Space Weather — OpenHamClock Manual

Why Space Weather Matters for Radio

Every HF contact you make relies on radio waves bouncing off the ionosphere — layers of charged particles in the upper atmosphere, sitting roughly 60–600 km above the Earth. The ionosphere is created and sustained by solar radiation. When the sun is active, the ionosphere is stronger and higher frequencies propagate further. When the sun is disturbed — by solar flares or geomagnetic storms — the ionosphere can be disrupted or completely absorbed, wiping out entire bands for hours or days.

Three numbers summarise solar conditions at any moment: SFI, K-index, and SSN. OpenHamClock displays all three in real time with colour-coded indicators so you can read them at a glance from across the shack.

Solar Flux Index (SFI)

The Solar Flux Index (also written as F10.7) measures the intensity of solar radio emission at 10.7 cm wavelength. It is the best single-number proxy for solar activity and HF propagation potential.

SFI value	Solar activity	HF propagation
Below 70	Very low — solar minimum	Higher bands (10–15 m) mostly closed. 40–80 m reliable regionally.
70 – 100	Low to moderate	20 m usually open. 15 m opens at solar noon. 10 m sporadic.
100 – 150	Moderate — rising cycle	15 m opens reliably. 10 m starts to come alive for long-haul paths.
150 – 200	High — solar maximum	10 m wide open. Worldwide contacts on 10 and 12 m are common.
Above 200	Very high — solar maximum peak	6 m can open globally. 10 m is outstanding. Magic band conditions.

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SFI and the solar cycle

SFI follows the roughly 11-year solar cycle. We are currently in Solar Cycle 25, which reached a stronger-than-predicted solar maximum around 2024–2025. Values above 200 are possible near the peak — conditions not seen since Cycle 23 in 2000–2001. The 30-day SFI history chart in the Solar panel shows where we are in the cycle.

K-index (Kp) — Geomagnetic Activity

The K-index (or planetary Kp index) measures disturbances in Earth's magnetic field caused by solar wind and coronal mass ejections (CMEs). Unlike SFI — where higher is better — a high K-index is bad news for HF.

Kp value	Condition	Effect on HF	Visible aurora
0 – 1	Quiet	Excellent — full ionospheric support	No
2 – 3	Unsettled	Good — minimal degradation	Very high latitudes only
4	Active	Minor polar path degradation	Above 60° latitude
5	Minor storm (G1)	Polar paths unreliable; mid-lat HF affected	Above 50° latitude
6 – 7	Moderate storm (G2–G3)	Significant HF degradation, blackouts possible at high latitudes	Above 45° latitude
8 – 9	Severe/Extreme storm (G4–G5)	HF blackout across most of globe. Radio may be useless.	Visible at mid-latitudes

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High K-index is not always bad

While a high Kp wrecks HF, it can open VHF and UHF via aurora backscatter. When Kp reaches 5 or above, check the Aurora map layer and look for other operators on 2 m CW using aurora scatter — a unique propagation mode with a characteristic buzzing, watery sound.

Sunspot Number (SSN)

The Sunspot Number (SSN, also called the Wolf Number) is a daily count of sunspot groups visible on the solar disc. Sunspots are cooler, magnetically active regions that correlate strongly with solar flux. More sunspots = higher SFI = better HF propagation.

SSN is slower-moving than SFI — it changes over days and weeks, not hours. It is best used as a medium-term trend indicator: is the sun becoming more or less active this month? The 30-day history chart shows this clearly.

Band Conditions Panel

Rather than asking you to interpret raw numbers, the Band Conditions panel translates SFI, K-index, and time-of-day into simple per-band indicators: open, marginal, or closed. Data comes from NOAA's N0NBH solar indices service.

📊 Screenshot: Band Conditions panel assets/img/screenshots/band-conditions.png

The Band Conditions panel. Each HF band gets a coloured indicator: green = open, yellow = marginal, red = closed. Day and night conditions are shown separately.

Each band shows two indicators: Day and Night.

Day — conditions during local daylight hours, when F2 propagation is active.
Night — conditions after dark, when the D-layer absorption disappears and lower bands extend their range dramatically.

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VHF indicators

The Band Conditions panel also includes VHF+ indicators for E-skip (sporadic-E ionisation, opening 6 m and sometimes 2 m to continental distances) and Aurora (as described above). These are less predictable than HF conditions but well worth watching in summer months.

Solar Panel — Four Views

The Solar panel cycles through four different views automatically, or you can click to step through them manually:

1 — Solar Image The latest EUV/X-ray image of the sun from NASA/SDO. Sunspot groups appear as dark spots. Active regions glow brightly in UV. Updated every 15 minutes.

2 — Solar Indices A 30-day history chart of SFI, SSN, and K-index. Excellent for spotting trends — is the cycle rising? Did a storm just pass? Are we on the downward slope?

3 — X-ray Flux Real-time X-ray flux from GOES satellites. X-ray flares appear as sharp spikes. A strong X-class flare can cause a sudden ionospheric disturbance (SID) — HF blackout on the sunlit side of Earth within minutes of the flare.

4 — Lunar Phase Current moon phase, illumination percentage, rise and set times, and distance. Useful for EME (Earth-Moon-Earth) operators — perigee (closest approach) means a stronger path and lower path loss.

Propagation Predictions

The Propagation panel (separate from Band Conditions) uses the ITU-R P.533-14 HF propagation model — the same international standard used by broadcast engineers worldwide — to predict per-band reliability percentages for your specific path to a selected DX target.

When you click a DX spot and open the DX Location panel, the propagation panel updates to show predicted signal reliability on each band for the current time of day on the specific path from your QTH to the DX location. This is not a generic "band open/closed" estimate — it is a point-to-point model corrected with real ionosonde data from the nearest measurement station.

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Using propagation predictions in practice

Suppose you see a rare Pacific DX spot on 15 m. Click it, and the propagation panel shows 15 m at 82% reliability for that path right now, 20 m at 65%, and 12 m at 91%. Consider calling on 12 m — the model says it is your strongest bet even if the cluster spot was on 15 m. Most DXpeditions operate on multiple bands simultaneously.