Weather, Time and Sky
	Weather
	The weather had a profound effect on U-boat operations.  A U-boat's low conning tower made it a poor observation platform and darkness and bad weather only made that worse.  On the other hand the low silhouette of a U-boat enabled it to take advantage of darkness and lighting conditions to maneuver unseen on the surface surprisingly close to the enemy.  Cold weather and heavy seas took a heavy toll on watchstanders who were far more exposed to the elements than on surface ships.  In heavy seas the U-boat's deck gun was unusable and anti-aircraft weapons were greatly reduced in effectiveness, the transfer of fuel from other U-boats was made more difficult or impossible as was the downloading of torpedoes from deck containers.  In extremely heavy seas, a U-boat's torpedoes could not maintain depth and a destroyer's depth charges and guns could not be brought to bear.  In those conditions the U-boat War came to a temporary stop while both sides simply rode it out.
	The center column of each KTB page was titled "Account of Position, Wind, Weather, Sea state, Humidity, Visibility, Moonlight, etc."  Weather was entered in the KTB every four hours when the boat was on the surface.  Some COs simply noted the bare facts with a few abbreviations and numbers, others were much more descriptive.  The standard items found in these weather notations are described below in the order they are usually seen in KTBs:
	Observation Examples Notes Wind direction and force ENE 6 The Beaufort/Petersen scale was used. Cloud cover 6/10, C 6, Teilweise bewölkt Most often expressed as 10ths of cloud coverage but several sets of descriptive words appear in KTBs. Sea state See 2, Seeg. 2 U-boats used the Douglas Sea Scale to report Sea state and Swell Visibility Sicht. 5 sm, 500m, gute Sicht Usually in nautical miles or in meters if the visibility is low, some COs referred to visibility in descriptive terms: excellent, good, medium, moderate, bad or very bad. Weather Nebel (f or n), Regen (r), Schnee (s) A description of weather phenomena such as drizzle, rain showers, snow, etc.  A modified version of the Beaufort notation was used often with the letter in parentheses The conditions below were sometimes noted Swell D 2, 2, leichte Dg. U-boats used the Douglas Sea Scale to report Sea state and Swell Darkness Helle Nacht Descriptive terms were used, reported only when the night was particularly light or dark Temperature -3 Grad, plus 7° C Reported only when the temperature was particularly cold Barometric pressure 1013mb abflauend Reported only when it was rising or falling as during storms
	Sea state refers to the height and character of waves on a large body of water (not what is observed at the shore where waves interact with the bottom).  Waves are locally generated by wind acting on the surface of the water.  Swells are generated remotely sometimes traveling long distances.  It is quite possible for the wind to be calm with no waves but a significant swell will be experienced.  It is also possible for swells to arrive from two directions at once.  During WWII wind force was reported using the Beaufort/Petersen Wind Scale and sea state and swells were reported using the Douglas Sea scale.  The Beaufort/Petersen Wind Scale is still in use today.  The Douglas Sea Scale has been replaced by the World Meteorological Organization Sea State Code.  For more information on these scales see the links below.
	Click the icons to proceed to the associated websites   Wikipedia page on the Beaufort Wind Scale   Wikipedia page on the Douglas Sea Scale
	The project has adopted the rules in the table below to convert the three sets of descriptive words found in U-boat KTBs to the modern official values for cloud cover.
	Terms for Sky conditions
	Modern Official Terms/Values     (Used in translated KTBs) Descriptive term Literal translation Term Tenths coverage Ganz bedeckt (bed.) completely overcast  (ovc.) overcast 10/10 Fast ganz bed. nearly completely ovc. broken 8/10 Zum grossen Teil bed. for the most part ovc. scattered 6/10 Halb bed. partial ovc. few 4/10 Unbedeckt or Klar clear clear 0/10         Sehr stark bewölkt (bew.) very strongly cloudy overcast 10/10 Stark bew. strongly cloudy broken 8/10 Mittel or Teilweise bew. medium or partly cloudy scattered 6/10 Leicht, Wenig, Kaum, Geringe, Schwach bew. lightly, or a little cloudy few 4/10 Nicht bew. not cloudy clear 0/10             overcast 10/10 Mehrere Wolken several clouds broken 8/10 Einzelne Wolken few clouds scattered 6/10 Fast wolkenlos Vereinzelte Wolken nearly cloudless few 4/10 Wolkenlos cloudless clear 0/10
	Distances for Visibility conditions (from the Wetterkurzschlussel)
	Descriptive Term Distance           Sehr schlechte Very bad As a result of precipitation or dust Up to 1/2 nm Schlechte Bad Horizon still completely obscured, as a result of precipitation, dust or mist Up to 1-1/2 nm Geringe Poor Horizon indistinct, as a result of precipitation, dust or mist Up to 3 nm Mäßige Moderate Horizon just barely visible Up to 5 nm Gute Good Horizon easily distinguished Up to 10 nm Sehr gute Very good Ships, the coast and other things well distinguished at appropriate distances Over 10 nm Außergewöhnlich klare Luft Exceptionally clear air   Over 20 nm Sehr dichter Nebel Very thick fog   Under 50 meters Dichter Nebel Thick fog   Up to 200 meters Mäßiger Nebel Moderate fog Up to 1/2 nm Nebel     Starker Seerauch Heavy sea smoke (a type of fog that forms in cold air above warm water) Under 1/2 nm Leichter Seerauch Light sea smoke Over 1/2 nm
	Beaufort weather notation - an adaptation of this international Convention was used to indicated weather phenomina in some U-boat KTBs with German adaptations such as n = fog.  Information below with thanks to the Weather Dr., Keith C. Heidorn, PhD.
	Weather Notation Code 1935 International Convention Weather Notation Code German adaptations observed in KTBs b Blue sky with clear or hazy atmosphere with less than one-quarter of sky clouded b Blue sky - The letter b preceeds cloud cover 1/10 - 3/10 cloud cover (in some KTBs the letter b stands for bedeckt = overcast) c Cloudy with detached opening cloud, where more than three-quarters of the sky area is clouded c Cloudy - The letter c preceeds 4/10 - 8/10 cloud cover (in some KTBs the letter c preceeds all cloud cover values 1/10 - 10/10) bc Sky area clouded over between one-quatrer and three quarter of total area   Not observed to date d Drizzle or fine rain d Drizzle or fine rain e Damp air without falling rain. Considerable water deposited on trees, buildings, riggings, etc. e Damp air f Fog f    or  n Fog (in some KTBs n = nebel is used) fc Wet Fog fc Not observed to date g Gloomy weather g Gloomy weather, turbid atmosphere h Hail h Hail kq Line squall kq Not observed to date l Lightning l Lightning m Mist visibility 1100 yards or more but less than 2200 yards m Mist o Overcast o Overcast (in some KTBs the letter o preceeds cloud cover 9/10 and 10/10) p Passing shower p Passing showers q Squall q Squalls r Rain r Rain rs Sleet rs Not observed to date s Snow s Snow t Thunder t Thunder rl Thunder rl Not observed to date u Ugly, threatening skies u Ugly, threatening skies v Unusually good visibility v Observed rarely but meaning unclear (not associated with unusually good visibility) w Dew w Dew x Hoar frost z Not observed to date y Dry air (relative humidity less than 60%) y Observed but meaning unclear (associated with wet weather in KTBs) z Dust haze z Not observed to date
	Lower case letters indicate moderate, capital letters indicate heavy intensity.
	Figures in parehtneses are believed to indicated intermittant conditions.
	Height/Length of Swells (from the Wetterkurzschlussel)
	Descriptive Term Height/Length Niedrig Low Under 2 meters Mäßige hoch Moderate 2-4 meters Hoch High Over 4 meters Kurz Short Up to 40 meters Mittellang Medium 40-50 meters Lang Long Over 50 meters
	Time and Sky
	Times in KTBs are in DGZ (Deutsche Gesetzliche Zeit/German Legal Time), which was usually MEZ/CET (Mitteleuropäische Zeit/Central European Time) = Universal Time/GMT +1 hour.
	Daylight savings time was introduced in Germany in May 1916.  During WWI, DGZ was changed to MESZ/CEST (Mitteleuropäische Sommerzeit/Central European Summer Time) = Universal Time/GMT +2 hours between the following dates:
	30.04.1916  23:00 MEZ   to   01.10.1916 01:00 MESZ
	16.04.1917  02:00 MEZ   to   17.09.1917 03:00 MESZ
	15.04.1918  02:00 MEZ   to   16.09.1918 03:00 MESZ
	Daylight savings time was reintroduced in Germany in April 1940.  During WWII, DGZ was changed to MESZ/CEST between the following dates:
	01.04.1940  02:00 MEZ   to   02.11.1942 03:00 MESZ
	29.03.1943  02:00 MEZ   to   04.10.1943 03:00 MESZ
	03.04.1944  02:00 MEZ   to   02.10.1944 03:00 MESZ
	02.04.1945  02:00 MEZ   to   16.09.1945 03:00 MESZ
	As a result of maintaining the ship's clocks and KTB in German Legal Time, sunrise and sunset became more and more offset from normal experience as the boat moved further east or west from base.  For example, off the east coast of the US in the winter months, the sun might rise at 14:00 and set at 24:00 according to the ship's clocks.
	Invisibility is the primary tactical advantage of a U-boat.  The vast majority of attacks took place during the hours of darkness for only then could the boat use high speed to maneuver into attack position without being detected.  Ideally the boat would maneuver to attack from the dark side (into the sunrise, sunset or moon) silhouetting the target against the dim light.  The phase of the moon and the degree of sky cover were key factors in determining how dark the night would be.
	The time of sun and moonset, rise and transit are important for navigation using the sextant.  A fix could also be generated by sighting the sun or the moon.  If the tactical situation permitted the boat was likely to surface when stars are visible and the horizon was distinct (between civil twilight and sunrise or sunset) or during the sun or moon transit.
	For all the above reasons, Sun and Moon Data have been provided for each day of the KTB.  To generate this data, the 1200 DGZ (Universal Time + 1 or 2 hours) position, given in the Kriegsmarine Quadrant system is converted to Latitude and Longitude (see the Navigation page for links to Quadrant converters).  The resulting position and the date are entered into the US Naval Observatory's web page form and the data is formatted for use on the U-boat Archive website.
	Click the icon to proceed to the US Naval Observatory website page for Sun and Moon Data