N Nansen bottle A device used by oceanographers to obtain subsurface samples of sea water. The "bottle" is lowered by wire, its valves open at both ends. It is then closed in situ by allowing a weight (called a messenger) to slide down the wire and strike the reversing mechanism. This causes the bottle to turn upside down, closing the valves and reversing the reversing thermometers which are mounted on it in a special thermometer case. If, as is usually the case, a series of bottles are lowered, then the reversal of each bottle releases another messenger to actuate the bottle beneath it.    Nautical mile The nautical mile is closely related to the geographical mile which is defined as the length of one minute of arc on the earth's equator. By international agreement, the nautical mile is now defined as 1852 meters.     Nasen cast A series of Nansen-bottle water samples and associated temperature observations resulting from one release of a messenger.       Nephelometer An instrument which measures the scattering function of particles suspended in a medium in order to determine the visual range through the medium. See visibility meter.     Nepheloscope (1) An instrument for demonstrating the temperature changes which occur in air that is rapidly expanded or compressed. (2) A laboratory instrument for the production of clouds by the condensation process. (3) Same as nephoscope.     Nephometer A general term for instruments designed to measure the amount of cloudiness.      Nephoscope An instrument for determining the direction of cloud motion. There are two basic designs of nephoscope, the direct-vision nephoscope and the mirror nephoscope. Also called nepheloscope.     Net pyranometer An instrument for measuring the difference of the solar radiation falling on both sides of a horizontal surface from the whole hemisphere.     Net pyrgeometer An instrument for measuring the difference between incoming and outgoing terrestrial radiation.     Net radiation The difference between downward and upward (total) radiation; net flux of all radiation.     Net radiometer An instrument for the measurement of the net flux of downward and upward total (solar and terrestrial) radiation through a horizontal surface.     Net solar radiation The difference between the solar radiation directed downward and upward; net flux of solar radiation.     Newtonian telescope A reflecting type telescope with a 45° mirror, so that the primary image is observed through a hole in the side of the tube.     NEXRAD Acronym for NEXt generation weather RADar. A network of advanced Doppler radars, known as the WSR-88D (Weather Surveillance Radar - 1988 Doppler), developed in the 1980s and implemented in the 1990s to replace the aging network of WSR-57 and WSR-74 radar systems.     Nine light indicator A remote indicator for wind speed and direction used in conjunction with a contact anemometer and a wind vane. The indicator consists of a center light, connected to the contact anemometer, surrounded by eight equally spaced lights which are individually connected to a set of similarly spaced electrical contacts on the wind vane. Wind speed is determined by counting the number of flashes of the center light during an interval of time. Direction, indicated by the position of the illuminated outer bulbs, is given to 16 points of the compass.     Nipher shield A conically shaped, copper rain gauge shield.     Nonlinear Not a linear function of the relevant variables.     Non-recording rain gauge A rain gauge which indicates but does not record the amount of precipitation captured.     Normal operating conditions The range of operating conditions within which a device is designed to operate and for which operating influences are stated. See operating conditions, reference operating conditions.     Normal-plate anemometer A type of pressure-plate anemometer in which the plate, restrained by a stiff spring, is held perpendicular to the wind. The wind-activated motion of the plate is measured electrically. The natural frequency of this system can be made high enough so that resonance magnification does not occur.     NRM wind scale A wind scale adapted by the U.S. Forest Service for use in the forested areas of the northern Rocky Mountains (NRM). It is an adaptation of the Beaufort wind scale. The difference between these two scales lies in the specification of the visual effects of the wind. The force numbers and the corresponding wind speeds are the same in both.     NWS National Weather Service. Administered by the U.S. Department of Commerce.