A heave fog hit Shanghai on early Thursday, reducing the visibility in some areas to less than 50 meters.

Shanghai's weather is likely to take a "roller coaster" as the high temperature topped over 27 degrees Celsius on Thursday, and will slump to around 18 from Sunday.

A new round of cold front is on the way and it is expected to hit the city from Saturday night with wind power increasing, forecasters said.

The mercury is forecast to remain high, ranging between 19 and 26 degrees, on Friday and Saturday with showers in some areas.

Sunday's temperature will significantly drop to between 12 and 18 degrees and keep for next week.

The city was hit by heavy fog on Thursday morning, which triggered an orange alert, the second highest in its weather warning system, at 2:40am. It led to the visibility in most areas in city reduced to less than 200 meters, some even less than 50 meters.

Vehicles running on city highways were required to follow a speed-limit of 60km per hour.

There were no serious traffic accidents or congestion in the morning, as the local traffic police stepped up to control the situation early on.

The ferry service in some suburban areas, such as Baoshan district, was suspended due to the heavy fog, police officers in those places escorted non-motor vehicles and vehicles across the Huangpu River via tunnels in batches.

In addition, Fengxian and Minhang police also safeguarded non-motor vehicles to cross the river from bridges.

Minhang police officers escort non-motor vehicles to cross the river from bridges amid Thursday's heavy fog.