Does the weather where you live change often, or is it fairly constant from day to day? Weather is the condition of Earth’s atmosphere at a particular time and place. But what is the atmosphere? Earth’s atmosphere is the envelope of gases that surrounds the planet. To understand the relative size of the atmosphere, imagine that Earth is the size of an apple. If you breathe on the apple, a thin film of water droplets will form on its surface. Earth’s atmosphere is like that water on the apple, a thin layer of gases Earth’s surface.
Composition of the Atmosphere
The atmosphere is made up of a mixture of atoms and molecules of different kinds. An atom is the smallest unit of a chemical element that can exist by itself. Molecules are made up of two or more atoms. Earth’s atmosphere is made up of nitrogen, oxygen, carbon dioxide, water vapor, and many other gases, as well as particles of liquids and solids.
Nitrogen is the most abundant gas in the atmosphere. It makes up a little more than three fourths of the air we breathe. Each nitrogen molecule consists of two nitrogen atoms.
Even though oxygen is the second most abundant gas in the atmosphere, it makes up less than on fourth of the volume. Plants and animals take oxygen directly from the air and use it to release energy from their food. Oxygen is also involved in many other important processes. Any fuel you can think of, from the gasoline in a car to the candles on a birthday cake, uses oxygen as it burns. Without oxygen, a fire will go out. Burning uses oxygen rapidly. During other processes, oxygen is used slowly. For example, steel in cars and other objects reacts slowly with oxygen to form iron oxide, or rust.
Most Oxygen molecules have two oxygen atoms. Ozone is a form of oxygen that has three oxygen atoms in each molecule instead of the usual two. Have you ever noticed a pungent smell in the air after a thunderstorm? This is the odor of ozone, which forms when lightning interact with oxygen in the air.
Each molecule of carbon dioxide has one atom of carbon and two atoms of oxygen. Carbon dioxide is essential to life. Plants must have carbon dioxide to produce food. When the cells of plants and animals break down food to produce energy, they give off carbon dioxide as a waste product. When fuels such as coal and gasoline are burned, they release carbon dioxide in the atmosphere.
Oxygen and nitrogen together make up 99 percent of dry air. Argon and carbon dioxide make up most of the other one percent. The remaining gases are called trace gases because only small amounts of them are present.
In reality, air is not dry because it contains water vapor. Water vapor is water in the form of a gas. Water vapor is invisible. It is not the same thing as steam, which is made up of tiny droplets of liquid water. Each water molecule contains two atoms of hydrogen and one atom of oxygen. The amount of water vapor in the air varies greatly from place to place and from time to time. Water vapor plays an important role in Earth’s weather. Clouds form when water vapor condenses out of the air form tiny droplets of liquid water or crystals of ice. If these droplets or crystals become heavy enough, they can fall as rain or snow.
Pure air contains only gases. But pure air exists only in laboratories. In the real world, air also contains tiny solid and liquid particles of dust, smoke, salt, and other chemicals. You can see some of these particles in the air around you, but most of them are too small to see.
Importance of the Atmosphere
Earth’s atmosphere makes conditions on Earth suitable for living things. The atmosphere contains oxygen and other gases that you and other living things need to survive. In turn, living things affect the atmosphere. The atmosphere is constantly changing, with gases moving in and out of living things, the land, and the water. Living things need warmth and liquid water. By trapping energy from the sun, the atmosphere keeps most of Earth’s surface warm enough for water to exist as a liquid. In addition, Earth’s atmosphere protects living thing s from dangerous radiation from the sun. The atmosphere also prevents earth’s surface from being hit by most meteoroids, or rocks from outer space.