Unit 2 Water and Weather - Spokane Public Schools

5.1 The AtmosphereEarth’s atmosphere is made of a mixture of gases called air. Because we can’t seeair molecules, sometimes we forget they’re there. However, air molecules createatmospheric pressure. This pressure affects the weather. Understanding theatmosphere helps us to understand the weather. This section is an introductionto Earth’s atmosphere.What’s in Earth’s atmosphere?NitrogenThe nitrogencycle and livingthingsOxygenTrace gasesYou may be surprised to learn that the most abundant gas inEarth’s atmosphere is nitrogen. Nitrogen gas makes up about78 percent of the atmosphere (Figure 5.1). Nitrogen is releasedinto the air by volcanoes and decaying organisms.Nitrogen is an important component of protein. Protein is anessential substance in the body tissues of all living things. Thenitrogen used to make protein in living things can’t be absorbeddirectly from the air. Instead, nitrogen is changed into nitrogencontainingmolecules by bacteria in the soil. Plants absorb thesemolecules from the soil and use them to make proteins. Animalsand people eat plants to obtain these proteins. The bacteria in thesoil eventually return nitrogen to the atmosphere (Figure 5.2).The second most abundant gas is oxygen, which makes up21 percent of Earth’s atmosphere. When we take a breath of air,the most important gas that we breathe in is oxygen. Humans andother living things need oxygen to survive.The remaining 1 percent of Earth’s atmosphere is made up of0.93 percent argon and 0.04 percent carbon dioxide. There arealso tiny amounts of neon, helium, methane, krypton, andhydrogen, which we call trace gases.air - the mixture of gases thatmake up Earth’s atmosphere.Figure 5.1: Gases in Earth’satmosphereFigure 5.2: The nitrogen cycle.98UNIT 2 WATER AND WEATHER