The dark red box indicates the current level of activity, at green and above there is a chance of seeing the Northern lights in the UK. The storm ratings range from G1 to G5 and the higher the number the better that chance is. For a full description of the categories and colours click here
To see more plots of current geomagnetic activity click here
Ovation Aurora forecast, courtesy of NOAA (www.swpc.noaa.gov/ovation)