What does A.A. stand for?

Anti-aircraft

Because much of World War II was fought in the air, antiaircraft weapons were widely used by both Allied and Axis forces in Europe and the Pacific.

Antiaircraft guns ran the gamut in size. Some were as small as machine guns while others, such as the German 128-mm Flak, were huge cannons. The 128-mm Flak was the largest antiaircraft weapon of the war and was usually mounted on a large tower. The guns were used to protect German-occupied territory from Allied bombing raids and inflicted quite a bit of damage on Allied planes.

U.S. ground troops in Europe and the Pacific relied on six antiaircraft weapons to bring down enemy planes. They included:

Quad .50-caliber machine gun. Capable of firing 2,300 rounds per minute, it was quite effective against low-flying planes.

37-mm gun. A large weapon, it was usually towed by truck and could fire 120 rounds per minute with an effective ceiling of 10,500 feet.

Bofors 40-mm gun. This was the most widely used antiaircraft gun in the war. Also towed by truck, it could fire 120 rounds a minute with an effective ceiling of 11,000 feet.

3-inch gun, also known as the 76-mm. Towed by truck, it could fire 25 rounds per minute and had an effective ceiling of 27,900 feet.

90-mm gun. A towed weapon, it could fire up to 25 rounds per minute in short cycles, with an effective ceiling of 33,800 feet.

120-mm gun. Used primarily for city defense, it had an effective ceiling of 56,000 feet.

U.S. forces generally referred to antiaircraft guns as AA, while the British called them ack-ack. German forces called them flak, as did Allied bomber pilots who wore protective flak jackets against their bursting shells, which were also called flak. (This is where the phrase “to take flak” comes from.)

The U.S. Navy had seven primary antiaircraft guns on large ships, as well as .30-caliber and .50-caliber machine guns on smaller ships and submarines. Antiaircraft cannon (meaning they fired explosive shells) ranged from 1.1-inch guns capable of firing up to 600 rounds per minute to 5-inch guns capable of firing up to 22 rounds per minute, with an effective ceiling of 37,200 feet.

One of the most important advances in antiaircraft weaponry during the war was the proximity fuse, also known as the Variable-Time (VT) fuse. Introduced in 1943, the VT fuse detonated a shell when it was close to the enemy plane, dramatically increasing the effectiveness of antiaircraft fire. Ammunition equipped with the VT fuse could be fired from guns that were three inches and larger.

Anti-Aircraft they were flax guns used to shot down enemy bombers and fighters, the UK and US still use them to this day their just more advanced, things like SAM sites (Surface-to-Air-Missile)

The daughter who served in this unit was Mary Soames, she was deployed in London, Belguim and Germany and reached the rank of Junior Commander. She is the last surviving child of Winston and Clementine Churchill

Anti-Aircraft

It must have been an American unit in the armed forces in England.

I always though it stood for Alcoholics Anonymous...