OVERVIEW Describe how the physical properties of matter relate to the science of meteorology and identify the events that take place when matter changes state.

Matter is around us in some form everywhere in our daily livesthe food we eat, the water we drink, and the air we breathe. The weather around us, such as hail, rain, invisible water vapor (humid-ity), etc., are all matter. Matter is present in three forms-solids, liquids, and gases. A good working knowledge of the physical properties of matter and how matter can change from one form to another can help you understand what is happen-ing in our atmosphere that produces the various meteorological occurrences we live with every day.

Learning Objective: Recognize how pressure, temperature, and density affect the atmosphere and describe how the gas laws are applied in meteorology.

Matter is anything that occupies space and has weight. Two basic particles make up the composi-tion of all matterthe atom and the molecule.

The molecule is the smallest particle into which matter can be divided without destroying its char-acteristic properties. In physics, the molecule is the unit of matter. Molecules are composed of one or more atoms. The atom is the smallest particle of an element of matter that can exist either alone or in combination with others of the same or of another element. The atom and atomic structure is constantly under study and has revealed a whole new array of subatomic particles. To date, a new definition for atom has not been developed. A compound is a substance (or matter) formed by combining two or more elements. Thus, ordi-nary table salt is a compound formed by combin-ing two elementssodium and chlorine. Elements and compounds may exist together without form-ing new compounds. Their atoms do not combine. This is known as a mixture. Air is a familiar mix-ture. Every sample of air contains several kinds of molecules which are chiefly molecules of the ele-ments oxygen, nitrogen, and argon, together with the compounds of water vapor and carbon diox-ide.

Ocean water, too, is another mixture, made up chiefly of water and salt molecules, with a smaller number of molecules of many other com-pounds as well as molecules of several elements.

Matter is found in all of the following three states:

1. Solid. Solids are substances that have a definite volume and shape and retain their original shape and volume after being moved from one container to another, such as a block of wood or a stone.

2. Liquid. A liquid has a definite volume, because it is almost impossible to put it into a smaller space. However, when a liquid is moved from one container to another, it retains its original volume, but takes on the shape of the container into which it is moved. For example, if a glass of water is poured into a larger bucket or pail, the volume remains unchanged. The liquid occupies a different space and shape in that it conforms to the walls of the container into which it is poured.

3. Gas. Gases have neither a definite shape nor a definite volume. Gases not only take on the shape of the container into which they are placed but expand and fill it, no matter what the volume of the container.

Since gases and liquids flow easily, they are both called fluids. Moreover, many of the laws of physics that apply to liquids apply equally well to gases.

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