm clear nights, and over water), it bends those signals back toward the ground and can trap them in a "duct" that carries them hundreds — occasionally thousands — of kilometres. That's the difference between hearing nobody past the next city and working stations several provinces or states away.

What the colours mean

The overlay shows how hard the atmosphere is bending signals right now. Where the map is clear, the air's behaving normally and only the usual faint scatter is in play.

How to read it

• Read the paths, not your pixel. What counts isn't the exact value over your head — it's whether a duct runs in a direction you care about.
• Brighter colour = more bending = stronger signals reaching farther.
• A duct is a road — signals travel along it. Aim your antenna down the duct and work whoever's under it.
• A path is only as open as its weakest point. One gap drops the whole thing — that's the "worst gap" the path tool flags.
• Distance is unforgiving: the farther you're reaching, the more robust the duct has to be the entire way.
• Time of day swings it. Land paths tend to be best around dawn and overnight and tail off by mid-afternoon; sea paths often run the opposite, peaking in the afternoon. Mixed land-and-water routes fall somewhere between, depending on the day.
• Terrain is a big deal. Ducting sits in the very bottom of the atmosphere, so the ground beneath shapes it — two spots a short drive apart can hear completely different things.
• A clear map isn't "dead air" — just no boost beyond the faint background scatter the index doesn't count. And even when you're not under a duct, a nearby one can leak you weak signals.

Tropospheric DX modes

Tropo ducting is the main DX mode on the higher ham bands — 2 m (144), 1.25 m (222), 70 cm (432) and microwave — and it also boosts FM and TV DX. During an opening, work it with SSB/CW for weak-signal, or FT8 to pull marginal paths out of the noise; aim the beam down the duct, chase grid squares and distant stations, and confirm conditions with beacons and live spotting.

Computed independently from public NOAA GFS weather data.

Data & credits: ducting model from NOAA GFS · beacons from WZ1V (newsvhf.com) · repeaters from hearham.com (open data) · TV stations from FCC/ISED · imagery & labels © Esri · basemaps © CARTO/OpenStreetMap · map by Leaflet.

⚠ About the “demo” forecast

The “now” frame is real. It's the live ducting field computed from the latest NOAA GFS run — your actual current conditions.

The future frames are a preview, not a real forecast — yet. Everything past “now” is synthesised from the current field two ways:

• A day/night curve. Ducting follows a daily rhythm — it tends to build overnight under temperature inversions and fade by mid-afternoon — so each future frame is scaled up or down by that cycle (it peaks around 3 a.m. local).
• A gentle eastward drift, a stand-in for weather generally tracking west-to-east.

So the animation shows a plausible shape of today's pattern over time, not a true hour-by-hour prediction. That's why every frame after “now” is tagged ⚠ demo.

What the real forecast will be

GFS publishes predicted atmospheres at forecast hours f006, f012 … f072+. Running the same ducting computation on each of those yields a genuine 0–72 hour forecast. Once those per-hour fields are wired in, the slider becomes a real forecast and the “demo” tag comes off.

The map mechanics — the colours, the time slider, and the path scoring — are all real; only the forward projection is a placeholder for now.