© Jarmo Koistinen
Janakkala, Finland
Latitude: 61° 0' 0'' N
Longitude: 25° 0' 0'' E
17 April 2010 1305 (Local Time)
Camera direction: towards SW
Image P/S code: P.5.7
Image I.D.: 5446
CL = 4, CM = 1, CH = /
Altostratus is a greyish, sometimes bluish cloud sheet or layer that has a striated, fibrous or uniform appearance and may totally or partly cover the sky. This photograph reveals an extensive layer of grey and fairly uniform Altostratus. At the top of the image, the cloud is sufficiently translucent that the Sun can be seen diffusely. This is the ground glass effect that defines the variety translucidus. However, the cloud on the right is somewhat darker and may be transitioning into the variety opacus as it thickens with the approach of an occluding front. Careful examination of the image reveals virga falling from the Altostratus. Near the bottom right corner, the lower cloud is rather diffuse due to the presence of virga, but virga can also be seen in other parts of the image (at 3 and 4). Small amounts of lower cloud are also present, including Cumulus humilis. In the bottom left-hand corner, small patches of Stratocumulus are visible, having developed from the flattening of Cumulus; hence, they are Stratocumulus cumulogenitus.
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The 1200 UTC synoptic chart shows a low of 989 hPa located over central Sweden and moving east. The associated occluding front was 200 km west of the image site, where cold surface air prevailed.