What is Fog?

Welcome to Fog 101!

We’ve all seen fog before, but do you know how it forms? What’s the difference between fog and mist, anyway?

This Weather 101 Guide on wind will answer all your questions. Our meteorologists helped create this guide so you can clear up the difference between fact and myth.

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What is Fog? 

icon of fog and a bridgeFog is a visible aerosol comprising tiny water droplets or ice crystals suspended in the air at or near the Earth’s surface. Nearby bodies of water, topography, and weather conditions are three factors that influence fog. 

You can think of it as a low-lying cloud. Fog most resembles stratus clouds, or low-lying, horizontally layered clouds. It is difficult to see through because of the varying concentrations of the water droplets. 

How is Fog Formed?

You can watch Earth Networks Meteorologist, Fred Allen, explain how it forms in the video below or you can scroll down to read about it.

 

Fog forms when the difference between air temperature and dew point is less that 4.5 degrees Fahrenheit or 2.5 degrees Celsius. When water vapor condenses, it turns into tiny droplets of varying concentration in the air. 

Dew Point: The temperature below which water droplets start to condense and form dew or frost. Dew point is a surface weather observation.

 

It usually forms at a relative humidity of about 100%, which occurs when there is increased moisture in the air or when the temperature is decreasing.

Fog can be a little tricky. Sometimes it forms at lower humidities. Other times, it fails to form with a relative humidity of 100%. 

More on Formation

Another way to look at fog formation is as when air at or near the earth’s surface becomes saturated. Air in this area becomes saturated by any of these three processes:

A dark blue "1" in a red circle Cooling

A dark blue "2" in a red circle Addition of moisture

Mixing with another air parcel

Since you’re probably not a meteorologist, we think this explanation is sufficient for you to understand the variable nature of fog and the different ways it can form.

Forecasting 

As we’ve mentioned before, this can be a little difficult to forecast. Dew and frost also form when relative humidity levels approach 100%. So how do forecasters know which condition you’ll experience?

To get fog, you need a thicker layer of saturated air at the surface. You also need a light breeze to help mix the atmosphere, but not too strong or drier air higher in the atmosphere will mix out the higher moisture near the surface.

Another factor to think about is that when it is calm, it’s harder for it to form.

What Causes Fog in the Morning? 

A graphic of the sun rising over two blue mountainsThis is one of the most frequently asked questions on the subject. Now that you know a little bit about what fog is and how it forms, do you think you could answer this question?

Answer: Fog forms in the morning because it is the coolest time of the day when the temperature drops to the dew point temperatures and the relative humidity approaches 100%. There are instances where dew points rise to the air temperature, but common morning fog is created as the atmosphere cools.

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Types of Fog

a sunrise over a foggy meadow

Since it forms in a number of ways, there are also various types of fog. This section includes definitions for several types of fog, including: evaporation, freezing, and radiation fog – just to name a few!

Fog vs. Mist

Before we get into different types, we’re going to address a common debate: Fog vs. mist!

This may come as a shock, but there really is no difference between fog and mist. These terms are interchangeable, with the only difference pertaining to observation visibility for aviation. This is how the National Weather Service’s glossary defines the two terms:

Mist: A visible aggregate of minute water particles suspended in the atmosphere that reduces visibility to less than 7 statute miles, but greater than or equal to 5/8 statute miles. It does not reduce visibility as much as fog and is often confused with drizzle.

Fog: Fog is water droplets suspended in the air at the Earth’s surface. Fog is often hazardous when the visibility is reduced to ¼ mile or less.

Now that we’ve got that out of the way, we can go through the different types!

Advection

This occurs when moist air passes over a cool surface by the wind and is cooled. Advection fog typically happens when a warm front passes over an area with significant snow pack.

It is even more common at sea when moist air encounters cooler waters, including areas of upswelling. We typically see this along the California Coast (specifically in San Francisco).

Although strong winds can prevent fog, this can form with windy conditions. Markedly warmer and humid air blowing over a snowpack can continue to generate advection fog at elevated velocities up to 50 mph or more. This fog moves in a turbulent, comparatively shallow layers.

Evaporation/Steam

fog over a lake

Also known as lake fog, evaporation or steam fog forms over bodies of water. It typically forms during the fall season when water temperatures don’t cool right away but air temperature does.

As a mass of dry, cold air moves over a warmer lake, the lake conducts warm, moist air into the air mass above. The transport between the lake and air events out and creates fog. This fluffy-looking fog is deep enough to block some sunlight.

Freezing

Freezing fog occurs when temperatures are below freezing and is composed of droplets of supercooled water that freeze to surfaces on contact.

Frontal

Frontal fog forms when raindrops, falling from a relatively warm air above a frontal surface, evaporate into cooler air close to the Earth’s surface. This causes it to become saturated. This can result from a very low frontal stratus cloud subsiding to the surface level in the absence of any lifting agent after the front passes. It forms in the same way a stratus cloud near a front does.

Ground

Fog covering the ground with mountains is the background

This type is close to the ground. It obscures less than 60% of the sky and does not extend to the base of any overhead clouds.

Hail

Hail fog sometimes occurs after hail accumulates due to decreased temperature and increased moisture leading to saturation in a very shallow layer near the surface.

It most often occurs when there is a warm, humid layer atop the hail and when wind is light. This is actually a ground fog that tends to be localized, however it can be dense and abrupt. 

Ice

Different from freezing, ice fog is only seen in the polar and artic regions when temperatures reach 14 degrees Fahrenheit. At this point, air is too cold to contain supercooled water droplets so it forms tiny ice crystals.

It can be beautiful, especially when associated with the diamond dust form of precipitation, in which tiny crystals of ice form and slowly fall. This often occurs during blue sky conditions, which can cause many types of halos and other results of refraction of sunlight by the airborne crystals.

Precipitation

red rain icon raindrops coming out of a cloudPrecipitation fog forms as precipitation falls into cold, drier air below the cloud and evaporates into water vapor. The water vapor cools and at the dew point it condenses. When it condenses, it creates fog.

This is common with warm fronts but can occur with cold fronts as well only if it’s not moving too fast. 

Radiation

Radiation fog happens after sunset when the land cools by infrared thermal radiation in calm conditions with a clear sky. The cooling ground then cools adjacent air by conduction. This causes the air temperature to fall and reach the dew point, forming fog. In perfect calm, the fog layer can be less than a meter thick. Turbulence can promote a thicker layer.

Radiation fog doesn’t typically last long after sunrise, but can linger all day during the winter months – especially in areas bounded by high ground. It is more common in the fall and early winter and when it rains the night before. Rain helps moisten up the soil and create higher dew points. This makes it easier for the air to become saturated and form fog.

Upslope 

UPSLOPE fog icon

The last type of fog on our list is upslope fog. The name of this fog describes it pretty well. Upslope fog forms when moist air is going up the slope of a mountain or hill. This movement condenses fog through adiabatic cooling and the drop in pressure with altitude.

Adabatic cooling causes sinking air to warm and rising air to cool. As moist winds blow toward a mountain, it up glides and this causes the air to rise and cool to meet up with the dew point temperature. This fog can be seen on the top of mountains.

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Fog Dangers

When most people think of severe weather, they think of thunderstorms, hurricanes, or tornadoes. However, fog can be dangerous too.

Although not as impactful as the conditions mentioned above, it presents its own set of risks and dangers to humans.

The major danger fog presents visibility. While visibility can be in an issue in industries like mining and construction, fog is most hazardous to drivers, marines, and aviation.

Motorists

A foggy toad with low visibility

Fog causes many car accidents each year. From 2016-2017, roughly 5.89 weather-related vehicle crashes occurred each year in the U.S., according to the Federal Highway Administration. These include wrecks during falling precipitation, slippery pavement, and fog.

How dangerous is fog to motorists? When breaking out fog-related accidents, the annual averages are as follows:

alert icon 25,451 crashes

alert icon 464 deaths

alert icon 8,902 injures

That means fog-related accidents claim more lives each year than tornadoes (60 people on average). This is because fog distorts your perception of speed and distance.

How can you drive safer?

Don’t speed! You may even have to drive slower than the speed limit so you have more time to judge your surroundings. The most important thing you can do while driving in foggy weather is to take your time and stay cautious.

Another good tip is to never use your high beams. High beams actually make it harder to see as the water vapor will scatter more light back at you. Use fog lights if you have them but never use high beams! There are also some vehicle telematics that protect drivers from bad weather, like fog,

Mariners

Fog is dangerous to those on the water because it can form quickly and catch boaters off guard. Because of the time it can take to stop or turn a marine vessel, we usually consider fog as “dense” for mariners if it reduces visibility to less than 1 mile.

Visibility can be reduced to a few feet, which can disorient even the most experienced boaters and create dangerous situations. The international standard for describing reduced visibility in marine forecasts are as followed:

alert icon Very Poor: Less than 0.5 nautical miles

alert icon Poor: 0.5 to less than 2 nautical miles

alert icon Moderate: 2 to 5 nautical miles

alert icon Good: Greater than 5 nautical miles

Boaters can stay safe weather by:

red check mark Slowing down

red check mark Turning on all running lights

red check mark Listening for sounds of nearby boats, buoys, and land

red check mark Using radar to help locate dangers

red check mark Staying put until it’s clear

 

Aviation

Flying in fog can be quite a challenge, even for the most experienced pilots. The dangers are most evident during landing and takeoff procedures and flying at lower altitudes.

According to the U.S. National Weather Service, around 440 people are killed due to weather-related aviation accidents including the conditions of low visibilities and ceilings each year. Our recent airport industry survey found that 43% of all delays are due to adverse weather conditions, like fog.

If you are planning a flight and it’s foggy or may become foggy, follow the following safety guidelines:

red check mark Get the latest forecasts, advisories, and meteorological advice to help make your flight safe

red check mark Consider changing plans to avoid flying in fog

red check mark Follow the Federal Aviation Administration’s mandated guidelines and flight rules for the specific flight category based on visibility and ceiling height

red check markKnow the layout of the airport you are departing from and arriving to, including the length and orientation of the runway

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Learn More

Fog is both a common and dangerous weather condition comes in various forms and has a serious impact on travelers by land, air, and sea. Feel like an expert? If you still have questions please let us know on Twitter and our meteorologists will get back to you as soon as possible.

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If you feel confident in your fog knowledge, it’s time to increase your understanding of another aspect of meteorology. Check out other topics like lightning detection, floods, and wind on our Weather 101 Resources Guide.

Click here to choose a new weather 101 topic to learn about

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