The Met Office has issued a clarification on the often-interchanged terms of mist, fog, and haze, providing a clear breakdown of their meteorological definitions and the conditions under which they occur. This guidance aims to help the public better understand weather warnings and atmospheric phenomena, particularly as visibility can significantly impact travel and daily activities across the UK.
According to the national weather service, the primary distinction between mist and fog lies in the level of visibility they permit. Mist is defined as a reduction in visibility to between 1,000 metres and 5,000 metres. This typically occurs when tiny water droplets are suspended in the air, often forming in damp, humid conditions or during a temperature inversion. While it can obscure distant views, it generally allows for safer travel than fog.
Fog, on the other hand, is a more severe reduction in visibility, defined as less than 1,000 metres. When visibility drops below this threshold, the Met Office often issues specific warnings, as it can pose significant hazards for motorists, pilots, and maritime traffic. Fog is essentially a cloud that forms at ground level and is composed of a higher concentration of water droplets than mist.
Haze differs fundamentally from both mist and fog. Unlike its water-based counterparts, haze is caused by dry particles suspended in the air. These can include dust, smoke, pollution, or even salt particles near coastal areas. Haze can scatter sunlight, giving the sky a milky or yellowish appearance, and can reduce visibility without the dampness associated with mist or fog. This distinction is important for understanding air quality and potential respiratory impacts.
Across the UK, these phenomena are common, particularly during autumn and winter. Regions prone to river valleys, such as parts of the Midlands and southern England, often experience dense radiation fog. Coastal areas, including parts of Scotland and Northern Ireland, can see sea fog or 'haar' roll in, significantly reducing visibility. Mountainous regions in Wales and northern England can also experience orographic fog as moist air is forced upwards.