A fossil fuel composed of the hydrocarbon methane.
natural levee
Coarse-grained deposits of sediments that build up along a stream’s banks as it floods.
natural selection
The mechanism for evolution. Natural processes favor some traits over others in a population causing those traits to be more common in subsequent generations. This results in change to a new species or subspecies.
neap tide
The smallest tidal range in a lunar month occurring at the first- and third-quarter moons when the Sun and Moon are at 90os relative to each other, relative to Earth.
nebula
An interstellar cloud of gas and dust.
nebular hypothesis
The hypothesis that our solar system formed from a spinning cloud of gas and dust, or a nebula.
neritic zone
The part of the ocean where the continental shelf gradually slopes seaward. Sunlight can penetrate to the bottom in much of the neritic zone.
net energy
The amount of usable energy available from an energy resource.
net-energy ratio
The ratio between the useful energy present in a type of fuel, and the energy used to extract and process the fuel.
neutron
A neutral particle in the nucleus of an atom.
neutron star
The remnant of a massive star after it explodes as a supernova.
niche
An organism’s “job” within its community.
non-consumptive water use
Water use that does not use up the water supply.
non-renewable resources
Resources that are being used faster than they can be replaced or their availability is limited to what is currently on Earth; e.g. fossil fuels.
nor’easter
Mid-latitude cyclones that strike the northeastern United States.
normal fault
A dip-slip fault in which the hanging wall drops down relative to the footwall.
nuclear energy
Energy that is released from the nucleus of an atom when it is changed into another atom.
nuclear fusion
The merging together of the nuclei of atoms to form new, heavier chemical elements; huge amounts of nuclear energy are released in the process.
nuclear fusion reaction
When nuclei of two atoms fuse together, giving off tremendous amounts of energy.
nucleic acid
Biological molecules necessary for life; includes DNA and RNA
nucleus
The center of an atom, made of protons and neutrons.