athodyd. A ram-jet engine. An athodyd (Aero-THermO-DYnamic-Duct) is an open tube whose inside is specially shaped to produce thrust when fuel is burned in it.
When the athodyd moves through the air at a high speed, the air in its front end is compressed. Fuel is added to this compressed air and burned, and the burning fuel heats the air and expands it. This expansion accelerates the air to the rear and produces thrust.
athwartships. A direction across, or at right angles to, the longitudinal axis of an aircraft.
ATIS (automatic terminal information service). Recorded noncontrol information continuously broadcast in selected terminal areas. ATIS information contains appropriate weather observations, altimeter setting, instrument approaches in use, and airport information needed by pilots approaching the airport.
The use of ATIS frees the controller of having to repeat this basic information on the initial call-up from each pilot.
atmosphere. The layer of gases surrounding the earth, from the surface upward to a height of about 22 miles. The atmosphere consists of a mixture of nitrogen, oxygen, and small quantities of a number of other gases.
The lowest level of the atmosphere, called the troposphere, extends upward to about 36,000 feet. This is the area in which clouds form. Above the troposphere is the stratosphere, where there is no water vapor; therefore clouds cannot form.
The atmosphere presses down on the surface of the earth with a pressure of 29.92 inches (760 millimeters) of mercury. Its standard weight at sea level is 0.07651 pound per cubic foot, and it becomes lighter, or less dense, as its height above the earth increases.
The temperature of the atmosphere drops as its altitude increases, to about 36,000 feet. Here, the temperature stabilizes at -67°F (-55°C), and it remains at this temperature until it reaches an altitude of about 82,000 feet.
atmospheric electricity. Static electricity that exists between clouds and between clouds and the surface of the earth.
atmospheric noise (radio reception). Noise produced in radio equipment by electricity in the atmosphere.
atmospheric pressure. The pressure exerted by the atmosphere because of the attraction of gravity exerted on the air directly above the measuring instrument.
Under standard conditions, at sea level, the atmospheric pressure is 14.69 pounds per square inch,
29.92 inches of mercury, 1013.2 millibars and 1.01325 newtons per square meter. This is called one
atmosphere of pressure (one atm). Atmospheric pressure is also called barometric pressure.
atmospherics (radio reception). A form of static in radio reception caused by atmospheric electrical phenomena such as electrical storms.
atom. The smallest particle of a chemical element that can exist, either alone or in combination with other atoms.
An atom is made up of a nucleus, which contains protons and neutrons and electrons, which spin around the nucleus. In a balanced atom, there are as many electrons spinning around the nucleus as there are protons in the nucleus.