Fog is essentially a cloud resting on the ground. Because fog significantly affects visibility, it is an important measurement for weather observation and forecasting. To ensure rapid and universal communication, meteorologists rely on standardized graphical symbols governed by the World Meteorological Organization (WMO). This system allows forecasters to quickly interpret atmospheric data regardless of language differences, which is crucial for transportation, particularly aviation and shipping.
The Standard Meteorological Representation
The official WMO symbol for current fog is represented by three short, horizontal, parallel lines or bars placed one above the other. This graphic indicates the presence of dense water vapor and is a component of the international code for present weather, known as the ww code figures.
The core definition of fog requires visibility to be reduced to less than 1,000 meters (1 kilometer) due to suspended water droplets or ice crystals. The three-bar symbol communicates this severe restriction. Variations of this symbol exist to specify the fog’s behavior, such as if it is patchy, thinning, or thickening over the observation period.
Meteorologists also track past weather using a separate code table. For past weather, the WMO uses code figure 20 to indicate that fog occurred during the preceding hours but is no longer present. This distinction between present and past conditions helps in understanding the stability and evolution of the local weather pattern.
Interpreting the Symbol on Weather Maps
The fog symbol is integrated into a complete graphical summary called a station model. The station model is a collection of numbers and symbols clustered around a central circle representing a weather station’s observations at a precise time. This model allows meteorologists to condense a large amount of weather data onto a synoptic map.
As a “present weather” indicator, the fog symbol is positioned directly to the left of the central station circle. Its placement is typically just below the recorded temperature and above the dew point. This standardized arrangement ensures that all observers can locate and interpret the condition uniformly.
A numerical value for visibility is often recorded near the fog symbol. This number represents the horizontal distance an observer can clearly see. When the three-bar fog symbol is present, the visibility value will be below the 1,000-meter threshold. The complete station model provides the context of temperature and wind, which are factors in the fog event.
Symbols for Related Conditions
Other atmospheric conditions that obstruct visibility use symbols distinct from the three-bar fog graphic. Mist is closely related to fog but is defined by a lesser reduction in visibility, typically between 1,000 and 7,000 meters. The symbol for mist (WMO code 10) is often depicted as two parallel horizontal lines or a horizontal bracket.
Haze is caused by extremely small dry particles rather than water droplets and is represented by a different symbol, typically a single, wavy, horizontal line. This distinction is important because haze does not involve the saturation of the air with water vapor like fog and mist.
Freezing fog is a hazardous type that occurs when suspended water droplets remain liquid at air temperatures below 0 degrees Celsius. The symbol for freezing fog combines the standard three horizontal bars with a small asterisk or ice crystal symbol. This composite symbol alerts observers to the potential for rime ice accumulation on surfaces.