'And here's today's space weather forecast...'

By BBC News Online's Damian Carrington in Washington DC

Space weather has joined earthquakes, hurricanes and gales in having official scales of severity. The worst events on the scale are accompanied by some severe impacts on human activities.

The most extreme warnings on the scales would precede "power grid systems collapse" and radiation bursts to high-flying jet passengers equivalent to 100 chest X-rays.

The scales have been announced just as the Sun's 11-year cycle of activity is reaching its maximum.

Geomagnetic storms

G5 Extreme - 4 per 11 years

G4 Severe - 100 per 11 years

G3 Strong - 200 per 11 years

G2 Moderate - 600 per 11 years

G1 Minor - 1700 per 11 years

Ernest Hildner, Director of the Space Environment Center at the US National Oceanic and Atmospheric Administration (NOAA), said: "The internal magnetic field of the Sun waxes and wanes and when the Sun has eruptions of plasma, it smacks into the Earth's magnetic field."

"When you go from a freezing spell to a heat wave, that's just 100 degrees Fahrenheit (38 Celsius) of change - when a space storm strikes, the flux of energetic particles jumps by 10,000 times."

Power grid collapse

His comments were made at the annual meeting of the American Association for the Advancement of Science (AAAS).

Solar radiation storms

S5 Extreme - Fewer than one per 11 years

S4 Severe - Three per 11 years

S3 Strong - 10 per 11 years

S2 Moderate - 25 per 11 years

S1 Minor - 50 per 11 years

NOAA has drawn up three scales, one for each of the threats associated with space storms.

Geomagnetic storms result from the Sun's magnetic field pushing into the Earth's.

The most severe of these, a G5 event, could cause power grids to collapse and transformers to be damaged. Satellite navigation systems would fail for days and aurorae could be seen at the tropics.

Commercial aeroplanes

Even G3 events, 50 times more common, would trigger false alarms and interrupt radio navigation signals.

Radio blackouts

R5 Extreme - Fewer than one per 11 years

R4 Severe - Eight per 11 years

R3 Strong - 175 per 11 years

R2 Moderate - 350 per 11 years

R1 Minor - 2000 per 11 years

Solar radiation storms are caused by a wave of highly energetic particles flooding over the Earth. The extreme S5 events pose risks to passengers and crew on commercial aeroplanes and to astronauts who are spacewalking. The protective nature of the Earth's atmosphere means little effect at the planet's surface.

Finally, a radio blackout scale has been produced. Here the extreme events would cause "complete high frequency radio blackout on the entire sunlit side of the Earth lasting for hours".

Even Moderate R2 events, of which 30 are expected every year, can lead to blackouts for mariners and aviators for tens of minutes.