how does fog form?

Fog is a meteorological phenomenon caused by moist supersaturation of air so that it can no longer hold steam and they form suspended water droplets. According to the Glossary of the American Meteorological Society, fog is defined as water droplets, or water crystals if the temperature is very low, suspended in the atmosphere in the vicinity of the earth's surface and reducing visibility below 1 km.¹. According to this definition, the only difference between a cloud and a fog bank is the location relative to the earth's surface.

How fog affects visibility depends primarily on the concentration of condensation nuclei (particles on which water condenses) and the size distribution of the droplets that make up the fog. In case of very dense fog, visibility can be reduced to a few meters. This is why fog can affect land transport, especially by road, and cause accidents.

Fog is practically the same as fog, but visibility is not reduced as much and it is always above 1 km.

The formation of the mist

If we are clear that fog is a cloud near the earth's surface, we can easily understand how it is formed. Air always contains some water vapor, which is often referred to as air humidity. The most common measure of air humidity is through the RHwhich is the amount of water vapor that a mass of air contains in relation to the maximum amount of water vapor that it can contain, that is, in relation to the saturation level.

The amount of water vapor that the air can contain depends on numerous factors, the most important being temperature. The higher the temperature, the greater amount of water vapor can be in the air and vice versa. Or what is the same, the higher the temperature the higher the saturation levelthe level of humidity from which the air no longer admits any more water in gaseous state and the water vapor begins to condense and form liquid water.

Thus, fog forms when the air reaches the saturation level of water vapor, or high relative humidity, and the excess begins to condense, forming drops of liquid water that remain suspended in the air. This situation occurs when an air mass cools and its capacity to contain water vapor decreases, with which lower your saturation level.

The cooling of the air can occur in several ways, depending on which the different types of fog. Among the most common we find:

Radiation fogs or by infrared cooling

Radiation fogs typically form at seasonal changes from summer to fall and winter in temperate regions. During the summer, the ground receives more solar radiation, especially infrared, and heats up. The heat from the ground in turn warms the air in contact with the earth's surface and increases its water vapor retention capacity, becoming more humid.

As the seasons change, warm, moist air meets cooler air masses. This causes water vapor in the air to rapidly condense and form fog banks. This type of fog is often known as radiation fog and is the most common type of fog, especially in temperate climates. It can also appear after sunset, regardless of the season, on warm days followed by a drop in temperature in the evening.

The valley mist it is a type of radiation fog confined by the orography of the valleys. If the weather is calm it can last several days, even weeks. It frequently occurs together with thermal inversion phenomena; cold air settles in the valley and warmer air masses pass over it.

advection fog

Advection occurs when masses of warm, moist air pass over cold ground. The air mass loses heat to the ground and cools, its relative humidity increases and the water vapor finally begins to condense. This type of fog is known by the generic name of advection fog and they are frequent on the coasts when the warm air from the mainland is cooled when it meets the colder masses of water.

Advection fogs can also form when warm fronts meet snow patches or hail accumulation in temperate regions. These mists are generally referred to as snow mists either hail fogsbut they are nothing more than a type of advection fog.

Unlike radiation fog, advection fog can typically be seen moving laterally near the ground.

evaporation mist

In Autumn and winter it is also typical to see fog over lakes and other bodies of water. This type of fog is due to the presence of a layer of air above the water that is warmer than the upper air. Warm air is more humid and, weighing less, rises to meet the cold air. The temperature of the rising air decreases and the moist air condenses, forming what is known as evaporation mist.

Precipitation mists

They are also known as frontal fogs and they occur when it rains and the air near the surface is very dry. The water in the raindrops evaporates and forms fog.

hillside mists

They occur on mountain slopes when the air rises gently until it meets cold fronts that lower the temperature to the saturation point.

ice mists

When temperatures in the lower layers of the atmosphere are very low, the condensation droplets that form fog are so cold that they can freeze on contact with subzero surfaces. Ice fog can be very dangerous on roads because of great reduction in visibility and due to the accumulation of ice on the asphalt.

If the temperature is cold enough, a fog bank can consist exclusively of small ice crystals in suspension. These ice fogs are common in the winter months in very cold regions near the poles.

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