Weather Conditions During the Breakthrough of the
  
 
Denmark Strait.
  
     
 
1)   Requirement for the weather conditions.
  In order to make the breakthrough through the Denmark Strait unnoticed by the enemy, special emphasis must be placed on suitable weather conditions in summer.  The following three main requirements must be placed on the weather situation:
    a) Low visibility, possibly fog,
    b) Low, close cloud decks to hinder enemy aircraft reconnaissance,
    c) Strong wind and seas to lower the enemy alertness and - if need be - to hinder the launching of boats.
  These weather conditions should exist as far as possible in the entire Denmark Strait and also for the whole duration of the passage, i.e. for 36 hours.
2)   Selection of suitable weather conditions.
  As things stand, the above requirements cannot be met by bad weather fronts that cross the Denmark Strait from SW to NE, since such bad weather zones are penetrated relatively quickly by a ship traveling in the opposite direction.  In addition, in view of the extremely sparse information available for weather reports, it is very difficult, if not impossible, to predict the direction and speed of displacement of such fronts with sufficient certainty that the main bad weather zone is encountered by the passing ship at the narrowest part of the strait.  Based on these considerations, a weather situation was waited for, which is characterized by a low pressure area to the south or over Iceland, as well as a strong front side south current in the sea area Scotland-Faroes and a strong north-east to east current in the Denmark Strait.  The moist warm air, which is led northwards by the south current at the front, penetrates the Denmark Strait with easterly winds and has to cause a severe deterioration in visibility by cooling over the cold water areas, although this deterioration in visibility will first set in in the eastern part of the Denmark Strait and then spread to the west, so that a accompanying ship encounters low visibility weather all along the strait.  The low clouds occurring on the north side of the low over Iceland, widespread precipitation and strong east to north-east winds also have a favorable effect, so that all of the above requirements can be met by such a weather situation.
 
     
     
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3)   Performing the weather briefing..
  The change of the general weather situation to conditions favorable for the breakthrough according to 2) could already be seen on the morning of 27 June, 1940 (Chart 1).  A pronounced low pressure area is approaching from the Atlantic on a north-easterly course, which is expected to pass south of Iceland.  It is accepted that the beginning of the favorable weather conditions will be on 27 June at noon on the basis of the information available for the night of 28 to 29 June.  The weather at the location of the ship on 27 June, still under the influence of a low over the North Sea, therefore there are moderate to strong north winds, good visibility, in the afternoon temporary decline in snow showers.  On 28 June in the morning the ship was at the east exit of Denmark Strait in a high pressure ridge with low cloud cover, weak winds and excellent visibility. The low approaching from the Atlantic had meanwhile advanced further to the northeast (Chart 2).  A pronounced southerly current has already set in between Iceland and England. Calculations show that the actual warm air will reach the Denmark Strait in about 48 hours. Since, as already stated, the deterioration in visibility must first begin in the eastern part of Denmark Strait, on 28 June at noon it was decided to first wait at the eastern exit of Denmark's Strait until visibility has decreased enough and then break through Denmark's Strait under the protection of the low visibility zone advancing to the west.
  On 29 June in the morning the time had come (Chart 3). The visibility at the ship's location is only 1 nm, it has been raining continuously from deep clouds for hours, the wind has increased from the east to strength 6-7. When passing through the Denmark Strait, the weather changed only slightly during the whole day and the following night. The visibility varied between 1/2 to 2 nm, only around midnight the visibility is temporarily 3-4 nm.  On the morning of 30 June the actual warm air with northeasterly winds reached the ship at the west exit of the Denmark Strait which brought thick fog, lasting until noon on 30 June.  During the following days the weather in the sea area east of Greenland was predominantly low visibility.
     
                                                                       
                                                                Regierungsrat und Marinemeterologe
 
     
     
 

 

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Chart1 Weather chart from 27 June 07.00 GMT
   
Chart2 Weather chart from 28 June 07.00 GMT
   
Chart3 Weather chart from 29 June 07.00 GMT