© Jüri Kamenik
Old Town of Tallinn, Tallinn, Estonia
Latitude: 59° 26' 12'' N
Longitude: 24° 44' 28'' E
07 April 2013 1545 (Local Time)
Camera direction: towards S
Image P/S code: P.6.1
Image I.D.: 5143
CL = 7, CM = 2, CH = /
An excellent image of precipitating Nimbostratus. An amorphous base, poor visibility and low light typify this cloud, particularly when precipitation has set in. Pannus was measured at 200 m (600 ft) and Nimbostratus at 500 m (1 600 ft). Snow fell for most of the day with the air temperature below 0 °C.
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A weak low (mid-latitude cyclone) passed the area with an occluded front.
The sounding from 170 km north-north-west and an hour before the photo shows an unstable boundary layer and very high moisture content from the surface to the tropopause at 353 hPa (7 535 m).
There are quite big but narrow bands of precipitation (intensity is more light than moderate), which is typical for frontal precipitation.
The image is a good example of a dark bluish Nimbostratus. It is an early morning view at the rear of a polar air mass surface pressure trough, located 100 km to the east and moving rapidly to the south-east. Light, continuous rain had lasted for two hours at the site and was about to cease. Other than colour, the cloud is typical Nimbostratus, devoid of any structure elements and measured at a height of 800 to 1 200 m.
The light grey band on the horizon is the level where snow is melting to rain and is due to the poorer visibility in snow than rain. The melting level is only seen when precipitation is not too heavy.
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Nimbostratus is a grey layer of cloud, often dark, with an appearance rendered diffuse by more or less continuously falling rain or snow that in most cases reaches the ground. It is thick enough throughout to obscure the Sun. In this image, the cloud is typically grey, with some variation, but has little form. As continuous light rain had lasted for several hours, we may add the supplementary feature praecipitatio. Low, ragged clouds frequently occur below the main layer as the air in the lower layers is moistened by the precipitation; such clouds can be seen just above the horizon and are Stratus fractus. More extensive Cumulus fractus can also be seen, the two forming the supplementary feature pannus.
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Nimbostratus is a grey layer of cloud, often dark, with a diffuse appearance caused by more or less continuously falling rain or snow that in most cases reaches the ground. It is thick enough throughout to obscure the Sun. Grey, uniform Nimbostratus is seen in the background of this image while light, intermittent snowfall occurs (supplementary feature praecipitatio). Below the Nimbostratus, we see ragged, fragmented cloud (Stratus fractus) at the lowest level and larger dark grey patches in the upper part of the image; together, these constitute the accessory cloud pannus. The cloud is forming in an increasingly moist layer beneath the Nimbostratus, caused by partial evaporation of the precipitation.
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