The black ice that caused crash chaos in Bradford this morning took many by surprise, coming as it did on the back of what had not seemed, on the face of it, a particularly cold night.

But the Met Office, which issued a yellow warning for ice overnight, has explained that the warmer air was a factor in creating the unusually slippery conditions.

A spokesman said: "Black ice forms on pavements and roads when the ground temperature is below freezing and when the air temperature is higher.

"It is caused when drizzle or rain hits the cold ground and freezes.

"During situations when the ground cools quicker or is colder than the air, a ground frost can occur without an air frost.

"Due to its transparent nature black ice can be mistaken for a wet surface and so can be highly dangerous."

The bad news is that there is another yellow warning for ice in place for overnight Thursday into Friday.

The spokesman said: "Once again, ice will be a hazard in places through this period. This will happen as heavy showers of rain, hail, sleet and snow alternate with clearer periods, allowing temperatures to dip sharply."

They added: "As a consequence, icy stretches are likely on untreated roads, pavements and cycle paths with some slippery surfaces likely."

A Bradford Council spokesperson said: "Our gritting crews were out gritting our Priority One routes across the district from 7pm last night and a full Priority One grit took place from around 4am this morning.

"On a Priority One grit, our guys are out in the freezing weather, covering around 700 miles of road trying their best to keep them clear. We treat over 60% of the roads in the district, which is more than any other council area in West Yorkshire.

"It is not feasible to grit all roads in the district or to respond to all requests from residents, of which we have had nearly 1000 a day in past week. Therefore we concentrate on main roads, bus routes, routes to hospitals, schools, isolated communities and ambulance, police and fire stations first.

"Once these Priority One routes are treated, our highways service starts gritting lower priority roads such as side roads and lanes.

"Grit can only do so much and there were particular issues this morning due to rain washing away grit that had been spread, before further rain came and froze on contact with road and pavement surfaces causing black ice.

"We would urge people to take extra care in winter weather, fit winter tyres to their vehicle, and allow extra time, plan journeys in advance by checking which routes are on grit runs and sticking to them.

"We would encourage people to help out in their neighbourhood by gritting their own street where they can."