_Antitrades._--Term formerly applied to the prevailing westerly winds of middle latitudes, but now more frequently applied to the westerly return-currents lying over the trade winds. Some writers prefer to call the former the _antitrades_ and the latter the _countertrades_. _Antitwilight Arch._--The pink or purplish zone of illumination bordering the shadow of the earth (_dark segment_) in the part of the sky opposite the sun after sunset and before sunrise. _Arcs of Lowitz._--A pair of rare halo phenomena. These arcs are directed obliquely downward from the parhelia of 22 degrees on either side of the sun toward the halo of 22 degrees. _Astraphobia._--A pathological condition experienced by certain persons before and during thunderstorms. _Atmometer._--An instrument for measuring evaporation; also called _atmidometer_, _evaporimeter_, etc. _Aureole._--(See _corona_. 1). _Aurora._--A luminous phenomenon due to electrical discharges in the atmosphere; probably confined to the tenuous air of high altitudes. It is most commonly seen in sub-Arctic and sub-Antarctic latitudes. Called _aurora borealis_ or _aurora australis_, according to the hemisphere in which it occurs. Observations with the spectroscope seem to indicate that a faint “permanent aurora” is a normal feature of the sky in all parts of the world. “_Backstays of the Sun._”--A sailor’s name for crepuscular rays extending downward from the sun. _Baguio._--The name current in the Philippines for a tropical cyclone. _Ballistic Wind._--A military term applied to a fictitious wind which, if affecting a projectile throughout its flight, would produce the same total effect in deflecting it from its course and altering its range as do the various winds that it actually encounters. _Ballon-sonde._--A sounding-balloon. _Bar._--A unit of pressure equal to 1,000,000 dynes per square centimeter. A bar = 100 _centibars_ = 1,000 _millibars_. A barometric pressure of one bar is sometimes called a “C. G. S. atmosphere,” and is equivalent to a pressure of 29.531 inches of mercury at 32 degree F. and in latitude 45 degrees. _Barisal Gun._--Same as _brontide_. _Barocyclonometer._--One of several instruments that have been devised for locating tropical hurricanes without the aid of a weather map. _Barograph._--A self-registering barometer. _Barometer._--An instrument for measuring the pressure of the atmosphere. The two principal types are the _mercurial_ and the _aneroid_. The _microbarometer_ is used to show minute changes of pressure. Certain forms of hygroscope are popularly miscalled “barometers.” _Barometer Well._--Same as _blowing well_. _Barometric Tendency._--The change of barometric pressure within a specified time (usually three hours) before one of the regular observations.