What is fog? Fog is a cloud that’s touching the ground. It forms because the air temperature and dew point just above the ground is very close together. So the air doesn’t need to rise and cool for condensation to occur as it does for clouds that form higher up. Also, it’s a natural phenomenon that indicates the winter season and cold weather.
As the sun goes down at night, the air over the ground cools faster than that over water. So we have relatively warm moist air over the ocean being affected over the cold land surface, and condensation occurs. Thus it’s named fog. Once the sun rises through the ground warms and the relative humidity drops, evaporation occurs, and the fog begins to clear.
What is fog made of?
Fog is just a cloud that forms at the ground instead of up in the sky. Fog is made of tiny droplets of water that are suspended in the air. But it can also consist of small particles of ice during icy conditions. Now, as the air cools, it loses its ability to hold water. When the air cools to the dew point temperature, it becomes saturated with water vapor, and that vapor condenses around tiny microscopic particles like dust. Then it forms water droplets. When enough of these water droplets create, fog is made.
Let’s focus on the ocean water temperature, which will be much cooler than the temperature of the land. Eventually, a warm moist air mass will flow across the ocean’s surface and all the while being cooled by the cooler water beneath. As this process happens, that lower level of air gets saturated by moisture, and you get the development of fog.
Let’s focus on the temperature of the air. The heat that is built up during the daytime leaves the ground. As this happens, a thin layer of moisture starts to form underneath drier air. Since moist air doesn’t absorb much heat, the air near the surface cools quicker. Moisture slowly rises, allowing for the water to turn into vapor, and fog is created.
Fog formation explains: Moisture happens when water evaporates into the air and allows for saturation near the surface. As the water evaporates, it warms the air layer closest to the ground allowing for sharp contrast and temperature. As the moisture rises, the water turns into vapor. It looks like steam and fog.
Have you ever noticed that fog can develop even after rain moves out? That’s because as it’s raining, a warm layer of air can move in that warms your raindrop. But then, eventually, it cools as it gets closer to the surface and moves through the cooler air. And the evaporation process starts a little bit sooner than in other circumstances. It creates an area of moist air closer to the ground, saturated, and the water turns into vapor.
Why do you see fog in the mountainous area? This happens when cold air in the lower elevations and the valleys begin to ride up the mountain’s slope. As this occurs, the air turns colder and eventually gets a thin layer of moisture to develop on that slope. It rises, and that will convert into water vapor and mist.
Radiation fog is the most common type during autumn mornings. Clear skies and crisp temperatures aid it. The cooling-off of land overnight is called thermal radiation. Also, It drops the temperature at the surface to the dew point temperature, allowing condensation and hence fog. Radiation fog usually dissipates soon after sunrise as the ground quickly warms up.
Where does the fog come from? In the mornings, the sky is clear, and the temperatures are crisp. During the evenings, the air cools, and it loses its ability to hold water vapor, then condenses, creating tiny liquid water droplets that hang in the air. After this occurs, fog appears. However, this kind of fog doesn’t last long once the Sun rises in the morning. The fog begins to burn off, and it then can appear as a low-lying cloud.
Different types of fog
There are different kinds of fog. It depends on temperature, air pressure, and the atmosphere. Now we are going to learn about different types of fog and their formation.
Radiation fog: The most common type of fog is radiation fog. It forms on clear nights with light winds. As the ground radiates heat away towards space, it absorbs heat from the air. This process cools the air throughout the night. If the air temperature drops to the dew point, the water vapor condenses out to form a cloud above the ground. If there is a slight breeze, then the water vapor will be able to spread vertically. However, if the wind is too strong, then the fog scatters into a state of invisibility.
Advection fog forms when warm, humid air blows in or add vets over a cool surface. The ground absorbs heat from that air lowering its temperature to the dew point. This is extremely common in areas with cold ocean currents, such as California’s Bay Area.
Precipitation fog: Precipitation fogs another fog that forms when air falls or when precipitation falls into cooler air. Precipitation or frontal fog occurs when light rain or drizzle evaporates into relatively dry air long before reaching the ground. This both cools the air and adds more moisture. It so a temperature and dew point get closer and closer together until fog forms.
The way that this fog form is by evaporation. So the dew point goes up to meet the temperature. What happens as precipitation falls into colder air? As that precipitation falls, it evaporates, and that evaporation increases the air’s water vapor content. As the water vapor content increases, it’ll finally reach saturation and produce a lower layer of fog. So the air can reach saturation and reduce visibility significantly in that area of precipitation.
Upslope fog: There’s also upslope fog that clings to the sides of hills and mountains as humid air blows up their slopes. In this case, like the previous cases, the temperature drops down to meet the dew point. Upslope fog results from adiabatic cooling as air rises. It expands in a cool through force descent.
Steam fog: Steam fog forms as cool air blows over warm water. The opposite of advection fog is steam fog. Warm waters are associated with warm, moist air. When a mass of cold air moves in, it cools the warm air’s moisture into a liquid. Usually, this type of fog does not get very high. Since cold air is required, it usually happens near the Arctic and Antarctica.
Freezing fog: There’s even freezing fog where supercooled water droplets from the fog at sub-zero temperatures and then freeze into ice when they come into contact with a surface.
Ice fog: The fog droplets freeze into ice crystals right there in the air. Ice fog can form either cooling the air to saturation. It disappears pretty quickly. But when you get to temperatures minus 40 Celcius or colder, that’s when the ice fog sticks around and doesn’t burn off as quickly.
What’s the foggiest place on earth? The Grand Banks off the east coast of Newfoundland, where the cold Labrador Current meets the warm Gulf Stream. And this mixture provides the perfect environment for the fog to form.
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Gultepe, Ismail, ed. “Fog Visibility and Forecasting.” Fog and Boundary Layer Clouds.
“Federal Meteorological Handbook Number 1: Chapter 8 – Present Weather”. Office of the Federal Coordinator for Meteorology.
Robert Penrose Pearce. Meteorology at the Millennium. Academic Press.
“Virga and Dry Thunderstorms.” National Weather Service Office, Spokane, Washington.