They say this is because the red warning lights aren't visible in fog or low cloud.

BBC: London helicopter crash: What are the rules for pilots?

Their solution was to yield most of the canvas or panel to the gray, fog-and cloud-filled sky (curator Emilie Gordenker, director of the Mauritshuis, calls them "our Dutch mountains"), with blue openings torn into the gray here and there so that spots of sunlight could illuminate parts of the land or water below.

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Horrified onlookers watched as the helicopter hit the crane amid low cloud and fog near Vauxhall station, south of the river and not far from landmarks such as the London Eye and the Houses of Parliament, and came plunging to the ground.

CNN: London helicopter crash prompts safety questions

We need to stop surrounding the cloud with a fog of tech-speak and explain to potential users what it can do for them, rather than focusing on how it works.

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Rox said its "highly sophisticated" new smoke cloud system released a heavy fog which reduced visibility when activated.

BBC: Rox in George Street in Edinburgh

At the time of the crash there was low cloud, poor visibility and patches of freezing fog.

BBC: London helicopter crash: Crane work resumes on tower