Seeing Through the Fog: a look at how we can distinguish between snow, fog and clouds in satellite images

Fog is a meteorological phenomenon that occurs when water vapor condenses into tiny water droplets or ice crystals in the air, reducing visibility to less than 1 kilometer.

Fog can occur at any time of the year and at any time of the day, but is most common in the early morning or late evening. While fog can be beautiful to look at from the ground, it can also be a nuisance for transportation and cause hazards for pilots. In this post, we’ll take a look at what fog looks like on satellite images, and how meteorologists use these images to study and forecast this meteorological phenomenon.

Satellite images are a powerful tool for studying weather patterns, including fog. There are different types of satellite images that can be used to observe fog, but the most common are visible and infrared images.

Visible images, as the name suggests, capture the visible spectrum of light that is reflected off the Earth’s surface. This type of image is useful for observing the location and extent of fog, as well as other types of clouds. On a visible satellite image, fog will appear as a thin, white layer that hugs the ground. This is because fog is composed of tiny water droplets that scatter light in all directions, making it appear white.

Fog in Northern India, date: 09/01/2023, Credit: European Union, Copernicus Sentinel-2 imagery

Infrared images, on the other hand, capture the infrared radiation emitted by the Earth’s surface. These images are useful for identifying the temperature of the Earth’s surface, which can help meteorologists determine the location and intensity of fog. On an infrared image, fog will appear as a cool, blue or purple color, since the temperature of the water droplets that make up fog is usually cooler than the surrounding air.

One important thing to note is that not all low clouds are fog. Stratus clouds are low clouds that often look similar to fog but they are formed at higher altitudes and they don’t bring visibility reduction. The temperature difference is a key factor to distinguish them, fog tend to have the same temperature as the surface while stratus clouds are warmer.

The first fog or low clouds of the month are persisting over the Po Valley, a sign of the arrival of the autumn season. date: 07/10/2022, location: Po Valley, Italy Credit: European Union, Copernicus Sentinel-3 imagery.

In addition to visible and infrared images, meteorologists also use radar images to study fog. Radar images, which are produced by bouncing radio waves off the atmosphere, can provide information about the precipitation and reflectivity of the atmosphere. On a radar image, fog will appear as a weak echo, since the water droplets that make up fog are too small to be effectively detected by radar.

How to distinguish fog, snow and clouds in a satellite image?

Visual interpretation of images is one of the most important skills that we must constantly practice in order to be able to understand well the processes and objects captured on Earth. Let’s take a look at what she looks like from space:

Landsat-8, USGS / NASA

On this image from 12/18/2019, captured by the Landsat-8 satellite, we can see the western part of Bulgaria – the mountains Vitosha, Verila, the valleys along Struma river and a very small part of Rila mountain in the lower part of the image.

On this day in the morning hours, when the image was taken, we can see the fog occupying the lowest parts of the Sofia city, a small part of the regions near Radomir and Kyustendil and the valley of Struma river.

The fog looks visibly fluffier than the snow that covers the high parts of Vitosha and Rila mountains.

At the same time, if we compare it with clouds, it can be more difficult to distinguish. However, on winter days, when there are typical temperature inversions in the valleys, the view as in the image above is very typical – in the valleys there is fog, which perfectly nests in the lower landforms, and on the high mountains it’s sunny and cloudless.

Fog can be beautiful to observe from the ground, but it can also be a nuisance for transportation and cause hazards for pilots. Satellite images are a valuable tool for meteorologists to study and forecast this meteorological phenomenon, and can be used to determine the location, extent, and temperature of fog, as well as its reflectivity and precipitation.

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