http://www.xcweather.co.uk/
this site may help explain why they happen some days and not others...
Yesterday the wind over London was coming from Essex and there were streaks everywhere... today the wind is blowing from sussex the other side of London and today the skies are completely dusty and over cast, so much so that the skies are not clear or cloudy... just misty almost...
We could almost do with a photo from space to help out
