Atmospheric Pressure - Also called barometric pressure or air pressure, this is the force exerted on a surface by the weight of the atmosphere. Informally, this is known as the "weight of the air."
Atom - The basic unit of matter, consisting of subatomic particles called protons, neutrons and electrons. The smallest unit of an element is one atom.
Celsius (C) scale - A temperature scale, also know as the centigrade scale. Water freezes at 0º C and boils at 100º C.
Cloud - A visible collection of very small water droplets or ice crystals suspended in the atmosphere at altitudes from near the ground to several miles above it.
Condensation - The process by which a gas changes to a liquid, for example water vapor changing into water droplets.
Condensation nuclei - Tiny particles in air, such as dust, dirt, and pollutants, that provide surfaces upon which water vapor molecules can condense into water droplets.
Conduction - The transfer of heat within a substance or from one substance to another due to a difference in temperature. Heat is always transferred from warmer to colder substances.
Convection - The transfer of heat due to molecular movement within a fluid where warmer, less dense parts of the fluid rise and cooler, more dense parts sink. An example of convection is the rising of warm surface air and the sinking of cold air from upper levels of the troposphere.
Density - The mass per unit volume of an object.
Deposition - A phase change in which a gas changes directly into a solid, without condensing into a liquid (see condensation), such as when water vapor forms frost on the ground. The opposite of deposition is sublimation, where a solid changes phase directly into a gas.
Element - A substance that cannot be broken down into simpler substances. Each element is distinguished by its atomic number, which defines the number of protons an atom of that element contains.
Evaporation - The process by which a liquid changes to a gas or vapor.
Fog - A thick cloud of water droplets at or near the Earth's surface which restricts visibility.
Force - Push or pull that gives energy to an object, sometimes causing a change in the motion of the object.
Funnel cloud - A rotating column of air extending from below a cloud but not reaching the ground.
Kinetic energy - The energy of motion; the energy an object has as a result of its motion.
Mesosphere - The layer of the atmosphere directly above the stratosphere ranging from 31 to 50 miles (50 to 80 km) above the Earth's surface. The mesosphere is the coldest and most mysterious layer of the atmosphere.
Molecule - A substance composed of two or more atoms held together by chemical bonds.
Phase change - The process by which a substance transforms from one phase (solid, liquid or gas) to another phase.
Precipitation - The formation of a liquid or solid from a solution. In the water cycle, this refers to rain, snow, sleet, or hail that falls towards the ground from a cloud.
Radiation - The emission or release of energy in the form of waves or rays.
Stratosphere - The layer of the atmosphere that extends from the top of the troposphere, 6.2 miles (10 km), to about 31 miles (50 km) above the Earth's surface. The stratosphere contains the ozone layer.
Sublimation - A phase change a solid directly into a gas, without passing throug the liquid phase, for example ice changing into water vapor without melting.
Temperature - A degree of hotness or coldness that can be measured using a thermometer. Also a measure of how fast the atoms and molecules of a substance are moving (see kinetic energy). Temperature is measured in degrees on the Fahrenheit, Celsius, and Kelvin scales.
Thermosphere - The thermosphere is above the mesosphere in the atmosphere, and is the hottest layer, where molecules absorb solar energy resulting in temperatures as high as 3,600 F.
Tropopause - A thin boundary layer between the troposphere and the stratosphere, defined by an area of uniform temperature between the two layers.
Troposphere - The layer of the atmosphere from the ground to around 33,000 feet (10 kilometers), on average. The troposphere contains most of the water in the atmosphere, and thus it's where most clouds and storms occur.
Volume - The amount of space occupied by a substance or object.
Vortex - A mass of whirling fluid or air. In weather, a vortex refers to the counter-clockwise flow of air within a funnel cloud, and also the force that keeps colder air near the Earth's poles.
Water cycle - The cycling of water between the Earth's oceans, atmosphere, land and living things. The water cycle involves evaporation, condensation, precipitation, and transpiration.
Wind shear - Any sudden change in wind speed or direction, a necessary factor in the formation of a funnel cloud or tornado.