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Nacreous cloud

Clouds that have a soft, pearly lustre and form at altitudes of around 25-30 km. Also called mother-of-pearl clouds.

Nautical mile

The distance of 1 minute of latitude. 1 nautical mile equals 1.85km.

NCEP

National Centres for Environmental Prediction; the modernized version of NMC.

Negative tilt trough

An upper level system which is tilted to the east with increasing latitude (i.e., with an axis from southeast to northwest). A negative-tilt trough often is a sign of a developing or intensifying system.

NEXRAD

NEXt-Generation Weather RADar. Technologically-advanced weather radar being deployed to replace WSR-57 and WSR-74 units in the US. NEXRAD is a high-resolution Doppler radar with increased emphasis on automation, including use of algorithms and automated volume scan. NEXRAD units are known as WSR-88D.

NGM

Nested Grid Model; one of the operational forecast models run at NCEP. The NGM is run twice daily, with forecast output out to 48 hours.

NGP

(NOGAPS) Operational forecast model run by the Fleet Numerical Meteorology and Oceanography Centre. Forecast output to 144 hours. The NOGAPS model is run primarily for operational use by the US Navy.

Nimbostratus

(Ns) Latin "nimbus" - violent rain, "stratus" - layer
A dark grey-looking cloud that often covers the entire sky. Some definitions of Nimbostratus include a required rainfall rate. Steady rain or snow generally falls from nimbostratus.

Nitrogen

The most abundant gas in air, comprising 78% by volume. It is colourless and odourless.

NMC

National Meteorological Centre (U.S.).

NMOC

National Meteorological Operations Centre (Aus). Responsible for the operation of the Australian weather service, issuing real-time analysis and prediction products, maintaining a nation-wide weather watch and support the operational communications and computing infrastructure.

NOAA

National Oceanographic and Atmospheric Administration (U.S.).

Noctilucent clouds

Wavy, thin clouds that are seen at twilight in polar regions. They form at altitudes of 80-90 km and are made visible by sunlight reflecting off the underside of the cloud.

Nocturnal

Related to night-time, or occurring at night.

NOGAPS

See NGP

Nowcast

A short-term weather forecast, generally out to six hours or less.

NSSL

National Severe Storms Laboratory, in Norman OK, USA. (Sometimes pronounced NES-sel.)

NWP

Numerical Weather Prediction. NWP uses simplified mathematical equations of motion to simulate various processes in the atmosphere.

NWS

National Weather Service (U.S.).

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