[Tomcattwo]

From Doenitz's UBoat Kommandanten Handbuch:

100.) Favorable conditions for attack.
a) With the sun behind: The torpedo control officer [the officer in charge of the discharging of the torpedoes is not dazzled, but can clearly see the sharp outline of the target. From the enemy ship, the periscope, raised no more than is absolutely necessary, is not to be seen among the reflections of the bright sun in the water; and also a trail of bubbles can not usually be seen until it is too late.
b) From the weather side (windward). The periscope should move with the sea. The waves, coming from behind, always wash over a periscope which is in the right position, that is to say, in a low position; and the splashes and spray water of the periscope are not easily made out. In addition, the enemy look-out to windward, especially in a strong wind, or in heavy rain, meets with difficulties.
As regards the weather side (windward), if the wind is moderate, torpedoes discharged from the stern form an exception (i.e., to the above inasmuch as the submarine, In moving with the waves (that is, in this case to leeward) comes near to riding on the waves, whereas to windward - driving against the sea - the periscope may cause conspicuous splashes even when the submarine is traveling at low speed. In a keen wind, however, the windward side is also more favorable for the stern launching of torpedoes, because the enemy, as a matter of course, then keeps a better lookout to leeward, and observation is better.
c) Wind 3 to 4 and sea 2 to 3 are most favorable for making the attack, because the sea then washes over the low-lying periscope, without interfering with the view of the target, while the action of the underwater (depth) steering gear is not adversely affected.
101.) Unfavorable conditions for attack.
a) Heavy seas or swell: It is difficult to keep the submarine at the right depth for attack, especially when the attack has to be carried out against the sea. According to the qualities (efficiency) of the underwater steering gear of the boat, this will soon put a limit to the possibilities of underwater attack (see Section I, D, No. 78). It is in a rough sea that an attack in a direction parallel to the waves is more likely to succeed most favorable for the underwater steering of the submarine and the depth course of the torpedo).
b) Sea as smooth as oil: The slightest ripple even of the low periscope is noticeable, and easily observable by the enemy. Exceptions: enemy coming out of a bright sun; conditions of twilight; moonlit nights.
c) Attack with dark thunderclouds in the background; even the most efficiently camouflaged (painted) periscope will then appear white against e dark black clouds. d) Against the sun: the estimation of speed, position and distance (range) is considerably more difficult; besides which, if the attack is carried out against the sun, there is the danger of attracting attention by the flashes of light proceeding from the objective (lens system of the periscope).