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June 2, 2019

How to interpret a satellite image

Satellite image of Northern Europe
Summary
In this article you can learn more about what the clouds are hiding.

Most people are somewhat familiar with satellite images, but fewer people know what kind of weather you can expect at the surface when looking at the different cloud formations on an image.

A satellite image can be very useful in order to understand the current weather situation. It is also a helpful tool in order to make a more accurate nowcasting, for example when there are thunderstorms or fog near your location. However, it is often necessary also to use surface observations, in order to better understand the weather situation. For example, it can be hard to see the difference between areas with low clouds and good visibility and areas with fog. Sometimes there are also several layers of clouds which makes it harder to determine the weather at the surface, when only using a satellite image.

Satellite Europe
March 9th, 2019 at 13:00 UTC - visual satellite image
Example – March 9th, 2019

The weather situation at the time of the above image:
A weak warm front over France and the Alps is separating the warm and moist air to the south from the colder and more unstable air to the north.
To the northwest, rather cold and dry air, with isolated showers.
Over central Europe and Denmark, somewhat moist air - locally with mist and showers/thunderstorm in connection with and south of a minor low located over the German Bight.
Over the Scandinavian Peninsula, cloudy conditions in connection with an occluded front with snowfall north of the front.

The below image from Blitzortung.com, is showing the lightning activity in connection with thunderstorms (cumulonimbus clouds) for the same time and area.

Lightning detections
March 9th, 2019 - lightning detection between 11:30 and 13:30 UTC

The dew point temperature (the temperature at which the air is saturated with water vapor) is a good indicator of the type of airmass. By comparing it to real temperature, you can determine if the air is humid (small difference) or dry (large difference).
As seen in the two pictures below, the air over France is rather warm and moist, while you have the opposite relation to the northwest with rather cold and dry air. To the northeast gradually colder air, but especially over southern Sweden also humid air.

Dewpoint measurements
March 9th, 2019 at 13:30 UTC - dewpoint temperature observations in 2m above the surface
Temperature measurements
March 9th, 2019 at 13:30 UTC - temperature observations in 2m above the surface
 
The associated weather

The weather at the time of the satellite image is shown below with classic weather symbols.

Weather symbols
March 9th, 2019 at 13:30 UTC - weather symbols. The yellow symbols are mist and fog, the green symbols are rain, the blue symbols are snow, and the green and blue triangles are showers of rain and snow respectively, and red symbols is thunderstorms.
 
Illustrated walkthrough

Below is a more detailed walkthrough of the weather in the areas marked in the image.

Satellite image with rings
March 9th, 2019 at 13:00 UTC – six selected areas with different clouds

 

Clouds at beach
The black area (Ireland/Scotland): Isolated showers, that could be with lightning, but also periods with partly blue sky. This cloud pattern indicates generally windy conditions, with gusty winds and is associated to cumulus, towering cumulus and cumulonimbus clouds.

 

Few clouds
The red area (UK east coast): No or only a few cumulus clouds due to the northwesterly winds over the British highlands, making the air on the leeside dryer.

 

Foggy conditions
The blue area (France/Bay of Biscay): Widespread low stratus clouds with mist/fog. Although the weather seems grey and gloomy, the sun is not far away as these clouds are rather thin and there are no clouds above.
During the summer, these low clouds typically disappear during the day with sunny weather as result, but during the winter they can cover the sun for days. Especially during the spring, there is a big temperature difference between the water and the land. The fog could be present all day over sea and along the coast, while people on land can enjoy a sunny day.

 

Dark clouds
The green area (Denmark/German Bight): A minor low pressure with cumulonimbus clouds with showers and risk of lightning embedded in frontal nimbostratus clouds, and somewhat misty conditions. Due to rather uniform cloud bases and the reduced visibility, you will not think of the weather as a typical thunderstorm weather.

 

Showers
The yellow area (Germany/Poland/the Netherlands): A combination of isolated and embedded showers. The thunderstorm cells are typically bigger than the ones in the black area described above. Also, larger areas with no or only a few clouds, as the sinking air between the cumulonimbus clouds will dry out the air and cover a larger area compared to the isolated showers northeast of the UK. Especially over Germany, the showers are partly embedded in lower stratocumulus clouds. It could therefore be difficult to determine if there is a risk of lightning, by only studying the sky from the ground.

 

Winter conditions
The orange area (Sweden): Mainly light precipitation. Due to the colder air to the northeast, the precipitation gradually turns into snow. Compared to the thunderstorm clouds over Germany, Poland, the Netherlands and the UK, the height of these clouds is lower and therefore producing less intense precipitation. Generally, rather poor visibility, over the southern part due to the rather moist air with mist and light drizzle, and over the northern part due to the visibility reducing snowfall.