(abbr) National Centers for Environmental Prediction (part of the US National Oceanic and Atmospheric Administration [NOAA], National Weather Service [NWS]). NCEP comprises 9 centres; one of those centres is responsible for running the atmospheric prediction models (see MRF). To find out more, visit: http://www.emc.ncep.noaa.gov/

Net radiation

The difference between absorbed and emitted radiation.




(abbr) see Noctilucent cloud.

Noctilucent cloud (NLC)

These are clouds in the high atmosphere (around 83km) which are thought to be composed of small ice-coated particles (possibly meteoritic debris), which can be observed in mid-summer (June/July in the Northern Hemisphere) against a twilit sky: the sun must be between 6 and 16 degrees below the local horizon. They appear as very tenuous, often interwoven filaments of cloud, mostly white or slightly blue in tone. (Also known as polar mesospheric clouds or PMC's.) [ For a link to more on this, and an aid to identification, see this entry in the main FAQ or this article.


(abbr) Navy Operational Global Atmospheric Prediction System (also abbreviated to NGP). This model is provided by the US Navy, Fleet Numerical Meteorology and Oceanography Center (FNMOC). To find out more, visit: https://www.fnmoc.navy.mil/PUBLIC/

Normal (environmental) lapse rate

The average change (decrease or increase) of temperature with height. In the troposphere it is 6.5°C / km, or ~2°C / 1000ft.

North Atlantic Oscillation

See "What is the North Atlantic Oscillation?".

Norwegian model

The classical idea of a travelling wave depression on the polar front running forward and deepening, with the cold front moving faster than the warm front, thus 'occluding' the warm sector, with the parent low slowing/turning to the left (in northern hemisphere), and filling up.


(abbr) Nimbostratus (NS in METAR/SIGWX charts etc., Ns otherwise); a deep cloud extending from low through to higher mid-levels, formed by vigorous and widespread rising motion, giving rise to heavy precipitation & a high risk of severe icing.


(abbr) No significant cloud - (No CB, no cloud < 1500m/5000ft or below the highest minimum sector altitude, whichever is greater, and CAVOK is not appropriate); used in aviation forecasts (and latterly in actuals).


(abbr) No significant weather; used with aviation weather forecasts, e.g. TAFs.


(abbr) Numerical weather prediction. The processes that make up the atmospheric system can be represented by fundamental equations, which can be solved in discrete time steps to achieve a numerical forecast. The concept was well understood from early in the 20th century, but needed the arrival of electronic computers in the early 1950's to be of practical use.