A、They were caused by the dirty smoke from burning coal. B、They were so thick that you could only see your own hand in front of

Britain has a variable climate. The weather changes so frequently that it is difficult to forecast. It is not unusual for people to complain that the weatherman was wrong. Fortunately, as Britain does not experience extreme weather conditions, it is never very cold or very hot. The temperature rarely rises above 32℃ in summer, or falls below—10℃ in winter.
    Summers are generally cool, but due to global warming they are starting to get drier and hotter. Newspapers during a hot spell talk of "heat waves" and an "Indian summer"—dry, hot weather in September or October. Hot weather causes terrible congestion on the roads as Britons rush to the coastal resorts.
    Winters are generally mild, with the most frequent and prolonged snowfalls in the Scottish Highlands, where it is possible to go skiing. If it does snow heavily in other parts of Britain, the country often comes to a standstill. Trains, buses and planes are late. People enjoy discussing the snow, complaining about the cold and comparing the weather conditions with previous winters!
    Contrary to the popular opinion, it does not rain all the time. There is certainly steady rainfall throughout most of the year, but the months from September to January are the wettest. Thanks to the rain, Britain has a richly fertile countryside which is famous for its deep green colour.
    Since the 1950s, most British cities have introduced clean air zones. Factories and houses cannot burn coal and must use smokeless fuel. The dirt caused by smoke used to cause terrible fogs, particularly in London. Londoners used to call their fogs "London Particulars" and you could not see your hand in front of you. Such fogs are now a thing of the past, but you can still see them in old films where they add mystery and atmosphere to murder stories and thrillers!