HF Propagation Forecast & Solar Data

Monitor real-time solar conditions and HF propagation forecasts. Check solar flux, K-index, sunspot numbers and band conditions to plan your next QSO.

What the solar numbers actually mean

Solar flux, measured at 10.7 centimetres and quoted in solar flux units, is the best single indicator of how well the higher HF bands will work. A flux below about 70 leaves 10 and 12 metres largely closed, values between 90 and 130 open them for regular daytime DX, and anything above 150 can keep 10 metres alive into the evening. The sunspot number tracks the same underlying activity and moves roughly in step with it.

The K index measures disturbance in the earth magnetic field on a scale from 0 to 9, updated every three hours. Values of 0 to 2 mean quiet conditions and stable paths. From K4 upward the ionosphere becomes disturbed, paths that cross high latitudes degrade first, and at K6 and above polar routes can close entirely while the lower bands become noisy. The A index is a daily average of the same disturbance, so it tells you whether the whole day has been settled or unsettled.

Which band to use and when

As a rule of thumb the higher bands follow the sun. Ten, twelve and fifteen metres are daytime bands that need decent solar flux and normally close after dark. Twenty metres is the workhorse, usually open somewhere in the world around the clock. Forty metres becomes useful in the late afternoon and is reliable all night, while eighty and one hundred and sixty metres are night bands that suffer heavily from summer static. When the K index rises, drop down a band rather than giving up.

Frequently asked questions

What is a good propagation score for DX?

Anything above 75 means the bands should be in good shape and even SSB will get out. Between 55 and 75 is workable with CW or FT8. Below 35, weak-signal digital modes are the only reliable option. The score combines solar flux, geomagnetic activity, flare level, solar wind and your local grey line into a single figure.

What is a good solar flux index for 10m?

10m generally needs a solar flux index above about 150 for reliable worldwide F2 openings, and above 200 the band can stay open for most of the daylight hours. Below 100 the band is usually limited to sporadic E in season and short skip.

Does a high K-index kill HF propagation?

A K-index above 5 indicates a geomagnetic storm and will badly degrade HF, especially on paths crossing the auroral zone and the poles. The high bands lose their maximum usable frequency first, while the low bands suffer raised noise. A K-index below 3 is quiet and best for DX.

What is the grey line and why does it matter?

The grey line is the twilight band between day and night that sweeps around the Earth. For roughly an hour either side of your sunrise and sunset the D layer, which absorbs low band signals during the day, decays faster than the F layer. That short window often gives exceptional 160m, 80m and 40m DX.

How often is the propagation data updated?

Solar and geomagnetic figures come directly from the NOAA Space Weather Prediction Center and refresh every ten minutes on this page. The planetary K-index itself is published every three hours, while solar wind and X-ray readings update every few minutes.

What is the difference between the solar flux index and the sunspot number?

They measure the same underlying solar activity in different ways. The sunspot number is a count of visible spots and spot groups, which is why it can jump around from day to day. The solar flux index is a physical measurement of radio noise from the Sun at 10.7 centimetres, taken the same way every day regardless of what is visible. For predicting HF, the flux is the more useful of the two, and it is the number this page leads with.

What is the MUF and why does it matter more than the flux?

The maximum usable frequency is the highest frequency the ionosphere will still bend back to Earth on a given path at a given moment. Above it, your signal goes through the ionosphere and into space. The flux tells you roughly how energetic the ionosphere is; the MUF turns that into a practical answer about which bands can work right now. If the estimated MUF is below 14 MHz, twenty metres is not going to give you F2 DX no matter how much power you run.

What is the difference between the K-index and the A-index?

Both describe disturbance in the Earth’s magnetic field. The K-index is a three hourly snapshot on a scale of 0 to 9, so it tells you what is happening now. The A-index is a daily figure derived from the day’s K values on a linear scale, so it tells you whether the whole day has been settled or rough. A quiet K with a high A usually means the field is recovering from a disturbance earlier in the day.

Why do polar paths close first when the field is disturbed?

Because charged particles from the Sun are funnelled into the atmosphere near the magnetic poles, where they raise absorption and make the ionosphere unstable. A path from Europe to Japan or to the west coast of North America crosses those latitudes, so it degrades while a path south over the equator is still perfectly usable. This is why a raised K-index can make the bands sound dead in one direction and normal in another.

What is IMF Bz and why does a negative value matter?

Bz is the north-south component of the interplanetary magnetic field carried by the solar wind. When it points south, it couples efficiently with the Earth’s own field and lets energy pour in, which is what turns a fast solar wind into a geomagnetic storm. A strongly negative Bz is an early warning that the K-index is about to rise, often hours before it does.

What is a radio blackout and how is it different from a geomagnetic storm?

A radio blackout is caused by X-ray flux from a solar flare, arrives at the speed of light, affects only the daylit side of the Earth, and clears within minutes to hours. A geomagnetic storm is caused by particles arriving a day or more later, hits the high latitudes hardest, and can last for days. The first kills the low bands and absorption-limited paths abruptly; the second degrades polar paths and raises noise for much longer.

Why is 10 metres closed when the flux looks high?

Solar flux sets the ceiling, not the floor. Ten metres also needs the path to be in daylight, the geomagnetic field to be reasonably quiet, and the season to help. A flux of 150 with a K-index of 5 will produce far worse conditions on ten than a flux of 120 with a K-index of 1. The score at the top of this page combines these rather than looking at flux alone, which is why it sometimes disagrees with the raw numbers.

What is sporadic E and does the solar data predict it?

Sporadic E is dense patches of ionisation in the E layer that reflect signals well above the normal MUF, giving strong short skip on 10, 6 and occasionally 2 metres. It has little to do with solar flux and is not predicted by any of the figures on this page. It follows a seasonal pattern, peaking in late spring and early summer in the northern hemisphere, and it appears and vanishes over minutes.

What conditions are typical rather than exceptional?

Across a normal week, twenty metres is open somewhere for most of the daylight hours and often well into the evening, and forty metres is reliable from late afternoon until dawn. Eighty and one hundred and sixty metres are night bands that are quiet in winter and heavily degraded by static in summer. Fifteen metres follows the flux closely, and ten and twelve metres are the first to close and the last to open as the solar cycle moves. A K-index of 0 to 2 is the normal quiet state; 4 or above happens on a minority of days and is what makes a weekend feel unusually hard.

Should I trust this score over my own ears?

No. The score is a prediction built from global measurements and your grid square, and it cannot know about your local noise floor, your antenna, or a sporadic E opening that started two minutes ago. Use it to decide which band to try first and when to be at the radio, then let the radio tell you what is actually happening.

More tools on this site

This page predicts. To see what stations are actually being heard right now, use the live DX cluster. To test one specific path on a fixed schedule, listen for the beacon shown by the NCDXF beacon monitor. To plan a weekend around activity rather than conditions, see the contest calendar, and for VHF work above the HF bands there is the satellite tracker.