Nacreous Cloud - A high altitude cloud that forms on rare occasions, 20 to 25 kilometers (12 to 16 miles) above the ground surface in the stratosphere. Nacreous clouds form when the atmospheric temperatures where they form fall below -78°C (-108°F). These clouds reflect sunlight in the early morning or late evening, producing a variety of iridescent colors against the dark sky. Because of their high altitude and the fact that the Earth’s surface is curved, these clouds are illuminated by sunlight originating from below the horizon.

Nadir - (1) The bearing direction straight below a specific location. (2) A term used in astronomy describing a point straight below the zenith for a particular location. 

Nanism - A term used to describe dwarfism in plants and animals. 

Nappe - A large mass of rock that has been pushed over other rocks by a recumbent fold, thrust fault, or both geologic processes combined.

Narrows - A water feature or valley that is geographically constricted.

Native Species - A species that normally exists and reproduces in a specific region of the Earth. Compare with immigrant species, indicator species, and keystone species.  

Natural Arch - An arch with an open area beneath it that forms naturally in a rock formation because of erosion. Also called a natural bridge.

Natural Bridge - An erosional feature in a natural rock formation that resembles a bridge.

Natural Disaster - A natural event that causes significant damage and loss of life, human and other. 

Natural Gas - A hydrocarbon-derived gas, mainly composed of methane, commonly found in the pores of sedimentary rocks of marine origin.

Natural Hazard(s) - (1) A natural phenomenon that can cause negative consequences to life. (2) The study of the dangers and risks associated with natural phenomena.

Natural Ionizing Radiation - A type of radiation from natural sources in the environment that can ionize (remove an electron) an atom or molecule. Ionizing radiation generally has a wavelength shorter than the lower end of the ultraviolet spectrum. Ionizing radiation can cause damage to DNA in living organisms.

Natural Region -  A region on the Earth’s surface that displays consistency in terms of climate, topography, landforms, and/or living organisms.

Natural Resource - A substance or thing (resource) derived from the natural environment. 

Natural Selection - The environment's influence on the reproductive success of individuals in a population. This influence results in the exclusion of maladapted genetic traits found within individuals in a population.

Natural Vegetation - An area on our planet where the various species of plants are not influenced by the presence of humans.

Nautical - Something that is of concern to maritime navigation or sailors.

Nautical Mile - A unit of measurement that equals 1852 meters,  6,076.1 feet, or 1.1508 statute miles.

Neap Tide -  A type of tide that occurs every 14 to 15 days and coincides with the first and last quarter of the Moon. This tide has a small tidal range because the Moon's and Sun's gravitational forces are perpendicular to each other. Contrasts with the spring tide.

Near Infrared - There are several different definitions for this term. (1) Form of electromagnetic radiation with a wavelength between 0.71 and 1.5 micrometers (µm). (2) Form of electromagnetic radiation with a wavelength between 0.75 and 1.4 micrometers (µm). (3) Astronomers typically define it as electromagnetic radiation with a wavelength between 0.70 and 1.0 and 5.0 micrometers (µm).

Near Threatened - One of the categories used by the International Union for Conservation of Nature's (IUCN) Red List to describe the status of a species. This category suggests the species has been evaluated and may be classified as being either critically endangered, endangered, or vulnerable in the near future.

Nearctic -  One of eight biogeographic realms found on Earth, where many species have a unique evolutionary history because of geographic isolation. Geographically, this realm covers most of North America, including Greenland (see Map). The climate of this realm ranges from subtropical to polar. Image Source: Wikimedia Commons. This image is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Nearest-Neighbor Analysis - A statistical measure determines the pattern of spread of something over a geographical area. A numerical value is calculated from this analysis that has two endpoints: the distribution of the phenomena is uniformly spaced or highly clustered.

Nearshore - A narrow zone extending seaward from the shoreline to a water depth of about 20 meters (60 feet). Compare with offshore.

Nearshore Current - The system of ocean currents found along the shoreline is caused by wave action. Normally consists of longshore and rip currents.

Neat Line - An embossed line that clearly identifies the edge of the area displayed on a map.

Nebula - An interstellar cloud of dust, hydrogen, helium, and other gases. Some nebulae are the remnants of a supernova. Some large nebulae are believed to be the nurseries for the formation of new stars, planets, and other Solar System objects. 

Nebular Hypothesis - A theory used to explain the formation of solar systems in the Universe. It suggests that random accumulations of interstellar gas contracted under gravity to form a disk-shaped, swirling mass of molecular matter (mainly hydrogen). Continued accretion of the swirling mass under gravity eventually produces a central star with orbiting planets. Estimated to take about 10 to 100 million years.

Neck - An ancient volcanic neck that has been exposed by the erosion of the rock that once surrounded it.

Needle - Can refer to a narrow pointed mass of rock associated with erosion in coastal or mountainous environments.

Needle Ice - A form of periglacial ground ice that consists of groups of ice slivers at or immediately below the ground surface. Needle ice is only about a few centimeters long.

Negative Feedback - A change in the state of a system that counteracts the measured influence of the initial alteration (input). Compare with positive feedback.

Nekton - Near-surface dwelling marine organisms that have the ability initiate their own movement against ocean tides and currents.

Neocatastrophism - (1) This term is used to describe the general idea that low-frequency catastrophes do occasionally occur in the backdrop of gradual uniform processes (see the idea of Uniformitarianism) operating everywhere on our planet. Further, the impact of such events can have a great impact, for example, the evolution of life. (2) This term is also used to suggest the idea that sudden mass extinctions of life in the past were caused by some low-frequency catastrophic event. (3) This term has also been used by astronomers to suggest that gamma-ray bursts act to regulate the development of life in the Universe.

Neogene - This geologic period contains two epochs: Miocene and Pliocene. This geologic period began about 23.03 ± 0.05 million years ago and ended 2.588 million years ago.  By the end of this period, Earth's climate cooled considerably, with the succeeding period being the Quaternary.  During the Neogene, birds and mammal species evolved into their modern forms, while humans first appeared in Africa. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.

Neoglacial - A period during the Holocene when temperatures became colder and resulted in sustained glacial advances. The Neoglacial occurred roughly between 4700 and 2500 years ago.

Neotropic - One of eight biogeographic realms found on Earth, where many species have a unique evolutionary history because of geographic isolation. Geographically, this realm includes South America, Central America and the various islands in the Caribbean. The climate of this realm ranges from tropical to mid-latitudinal.

Nephoscope - A meteorological instrument used to measure the altitude, direction of movement, and speed of clouds.

Neritic - The area of the ocean that is directly above the continental shelf.

Nesting - A term used in data sampling that describes a situation where samples and sub-samples are arranged hierarchically.

Net Longwave Radiation (Balance) - The balance between incoming and outgoing longwave radiation. Mathematically expressed as: L* = (L⬇ - L⬆) where L* is net longwave radiation at the surface, L⬇ is atmospheric counter-radiation (greenhouse effect) directed to the Earth's surface, and L⬆ is longwave radiation lost from the Earth's surface.

Net Primary Productivity (NPP) - Total amount of organic chemical energy or biomass fixed by the processes of photosynthesis minus the chemical energy or biomass lost through cellular respiration. Compare with gross primary productivity.

Net Radiation (Balance) - The balance between incoming and outgoing shortwave and longwave radiations. Mathematically expressed as:

Q* = (K + k)(1 - a) - L⬆ + L⬇

where Q* is surface net radiation (global annual values of Q* = 0, because input equals output, local values can be positive or negative), K is surface direct shortwave (solar) radiation, k is diffused shortwave (solar) radiation (scattered insolation) at the surface, a is the albedo of surface, L⬇ is atmospheric counter-radiation (greenhouse effect) directed to the Earth's surface, and L⬆ is longwave radiation lost from the Earth's surface.

Net Radiometer - Is a meteorological instrument (radiometer) that measures the difference between downward/incoming radiation and upward/outgoing radiation from a surface.

Net Secondary Productivity -  The total amount of organic chemical energy or biomass assimilated by consumer organisms minus the chemical energy or biomass lost through cellular respiration. Compare with gross secondary productivity.

Net Shortwave Radiation (Balance) - The balance between incoming and outgoing shortwave radiation. Mathematically expressed as:

K* = (K + k)(1 - a)

where K* is surface net shortwave radiation, K is surface direct shortwave (solar) radiation, k is diffused shortwave (solar) radiation (scattered insolation) at the surface, and a is the albedo of the Earth's surface. 

Nets - A type of patterned ground found in periglacial environments that has a weblike arrangement on the ground surface.

Network - A system that consists of a number of parts that work together because of some type of physical linking. For example, a stream network consists of many stream channel segments that are linked and work together to transport water and sediment.

Neutral (pH) - Any substance with a pH around 7.0.

Neutral Atmosphere - Condition in the atmosphere where isolated air parcels do not tend to rise or sink. The parcels of air tend to be at same temperature as the air that surrounds them. Compare with a stable atmosphere and a unstable atmosphere.

Neutral Shoreline - A coastline that is not undergoing landform transformation because of the upward or downward elevational movement of the associated land surface.

Neutral Soil - A soil that has a pH at or very near neutral (7.0).

Neutral Stability - A meteorological term that describes a situation where isolated air parcels in the atmosphere do not tend to rise or sink.

Neutral Solution - Any water-based solution that is neutral (pH approximately 7.0) or has an equal quantity of hydrogen ions (H⁺) as hydroxide ions (OH^-). Also see acidic solution and basic solution. 

Neutralism - An interspecific interaction where the species do not directly influence each other's fitness.  

Neutron - A type of subatomic particle found in the nucleus of an atom. This particle is similar in mass to a proton but lacks an electromagnetic charge. Compare with an electron and a proton.

Neutron Probe - An instrument designed to measure soil moisture content. Uses the emission of neutrons from americium-241 and beryllium in its process of taking the measurement.

Névé - A partially melted and compacted snow that has a density of at least 500 kilograms per cubic meter.

Newton (N) - A unit of force that creates an acceleration on a mass of 1 kilogram equal to 1 meter per second per second with no friction and under the conditions of a vacuum.

Niche - The role that a species has in a habitat because of its evolutionary adaptation to this particular environment. This includes its behavior and interactions with other species.

Niche Glacier - Is a very small glaciers in size that form where snow and ice accumulate in a mountainside depression. Smaller than a cirque glacier.

Niche Specialization - A process where evolution, through natural selection, adapts a species to a particular set of abiotic and biotic characteristics within a habitat.

Nickpoint (Knickpoint - British spelling) - A point on a stream long profile where the gradient is interrupted by a sudden drop in elevation. Nickpoints are the locations of rapids and waterfalls.

Nightglow - See airglow.

Nimbostratus Cloud - Dark gray, low altitude cloud that produces continuous precipitation in the form of rain or snow. Found in an altitude range from the surface to 3,000 meters (9,840 feet).

Nimbus - A term that relates to a cloud that is producing rain.

Nitrate - A form of nitrogen commonly found in the soil and used by plants for building amino acids, DNA, and proteins. It is commonly produced by the chemical modification of nitrite by specialized bacteria. The chemical formula for nitrate is NO₃^-.

Nitric Acid - Acid with the chemical formula: HNO₃.

Nitric Oxide - A gas produced by bacterial action in the soil and by high-temperature combustion. Nitric oxide is a component in the production of photochemical smog. This colorless gas has the chemical formula NO.

Nitrification - The biochemical oxidation of ammonium to nitrite, and then nitrite to nitrate. This process is carried out by specialized bacteria.

Nitrite - A form of nitrogen commonly found in the soil. It is commonly produced by the chemical modification of ammonium by specialized bacteria. This form is toxic to plants and animals at high concentrations. The chemical formula for nitrite is NO₂^-.

Nitrogen - An important chemical element used in a variety of processes associated with life on Earth. The atmosphere is probably the most important store of nitrogen, where it exists in abundance as a diatomic gas (N₂). Nitrogen occurs in all organisms, playing an important role in the chemistry of amino acids, proteins, and nucleic acids.

Nitrogen Cycle - A chemical model showing the storage and cyclic movement of organic and inorganic forms of nitrogen between the atmosphere, biosphere, hydrosphere, and lithosphere. 

Nitrogen Dioxide - A gas produced by bacterial action in the soil and by high-temperature combustion. Nitrogen dioxide is a component in the production of photochemical smog. This reddish-brown gas has the chemical formula NO₂.

Nitrogen Fixation - A biological or chemical process where gaseous nitrogen is converted into solid forms of nitrogen. Biological fixation of nitrogen is done by specialized organisms, such as microorganisms like bacteria, actinomycetes, and cyanobacteria. Chemical fixation occurs at high temperatures. One natural process that can produce enough heat to fix atmospheric nitrogen is lightning.

Nitrogen Oxides - Consists of two gases, nitric oxide (NO) and nitrogen dioxide (NO₂). These gases are produced by bacterial action in the soil and by high-temperature combustion. Both gases are component parts in the production of photochemical smog.

Nitrogen Saturation - An overabundance of nitrogen in the forms of ammonium and nitrate in a natural ecosystem is usually caused by human-induced inputs related to agriculture and fossil fuel combustion. Nitrogen saturation can cause the artificial fertilization of plants, acidification of water bodies, lower soil pH, changes in species composition in ecosystems, increased soil-solution aluminum concentrations, altered ecosystem nutrient cycling, and decreased soil fertility.

Nitrous Oxide - A gas found in Earth's atmosphere that contributes to the greenhouse effect. Sources for nitrous oxide include land-use conversion, fossil fuel combustion, biomass burning, and soil fertilization. The chemical formula for nitrous oxide is N2O.

Nivation - A process where snow patches initiate erosion through physical weathering, meltwater flow, and gelifluction.

Nivation Hollow - A type of ground depression found in periglacial areas that is created by nivation.

Noctilucent Cloud - High altitude cloud composed of ice crystals that appear to glow silver or bright blue shortly after sunset. These clouds are commonly seen during the summer season at latitudes between 50 and 70° and are about 80 kilometers (50 miles) above the Earth's surface.

Node - (1) A point on a line or in two or three-dimensional space. (2) A point where lines or pathways cross or bifurcate.

Nodule - In geology, the term refers to an aggregation of some type of mineral inside a large mass of sedimentary rock.

Noise - (1) Term used in statistics to describe a significant amount of observational measurement variation in data. (2) Related to instrumentation and measurement, noise refers to unwanted data fluctuations that obscure the desired measurement signal.

Nominal Scale - A method for measuring data where the potential values are qualitative. Units of measurement in this scale do not overlap, and the differences between units are not mathematically measurable.

Non-Clastic Sedimentary Rock - Sedimentary rocks that are created either from chemical precipitation and crystallization, or by the lithification of once living organic matter.

Non-Ionizing Radiation - A form of electromagnetic radiation that does not have enough energy to cause ionization of atoms in living tissue. Examples of this type of radiation include radio waves, microwaves, infrared radiation, and ordinary light. Also see ionizing radiation.

Non-Linear System - A system where the operating cause and effect processes do not have simple linear outcomes, such that output is not directly proportional to input. 

Non-Parametric Statistical Test - A statistical test that does not require the sample data to be normally distributed.

Non-Relic Slope - A slope that was created by processes in the past that are identical to the ones acting on it currently.

Non-Renewable Resource - A resource that is finite in quantity and is being used faster than its ability to regenerate itself. This resource can be depleted until it no longer exists in the environment.

Non-Selective Scattering - One of three types of atmospheric scattering. In this type, all wavelengths in the visible and near infrared spectrum are scattered, producing a white response. The particles responsible for non-selective scattering are larger than the wavelengths of radiation they influence. Water droplets and dust particles with diameters between 5 and 100 micrometers are very effective at this type of scattering. This type of scattering causes fog and clouds to appear white when they interact with sunlight. Also see mie scattering and rayleigh scattering.

Non-Sequence - An interruption in the sequence of sedimentary strata layers where a period of no deposition of material occurred.

Non-Symbiotic Mutualism - Mutualistic biotic interaction where the mutualists live independent lives, yet cannot survive without each other. For example, pollinating insects like bees and some flowering plants. Compare with a symbiotic mutualism.

Nonconformity - A type of sequence in strata layers where underlying older rocks are of metamorphic or igneous origin rather than sedimentary.

Normal - A term used in climatology and meteorology for a period of usually 30 years from which mean, standard deviation, and extreme values for a climate variable are calculated. Sometimes referred to as a climate normal.

Normal Distribution - A common probability distribution displayed by a representative data sample or the whole population of some quantitatively measurable variable. If the values of this distribution are plotted on a graph's horizontal axis and their frequency on the vertical axis, the pattern displayed is symmetric and bell-shaped. The central value in this type of frequency distribution is usually the mean (arithmetic average of all the values measured for the variable), and this value represents the central peak of the distribution and the most frequently occurring value. Also called the normal curve or the bell-shaped curve. Also see standard deviation, kurtosis, and skewness.

Normal Fault - A vertical fault where one slab of the rock is displaced up and the other slab down. This type of fault is created by tectonic tensional forces acting in opposite directions.

Normal Lapse Rate - Average rate of air temperature change with altitude in the troposphere. This value is approximately a decrease of 6.5°C per 1,000 meters (3.6°F per 1,000 feet) rise in elevation. Also called an environmental lapse rate.

Normal Stress - A type of stress caused by forces applied to an object’s surface at 90°, possibly producing strain and deformation. Compare with shear stress.

North Atlantic Drift - A strong, warm western boundary current in the Atlantic Ocean. The North Atlantic Drift is an extension of the Gulf Stream. Originates southeast of the Grand Banks of Newfoundland (about 40° North latitude and 51° East longitude), transporting warm tropical ocean water to just west of Ireland. Also called the North Atlantic Current.

North Magnetic Pole - A location in the Northern Hemisphere where the lines of force from Earth's magnetic field are at right angles to the Earth's surface. This point on the Earth gradually changes its position with time.

North Pole - Surface location defined by the intersection of the polar axis with Earth's surface in the Northern Hemisphere. This location has a latitude of 90° North.

North Star - Is a bright star that is aligned approximately with the Earth's axis of rotation. This fixed point in the night sky was used to take celestial navigation measurements. Also called Polaris.

Northeast Trade Winds - Northern hemisphere surface winds found in the tropics that blow from about 30° North latitude (Subtropical High Pressure Zone) to the equator (Intertropical Convergence Zone). These winds blow from northeast to southwest. Also see Southeast Trade Winds.

Norther - A term in meteorology referring to the occurrence of a cold air mass originating from higher latitudes. This climate event is characterized by cold winds, a sudden drop in air temperature, and precipitation (often snow). There are different types of northers, with specific characteristics depending on geographical location.

Northing - Second measurement of a grid reference used to specify the location of a point on a rectangular coordinate system. The distance measured northward from the origin of a rectangular coordinate system. Also see easting.

Notch - A common coastal landform created when wave action and weathering cut into the base of a cliff, creating an overhanging mass of rock.

Nubbin - Small-scale mounds of soil or sediment created by the heaving action of subsurface ice crystal growth. 

Nuclear Energy - Energy released when the nucleus of an atom experiences a nuclear reaction like the spontaneous emission of radioactivity, nuclear fission, or nuclear fusion.

Nuclear Fission - A process where the mass of an atomic nucleus is made smaller by removing subatomic particles. This process releases atomic energy in the form of heat and electromagnetic radiation.

Nuclear Fusion - A process where the mass of an atomic nucleus is made larger by the addition of subatomic particles. This process releases atomic energy in the form of heat and electromagnetic radiation.

Nuclear Waste - See radioactive waste.

Nuclear Winter - A theoretical model that suggested that the detonation of numerous atomic bombs could create a stratospheric smoke and soot cloud. In turn, this cloud would significantly reduce the amount of sunlight reaching the Earth's surface and the heat generated at the Earth's surface and in the lower atmosphere. As a result, the Earth's climate will quickly turn colder for many months or even years. A similar climate change could occur due to the impact of an asteroid or comet on Earth, or the eruption of an enormous volcano. Also called atomic winter.

Nucleic Acid - Is an organic compound composed primarily of different combinations of carbon, hydrogen, nitrogen, oxygen, and phosphorus. They are very complex compounds formed by the atomic bonding of thousands of individual atoms. DNA or deoxyribonucleic acid, the genetic blueprint of life, is an example of a nucleic acid.

Nucleus - (1) The dense central part of an atom that is composed of neutrons and protons. (2) A structure found in eukaryotic cells that contains the chromosomes.

Nuée Ardente - A release of a glowing cloud of dense hot volcanic ash and gas by a volcano that moves downslope at high speeds, incinerating the landscape.

Null Hypothesis (H0) - This is a hypothesis that has been suggested by a researcher because it is believed to be true or because it is being used as a starting point for developing a logical scientific argument. Used in statistical testing to help organize the testing of reasoned arguments. Compare with an alternative hypothesis.

Nunatak - An isolated piece of bare ground (usually a mountain) surrounded by snow and ice of a glacier or ice sheet.

Numerical Modeling - These are mathematical models that simulate real-world systems using a time-stepping procedure to generate the required output. Weather forecast models are examples of numerical models.

Numerical Weather Forecasting - A meteorological forecasting technique that uses mathematical models of the atmosphere and oceans and high-speed computers to make future weather predictions based on current conditions.

Nutation - The periodic minor shift in the orientation of the axis of rotation of a largely axially symmetric object. Related to the Earth’s axis, nutation is caused by the opposing gravitational attraction of the Sun and the Moon and has a cycle of about 19 years.

Nutrient - Any food, chemical element, or compound an organism requires to live, grow, or reproduce.

Nutrient Cycle - A chemical model showing the cycling of a single element by various abiotic and biotic processes through the various stores found in the biosphere, lithosphere, hydrosphere, and atmosphere.

Nutrient Cycling - Is the movement of nutrient elements and/or compounds within and between various biotic or abiotic sinks found in the biosphere, lithosphere, hydrosphere, and atmosphere.

Nutrient Status - A term used to describe the potential availability of nutrients in a soil, river, lake, ecosystem, etc.