A severe weather warning remains in place for Norfolk with further snowfall expected overnight.
Driving conditions on main roads including the A47, A11 and A140 have been described as treacherous following heavy snowfall in the early hours.
It is predicted up to 10cm (3.9in) of snow could fall in places.
The Broads Authority said people have been "risking their lives" by trying to walk across the frozen water at Whitlingham, near Norwich.
Schools closed
Police have warned motorists not to take unnecessary journeys.
Norwich International Airport (NIA) reopened at about 1100 GMT once the runway was cleared of snow.
A spokeswoman said: "We would like to advise passengers to get in touch with their airline or tour operators for up-to-date information on their flights."
Driving conditions in Norfolk have been described as treacherous
Across the county 364 out of 438 schools closed for the day.
Meanwhile hundreds of homes in the south of Norfolk have been without power following a problem with an overhead line.
Servicemen travelling from Norwich to work at RAF Marham turned back earlier because of the poor road conditions.
On Wednesday Norfolk County Council shipped 1,600 tonnes of salt from Cheshire to grit the county's roads.
The East of England Ambulance service has hired extra four-wheel drive vehicles to get to emergency callouts during the icy weather.
Boats warning
The service has seen a 20% increase in 999 calls since the snow began and it has cut down on non-emergency patient transport to cope.
The Met Office said the cold weather had been the most prolonged spell of freezing conditions across the UK since December 1981.
The wintry weather is forecast to continue for the next couple of weeks, with further snowfall expected at times.
The water is obviously icy cold and people can die of hypothermia very quickly or drown if they are caught under the ice
Russell Wilson, Whitlingham park ranger
The Broads Authority has urged people not to go boating or set foot on frozen rivers and broads.
Oulton Broad, Barton Broad, Ranworth Broad and Womack Water, as well as other broads, marinas and many side dykes are iced up which has left some boat owners stranded and unable to return to their moorings, it said.
Whitlingham park ranger Russell Wilson said the area had become a "winter wonderland" but visitors should not attempt to venture on to the ice.
Little Broad was cordoned off earlier when a park bench was uprooted and placed on the ice.
"The water is obviously icy cold and people can die of hypothermia very quickly or drown if they are caught under the ice," he said.
"Please keep your dogs under control too and don't go after them if they stray on to the ice."
Head navigation ranger Adrian Vernon said: "Slippery decks make navigating hazardous and boats could get damaged or holed if they hit ice.
"Even if they get out of their mooring the cold weather may mean that they may not get back because of the ice.
"There are already several boat owners unable to return to moorings at Ranworth and Horning."
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