Space weather for CW
The sun decides how far your signal gets. Here are the numbers from right now - and what they mean.
How the bands stand today
| Band | Day | Night |
|---|---|---|
| 80m-40m | fair | good |
| 30m-20m | good | good |
| 17m-15m | fair | fair |
| 12m-10m | poor | poor |
What the numbers mean
Solar flux (SFI) measures the sun's radiation at 2800 MHz and is the handiest indicator of how strongly the upper atmosphere is ionised. Below 90 there is little going on and the high bands stay shut; above 140, 15, 12 and 10 m open up over long distances. The value follows the eleven-year solar cycle.
K index measures the disturbance of the earth's magnetic field, renewed every three hours, on a scale from 0 to 9. 0 to 1 is quiet, from 4 it turns unpleasant, from 5 it is called a storm. For CW it shows up as flutter and fading - the tone goes rough and wanders in volume.
A index is the same thing averaged over a whole day. It reacts more slowly and says more about how the day went overall.
Sunspots are the visible side of the same thing. Many spots usually mean a high SFI - but not immediately; the ionosphere takes hours.
Noise floor as an S reading: what is left on the band when nobody is transmitting. At S0 to S1 you hear very weak stations; at S7 they vanish into the noise.
Why CW copes better with all this
A Morse signal at ordinary speed occupies about 100 Hz. A voice signal needs roughly 2700 Hz - twenty-seven times as much. Noise spreads across the bandwidth: narrow the receiver to 100 Hz and you keep almost all of the noise out while letting the wanted signal through.
That is why CW still works when voice has long given up - and why bad space weather hurts Morse less than anything else. You can hear how much it matters in the interference training: the same message, once with a wide filter and once with a narrow one.
Where the values come from
Our own server fetches the figures hourly from hamqsl.com (N0NBH). A second request to the US agency NOAA/SWPC returns the same K index independently - it is not displayed, only used to notice when the first source stops moving. A value that has not changed for two days otherwise looks just like a fresh one.
Your browser talks to none of these servers. That is not a detail: a request to a third party from within the page would be a transfer to that party and would require a consent banner on every page of this site.
Frequently asked questions
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At which values is it worth switching on?
For CW, almost always. A high SFI opens the upper bands, a low one pushes traffic to 40 and 80 m - but something is always open. The K index matters more: from 5 upwards every band gets noisy and fluttery, and then the lower bands are the calmer ones.
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Why do the band figures differ so much between day and night?
The D layer of the ionosphere is created by sunlight and absorbs low frequencies. During the day little gets far on 80 m; at night the layer disappears and the same band carries across continents. On 10 m it is the other way round: there you need the sun for anything to be reflected at all.
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How current are the figures?
The time of measurement is shown under the values, together with the age in hours. If they are older than six hours the page says so explicitly - a number without its age is a claim.