On these chilly winter days, Hanoi and several provinces experience dense fog in the early morning or late evening. Many have observed that fog in recent years seems to be thicker than before. So, what exactly is fog? Does it have any connection to environmental pollution?
Fog is a phenomenon where water vapor in the atmosphere condenses into tiny water droplets that float in the air, reducing our visibility.
Types of Fog
Depending on the conditions under which fog forms, it can be classified into several types:
- Advection Fog
- Radiation Fog
- Evaporation Fog
- Frontal Fog
Fog vs. Haze
Haze occurs when particles of dust and smoke suspended in the air reduce horizontal visibility. Dense haze can lower visibility to a few hundred meters or even a few dozen meters, much like thick fog. Haze is often caused by local factors such as forest fires or environmental pollution.
Both fog and haze are hazardous meteorological phenomena. They particularly pose risks to road, river, maritime, and air transportation, causing significant disruptions and losses each year.
In Vietnam, fog typically occurs from late autumn to late spring, with the heaviest and most frequent fog appearing during winter months. Due to increasing air pollution, fog and haze have become more prevalent and intense in recent years.
How Does Fog Form?
Fog is a meteorological phenomenon that is both common and noteworthy. However, it doesn’t occur frequently. Certain conditions must be met for fog to form:
- High relative humidity in the air.
- Low air temperatures.
- Weak wind speeds or calm air.
Fog often forms when moist air from bodies of water such as lakes, rivers, or ponds moves to areas with cooler ground temperatures.
Types of Fog Explained
1. Evaporation Fog
This fog forms when the water surface temperature is lower than the air temperature above it. In such cases, evaporation does not occur because the vapor pressure in the air above is lower than that at the water surface, preventing saturation. However, if the air temperature above the water surface drops below the water surface temperature, evaporation resumes. Once the air reaches saturation, excess water vapor condenses, forming fog.
2. Advection Fog
Advection fog forms when warm air moves over cooler surfaces. This creates a temperature inversion within the air mass, forming fog from the ground up to the inversion boundary. There are two main types of advection fog:
- Warm Advection Fog: Common when warm, moist air moves over cooler surfaces, such as tropical maritime air masses moving over cooler land in winter. This type is frequently observed in Vietnam as tropical air from the East Sea moves inland.
- Cold Advection Fog: Occurs over water when warm water evaporates into cooler air, or on land from evaporation over rivers, lakes, or ponds.
3. Frontal Fog
Frontal fog appears when a warm front with rain passes through. The evaporation of raindrops causes the air near the ground to become saturated. As pressure drops quickly, the air undergoes adiabatic cooling, allowing water vapor near the surface to condense into fog.
4. Radiation Fog
This type of fog typically forms in cold continental high-pressure zones during early to mid-winter when the air is relatively moist, temperatures are low, and the sky is clear.
- In calm conditions, radiation fog forms in the air closest to the ground (2–5 meters) over water and in valleys.
- At wind speeds of 2–3 m/s, radiation fog can develop into a thicker layer, extending up to 100–150 meters in height.
- Radiation fog near the ground usually dissipates along with the inversion layer when the sun rises.
Radiation fog can occur in patches. If it merges with low stratus clouds, it may persist for longer periods.
