During these cold winter days, Hanoi and the northern provinces experience thick fog in the early mornings or late evenings. Many people have noted that the fog in recent years has been denser compared to previous years. So, what is fog? Is it related to environmental pollution?
Fog is a meteorological phenomenon where water vapor in the atmosphere condenses into very small water droplets that float in the air, reducing our visibility.
Depending on the conditions under which each type of fog forms, fog can be classified into several types:
– Stratus fog
– Radiation fog
– Evaporation fog
– Front fog…
Mist is a phenomenon characterized by a collection of dust and smoke particles suspended in the air, which reduces horizontal visibility. Dense mist can decrease visibility to a few hundred meters, or even down to tens of meters like heavy fog. Mist is often caused by local factors such as forest fires and environmental pollution.
Both fog and mist are dangerous meteorological phenomena. Particularly for road, river, sea, and air transportation, fog causes significant obstacles and losses every year.
In our country, fog typically occurs frequently from late autumn to late spring, with the most intense occurrences happening in the winter months. Nowadays, as air quality deteriorates, fog and mist occur more often and with greater intensity.
Causes of Fog Formation
Fog is a meteorological phenomenon that we often observe and is of great concern. However, fog does not always occur regularly. Essentially, for fog to form, certain conditions must be met:
– The relative humidity of the air must be high.
– The air temperature must be relatively low.
– Wind speed must be weak or calm.
Fog usually appears when moist air from water bodies, ponds, lakes, or rivers moves to an area with a lower surface temperature.
Types of Fog
1. Evaporation Fog
Evaporation fog forms when the temperature at the water surface is lower than the temperature of the air layer above it; at this point, evaporation hardly occurs because the vapor pressure in the air layer above is less than the vapor pressure at the water surface, thus the saturation state of water vapor in the air layer cannot be achieved. Conversely, when the temperature of the air layer above the water is lower than the water surface temperature, evaporation continues to occur. When the humidity in the air above reaches saturation, the excess water vapor condenses to form fog.
2. Stratus Fog
This type of fog forms when a body of warm air moves over a colder surface; within the air mass, a temperature inversion layer is created, and fog forms from the ground up to the boundary of the inversion layer. In practice, the causes of stratus fog formation are quite varied, and it is generally classified into two main types:
a. Warm Stratus Fog: This occurs when the air cools down over a colder surface, and is the most common form of stratus fog, forming in warm, moist air masses, such as tropical maritime air moving inland to cooler temperatures. In our country, this type of fog is often observed moving from the East Sea inland during the cold months.
b. Cold Stratus Fog: This appears over a body of water when water vapor evaporates from the warm water surface into the cold air; on land, we can encounter evaporation fog rising from rivers, lakes, and ponds…
3. Front Fog
This type of fog appears when warm fronts pass through with rain; due to the evaporation of raindrops, the air near the surface becomes saturated, and simultaneously, pressure drops rapidly, allowing the air to expand and cool, making it easier for water vapor close to the ground to condense into fog.
4. Radiation Fog
This type of fog typically occurs in cold continental high-pressure ridges, appearing in the early to mid-winter when the air is relatively humid, temperatures are low, and skies are clear. It usually forms between midnight and morning.
– In calm conditions, radiation fog forms in the layer of air close to the ground (2-5 meters above ground) over water bodies and in valleys.
– When wind speeds are around 2-3 m/s, radiation fog can develop into a thicker layer, potentially reaching heights of 100-150 meters.
– Ground radiation fog typically dissipates as the inversion layer near the surface breaks down and usually does not last long once the sun appears.
– This type of fog tends to form in clusters; if there is radiation fog above, it may merge with low stratus clouds and persist longer.
