Atom - The smallest part of an element that has all the properties of the element.

Celsius (C) scale: A temperature scale where water freezes at 0º C and boils at 100º C.

Cloud - A visible collection of very fine water droplets or ice crystals suspended in the atmosphere at altitudes from just above the ground to several miles above sea level.

Condensation - The process by which a gas or vapor changes to a liquid.

Conduction - The transfer of heat within a substance or from one substance to another as a result of molecular motion. Always from warm to cold areas.

Convection - transfer of energy and mass by motions in a liquid or gas. In the atmosphere, convection usually refers to the vertical interchange of air masses. An example of convection is the rising of warm surface air and the sinking of cold air from upper levels of the atmosphere.

Density - The mass per unit volume of an object.

Deposition - A phase change of water directly from water vapor to ice without condensing (see condensation).

Element - The purest type or form of a substance

Evaporation - The process by which a liquid changes to a gas or vapor.

Fog - Condensed water vapor or a cloud floating close to the ground and likely to obscure 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 a cloud but not reaching the ground.

Kelvin (K) Scale - A temperature scale where 0º K represents absolute zero, the freezing point of water is 273º K, and the boiling point of water is 373º K.

Kinetic Energy - The energy of motion; the energy an object has as a result of its motion.

Liter - Basic unit of volume in the metric system.

Mass - The amount of matter in an object. It can be expressed in math as the total weight of the atoms or molecules in the object.

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.

Molecule - a substance composed of two or more atoms held together by chemical bonds.

Phase Change - The process by which a substance transforms from solid to liquid or liquid to gas, as well as transformations in the reverse direction.

Radiation - The emission or release of energy in the form of waves or rays.

Stratosphere - The layer of the atmosphere just above the troposphere. It ranges from about 6 to 31 miles (10 km to 50 km) above the Earth’s surface. This layer generally experiences a increase in temperature with height.

Sublimation - A phase change of water directly from ice to water vapor without melting.

Temperature - A degree of hotness or coldness the 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 highest layer in the atmosphere, where atoms absorb solar energy resulting in warming of the air to as high as 3,600 F.

Troposphere - The layer of the atmosphere from the ground to around 33,000 feet (10 kilometers). This is where much of the weather we experience occurs.

Volume - The amount of space occupied by a substance or object.

Vortex - Whirling motion or mass.

Wind shear - Any sudden change in wind speed or direction.