NOAA Space Weather Scales: G, S and R Levels for Hams

When a magazine headline says that a severe geomagnetic storm is on its way, what should a radio amateur actually expect? The answer is easier to find if you know the code used by forecasters. The NOAA space weather scales turn complicated physics into numbered levels, much like the categories for hurricanes and earthquakes, and each level comes with a description of likely effects. This guide explains the three scales published by the NOAA Space Weather Prediction Center: G for geomagnetic storms, S for solar radiation storms and R for radio blackouts. It focuses on what each means for HF, VHF aurora and polar paths, how often the events occur, and how to use alerts when planning a session.

What the Scales Are

The NOAA Space Weather Scales page explains that the scales were introduced to communicate to the general public the current and future space weather conditions and their possible effects on people and systems. Many Space Weather Prediction Center products describe the space environment, but few described the effects that can be experienced as a result of disturbances, and the scales fill that gap. They describe disturbances for three event types: geomagnetic storms, solar radiation storms and radio blackouts. Each scale has numbered levels, from 1 for minor to 5 for extreme, analogous to the scales for hurricanes, tornadoes and earthquakes, with a list of possible effects, a physical measure and the average frequency of events.

The frequency column is given per solar cycle of 11 years, so it tells you how often to expect each level over a full cycle. That is useful to keep in mind with our article on solar flux, A-index and K-index, which covers the daily values that sit underneath these levels. The scales are the plain-language layer on top.

The G Scale: Geomagnetic Storms

Geomagnetic storms are disturbances of the Earth’s magnetic field. They are measured with the planetary K-index (Kp), and the G scale maps Kp to levels. The effects listed by NOAA for HF radio are the most relevant for amateurs.

The table reproduces the HF and aurora entries from the NOAA page; the page also lists effects on power systems, spacecraft, satellite navigation and pipelines at each level. For radio amateurs the main message is that the higher the G level, the more the HF bands are disturbed, the lower in latitude the aurora can be seen, and the better the chances for radio aurora on VHF. Our guide to radio aurora on VHF explains how to use those events.

The S Scale: Solar Radiation Storms

Solar radiation storms are bursts of energetic particles from the Sun, measured by the flux of particles at energies of 10 MeV or more. The S scale runs from S1 to S5. Their main effects on radio are on paths that cross the polar regions. According to NOAA, S1 has minor impacts on HF radio in the polar regions, S2 gives small effects on HF propagation through the polar regions, S3 gives degraded HF propagation through the polar regions, S4 is likely to blackout HF radio through the polar regions, and S5 can cause a complete blackout of HF communications through the polar regions.

• S1 (minor): about 50 events per cycle; minor impacts on HF in the polar regions.
• S2 (moderate): about 25 per cycle; small effects on polar HF propagation.
• S3 (strong): about 10 per cycle; degraded polar HF propagation.
• S4 (severe): about 3 per cycle; blackout of HF radio through the polar regions for several days is likely.
• S5 (extreme): fewer than 1 per cycle; complete HF blackout through polar regions possible.

The practical meaning for hams is that radio contacts that go over the pole, such as paths from Europe to the west coast of North America or to Japan, may suffer during radiation storms even if the rest of the bands seem normal. Stations at high latitudes feel the effect most. If you use a path predictor, you can check the radiation storm level before blaming your antenna.

The R Scale: Radio Blackouts

Radio blackouts come from solar flares. The R scale uses the peak X-ray flux of the flare, measured in the GOES X-ray classes, to define levels from R1 to R5. The effects occur on the sunlit side of the Earth and last for minutes to hours, which matches the time the flare keeps the lower ionosphere heavily ionized. NOAA lists the physical measure and the frequency per cycle for each level.

• R1 (minor), X-ray class M1: about 2000 events per cycle (about 950 days); weak degradation on the sunlit side, with occasional loss of radio contact.
• R2 (moderate), M5: about 350 per cycle (about 300 days); limited blackout on the sunlit side, with loss of contact for tens of minutes.
• R3 (strong), X1: about 175 per cycle (about 140 days); wide-area HF blackout and loss of contact for about an hour.
• R4 (severe), X10: about 8 per cycle (about 8 days); blackout on most of the sunlit side for one to two hours.
• R5 (extreme), X20: fewer than 1 per cycle; complete HF blackout on the entire sunlit side for a number of hours.

A flare blackout is sudden and short-lived, and it only affects the sunlit hemisphere, so a contact between stations in darkness may continue normally. We cover the mechanism and the D-Region Absorption Prediction in our guide to HF radio blackouts, which is a useful companion to this article.

How Forecasts and Alerts Reach You

The scales are used in the watches, warnings and alerts that the Space Weather Prediction Center issues. A watch means conditions are possible in the coming days, a warning means they are expected or beginning, and an alert reports that a threshold has been reached. The alert text names the scale level, for example a geomagnetic storm watch at G2 or an alert for an R1 radio blackout, so once you know the scales you can read an alert at a glance. Many hams follow these messages through the NOAA website, through apps and through space weather feeds that scroll in logging software.

One practical habit is to separate the forecast from the observation. A forecast of G3 tells you to be ready, but the observed Kp tells you what happened. If the storm arrives and Kp rises to 7, check the bands on that day and make a note. Over a season you will see how closely the prediction and the experience match for your own antennas and paths.

Planning Around Space Weather

Experienced DXers keep a simple plan for disturbed days. On the first day of a geomagnetic storm, when the bands are unsettled, they often switch to lower frequencies, because the higher bands tend to suffer most when the F2 layer is depressed. On the following day, as the ionosphere recovers, they look for enhanced propagation on certain paths. Contest operators watch the K-index in the hours before a major event and decide whether to emphasize low bands or high bands at the start. Portable operators who plan to operate near the poles, for example on a polar expedition or in a high-latitude park, check the S scale before they go, since a radiation storm can close the very paths they are trying to use.

It is also worth remembering that space weather conditions change on a timescale of hours. The scales do not tell you what will happen next week, only what is expected now and in the next few days. Combine them with the daily solar flux and your own listening, and you will have a better picture than any single number can give you.

Comparing the Three Scales

The three scales describe different physical processes, and they do not always occur together. A big flare can cause an R3 blackout, and a few hours or days later a coronal mass ejection may cause a G3 storm, and in some cases a solar radiation storm at the same time. In other cases only one of the three occurs. For a radio amateur, this means checking all three before you decide what is wrong with the bands. A good rule is to look at the R scale for sudden loss of signals, at the G scale for several hours or days of poor or unusual conditions, and at the S scale when your path crosses the polar region.

Using the Scales in Practice

1. Check the current levels before a contest or DXpedition chase. The NOAA page and the SWPC alert feeds list the current events and the forecast.
2. Match the symptom to the scale. Sudden fade-out in daylight: R. Several days of flutter, noise and unusual openings: G. Polar-path trouble: S.
3. Use bad days for different activities. A G3 storm is a bad day for HF DX on high-latitude paths, but a good day to listen for aurora on 6 and 2 meters, and to try low-band contacts that are sometimes enhanced.
4. Do not panic about the headlines. A G1 or R1 event occurs often and has minor effects on most stations; the levels marked as extreme are rare.
5. Keep a record. Note the conditions and what you observed; over time you will learn how your own station responds.

For general reading about the broader topic, the Wikipedia article on geomagnetic storms provides historical examples and the physics behind the numbers. To see how disturbance depends on the position in the 11-year cycle, our guide to HF propagation explains why quiet and disturbed days both matter.

Common Misunderstandings

• “G5 means the end of radio.” NOAA says HF radio propagation may be impossible in many areas for one to two days at G5, which is serious but temporary, and G5 events are very rare.
• “Every flare ruins the bands.” Many flares are small, and R1 and R2 events typically cause short degradations on the sunlit side only.
• “A storm always means no DX.” Geomagnetic storms reduce some paths and improve others, and they can open VHF aurora paths that are otherwise closed.
• “The scales predict the next hour exactly.” They describe levels and likelihoods; forecasts can be wrong, and the real event may arrive early or late.

The Bottom Line

The NOAA space weather scales give the plain-language levels of three types of disturbance: G for geomagnetic storms (measured by Kp), S for solar radiation storms (important for polar paths) and R for radio blackouts (sudden daytime fade-outs after solar flares). Each runs from level 1 to 5, and NOAA lists how often each level occurs per 11-year cycle, from about 1700 G1 storms down to fewer than one R5 or S5 event. Use the scales to decide whether a bad day is due to space weather, to pick the right band or activity, and to avoid blaming your station for something that comes from the Sun.

Frequently Asked Questions

What do G, S and R stand for?

G is for geomagnetic storms, S for solar radiation storms and R for radio blackouts. Each has five levels, from 1 (minor) to 5 (extreme).

Which scale affects HF the most?

All three can. R causes sudden daytime blackouts on the sunlit side, G causes disturbed conditions for hours or days, and S affects polar paths.

What Kp level is a G1 storm?

Kp 5. G2 is Kp 6, G3 is Kp 7, G4 is Kp 8 including a 9-, and G5 is Kp 9.

How often do the strongest events occur?

NOAA lists G5 storms at about 4 per 11-year cycle, R5 blackouts at fewer than 1 per cycle and S5 radiation storms at fewer than 1 per cycle.

Does a geomagnetic storm help VHF operators?

It can. Stronger storms push the aurora to lower latitudes, so radio aurora contacts on VHF become possible in places that rarely see them.

Where can I see the current levels?

On the NOAA Space Weather Prediction Center website, which publishes the scales together with current conditions and forecasts.