MUF, foF2, and Ionosondes — How HF Propagation Forecasting Works

MUF, foF2, and Ionosondes — How HF Propagation Forecasting WorksMaximum Usable Frequency explained — what foF2 is, how ionosondes measure it, and how the BloomWX live MUF heatmap interpolates a global propagation field from worldwide GIRO stations.MUF — Maximum Usable Frequency — is the highest HF frequency that the ionosphere will refract back to Earth for a given path. Frequencies BELOW the MUF make the bounce; frequencies above punch through into space. Higher MUF means MORE bands open.Where the data comes fromThe ionosphere map plots live readings from the GIRO (Global Ionospheric Radio Observatory) network — about 50 ionosondes worldwide that fire a vertical-incidence radar pulse every 15 minutes. The key output is foF2, the F2-layer critical frequency. We multiply by an empirical M-factor (~3.3 for mid-latitudes) to get MUF for a typical 3,000 km path.Reading the heatmapBetween stations, IDW-interpolated MUF across the globe, smoothed with a Gaussian blur. Color buckets:

< 7 MHz: 80m only (polar night)

7-14 MHz: Up to 40m

14-21 MHz: Up to 20m

21-28 MHz: Up to 15m

28-50 MHz: Up to 10m, DX paradise

> 50 MHz: Up to 6m, solar-cycle peak F2 conditions

Part of the BloomWX learn library — beginner-friendly explainers covering every surface of the BloomWX weather dashboard. Open BloomWX to see live data for any U.S. county.