Weather conditions will be central to the inquiry into today's helicopter crash by accident investigators.
The key question that the Air Accidents Investigation Branch (AAIB) team will want an answer to is: "Was visibility suitable for a helicopter flight over London?"
Battersea Heliport in south London has said that the pilot of the helicopter had requested to divert and land there due to bad weather.
Earlier, it is thought that the pilot had had to abandon his plan to land at Elstree in Hertfordshire due to fog.
The AAIB will check with weather forecasters and find that cloud in central London was low at the time of the accident.
The investigators will also want to find out what conditions were like at Redhill in Surrey from where the helicopter is thought to have taken off.
Forecasters at MeteoGroup, the weather division of the Press Association, said there were very misty conditions over south east England this morning, with some dense fog in places.
It added that areas around London and to the north of the capital were shrouded in low cloud, down as low as 200ft, and areas of freezing fog.
Weather stations at Stansted in Essex, Luton in Bedfordshire and High Wycombe in Buckinghamshire all reported foggy conditions this morning.
The conditions were bad enough for London City Airport - at London Docklands - to announce flight delays.
Although helicopter flight over London is governed by strict regulations and pilots are under the control of air traffic control company Nats, decisions on whether to take off are down to pilots.
Peter Norton, chief executive of civilian helicopter trade organisation the British Helicopter Association, said: "Rules, routes and regulations concerning flying over London are well documented and familiar to pilots.
"It is a captain's decision on whether the conditions are fit for flying. Captains will be in touch with air traffic controllers but it's a decision for captains."
The Civil Aviation Authority lays down the routes that helicopters must take over London. Normally flights by single-engined helicopters are prohibited except along the route of the River Thames.
Some helicopter pilots are licensed to fly when conditions dictate the use of instruments.
But helicopters travelling over London are subject to visual flight rules.
There are various agreed starting-off points for helicopters entering the London area, with places from which they must report their position and other places where they may be put in a holding position.
It could be that the AAIB, as it normally does in major incidents, will issue a short interim report within days.
This report is likely to outline the basic facts of the incident, with a fuller report possibly taking some time to come out.
A Civil Aviation Authority (CAA) spokesman said: "Helicopter operations in central London are strictly controlled. Single-engine helicopters are required to fly along designated routes, which have been selected to provide maximum safety by routing helicopters along the River Thames, avoiding flying over built-up areas as much as possible.
"Twin-engine helicopters can operate in wider areas. However, all aircraft operating in central London are subject to air traffic control clearance."
He went on: "There are requirements for lighting on tall structures. In addition, where appropriate, very tall structures are also notified to pilots for flight planning purposes, as was the case with the crane that was involved in this morning's accident."
The CAA added that it would provide any assistance required by the AAIB.
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