Bulgaria's National Institute of Meteorology and Hydrology (NIMH) warned of fog for Tuesday, 10 November, in its weekly forecast issued on 6 November, forecaster Anastasia Stoycheva told Bulgarian National Radio (BNR).

For the lowlands, the institute forecast fog or low cloud from the night before Tuesday until noon. "I can't recall another case where fog led to such a tragedy," Stoycheva said.

She said rain fell until 5am and left moisture in the lowest layer of air, which formed what is known as radiation fog.

"These fogs are short-lived and local," she said. Fog can form at any time of day or night.

Meteorologists speak of fog when visibility is under 1,000 metres, Stoycheva said. If visibility falls below 100 metres, or even below 50 metres, the fog is very thick.

In daylight, areas of thick fog can be spotted by their clear white edges, she added.

Stoycheva said radiation fog is common in the Danube Plain and the Upper Thracian Plain, where conditions favour it.