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Glossary

There are currently 37 names in this directory
ABSOLUTE HUMIDITY
is the weight of water vapor per unit volume of air.

ABSOLUTE INSTABILITY
When the lapse rate of a column of air is greater than the dry adiabatic lapse rate. The term absolute is used because this applies whether or not the air is dry or saturated.

ABSOLUTE TEMPERATURE SCALE
A temperature scale with a freezing point of +273°K (Kelvin) and a boiling point of +373°K.

ABSOLUTE ZERO
Considered to be the point at which theoretically no molecular activity exists or the temperature at which the volume of a perfect gas vanishes. The value is 0° Kelvin, -273.15° Celsius and -459.67° Fahrenheit.

ABSORPTION
The process in which incident radiant energy is retained by a substance. The absorbed radiation is then transformed into molecular energy.

ABYSSAL PLAIN
The flat, gently sloping or nearly level region of the sea floor.

ACID PRECIPITATION
Rain or snow with a pH value of less than 5.6; (sometimes caused by air pollutants.)

ADIABATIC PROCESS
A thermodynamic change of state in a system in which there is no transfer of heat or mass across the boundaries of the system. In this process, compression will result in warming and expansion will result in cooling.

ADIABATIC TEMPERATURE CHANGE
A cooling or heating of the air caused by the contraction or expansion of air molecules, as opposed to the loss or gain of heat. For example, adiabatic cooling takes place as air rises.

ADVECTION
The horizontal transfer of any property in the atmosphere by the movement of air (wind). Examples include heat and moisture advection.

ADVECTION
Horizontal movement of air, moisture, or heat.

ADVECTION FOG
Fog that develops when warm moist air moves over a colder surface, cooling that air to below its dew point.

ADVECTION FOG
Horizontal movement of warm, humid air over colder ground or water.

ADVISORY
Statements that are issued by the National Weather Service for probable weather situations of inconvenience that do not carry the danger of warning criteria, but, if not observed, could lead to hazardous situations. Some examples include snow advisories stating possible slick streets, or fog advisories for patchy fog condition causing temporary restrictions to visibility.

AFOSAFOS
Acronym for Automation of Field Operations and Services. It is the computer system that links National Weather Service offices together for weather data transmission.

AFTERGLOW
The glow in the western sky after sunset.

AIR MASS
A large body of air with nearly uniform temperature and moisture content.

AIRSTREAM
A significant body of air flowing in the same general direction.

ALTITUDE
Height expressed as the distance above a reference point, which is normally sea level or ground level.

AMIHAN
Local word for "northeast monsoon". It affects the eastern portions of the country from October up to late March, starts over Siberia as a cold, dry air mass but gathers moisture as it travels across the Pacific Ocean before reaching the eastern sections of the Philippines is characterized by widespread cloudiness with rains and showers. The North Pacific Trades gradually replaces the NE Monsoon in March, appears in all seasons and blows dominantly from March to April, giving strong convective activity.

ANEMOMETER
An instrument that measures wind speed.

ANEROID BAROMETER
An instrument built around a metal structure that bends with changing air pressure. These changes are recorded on a pointer that moves back and forth across a printed scale.

ANISALLOBAR
Isogram with the same rise of barometric pressure in a given time.

ANOMALY
Also known as departure from normal. The deviation of (usually) temperature or precipitation from the normal values in a given region over a specified period.

ANTICYCLONE
A closed wind circulation of high barometric pressure that rotates clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. With respect to the relative direction of its rotation, it is the opposite of a cyclone.

APHELION
the point in the path of a celestial body (as a planet) that is farthest from the sun.

ARCTIC AIR
A mass of very dry, very cold air that develops over the snow-and-ice-covered regions of the Far North.

ARIDITY
The degree to which a climate lacks effective, life promoting moisture; the opposite of humidity.

ASTEROID
A relatively small, inactive, rocky body orbiting the Sun.

ATMOSPHERE
The mass of air surrounding the Earth.

ATMOSPHERIC PRESSURE
The amount of force exerted on a unit surface area. Also called air pressure.

AURORA
A luminous phenomenon that consists of streamers or arches of light and is caused by electrical discharges in the atmosphere, mostly confined in the tenuous air of high altitude. It is most commonly seen in sub-Arctic and sub-Antarctic latitudes. However, 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.

AURORA AUSTRALIS
An aurora that occurs in the southern hemisphere; also called southern lights.

AURORA BOREALIS
An aurora that occurs in the northern hemisphere; also called northern lights.

AVIATION WEATHER FORECAST
A forecast of weather elements of particular interest to aviation. These elements include the ceiling, visibility, upper winds, icing, turbulence, and types of precipitation and/or storms. It can be divided into four basic categories, area forecasts, terminal forecasts, route forecasts,and flight forecasts.

AVIATION WEATHER OBSERVATION
An evaluation, according to set procedure, of weather elements which are most important to aircraft operations. It includes the cloud height or vertical visibility, sky cover, visibility, obstructions to vision, certain atmospheric phenomena, and wind speed and direction that prevail at the time of the observation.

AZIMUTH
The length of the arc of the horizon intercepted between a given point and an adopted reference direction, usually true north, and measured clockwise from the reference direction. It is a horizontal direction expressed in degrees. It is sometimes synonymous with bearing, but the latter is a navigation term and can be modified in several ways. Any point on or above the horizon can be located by its angles of azimuth and elevation plus either height or distance (or slant range) data.


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