Authorities issued their highest warnings for fog and haze on Tuesday as smog enveloped major cities in northern China, warning the public that visibility could drop to less than 50 metres (164 feet).

Northern province Hebei launched an anti-pollution emergency response, listing traffic safety controls for when necessary including suspending flight takeoffs and landings, temporarily closing highways and suspending ferries, China’s meteorological bureau said in a notice.

Authorities also warned road users to stop in safe parking areas when conditions required and asked people to stay indoors.

Beijing said it would implement traffic control measures if the capital activates its highest air pollution warning.

Heavy smog has engulfed the country’s north for a few days while autumn temperatures soared to typical early summer levels close to 30 degrees Celsius (86 degrees Fahrenheit) in some areas.

Experts said weak cold air currents from the north pole were a key factor behind the unusual weather.

As air pollution levels in the wider Beijing-Tianjin-Hebei area and northern part of Henan province reached moderate to severe, pollution control experts said increased industrial activities, heavy trucking and crop fires had contributed to the haze, state media CCTV reported.

Regional power consumption in late October was up 5% compared with first half of the month, more significantly in the cement, brick and tile industries, worsening conditions, CCTV said.

Parts of Tianjin municipality and Hebei and Shandong provinces, as well as eastern areas of Jiangsu province saw heavy fog resulting in visibility of less than 1 km (0.62 mile) on Tuesday morning, the National Meteorological Center (NMC) said.

Until Thursday, light to moderate haze will continue to cover the central and southern parts of China’s northern region, with severe haze forecast in the central parts, NMC said.

Cold air currents are forecast to stream in the north from Thursday night, creating conditions that could weaken and dissipate the haze, the weather forecaster said.