1. Earth’s atmosphere is a layer of gases surrounding the planet, protecting and sustaining life. - It insulates us so that we don’t freeze at night. - Its ozone layer protects us from the sun’s ultraviolet rays. It also contains primarily carbon dioxide needed by plants and the oxygen we need to breathe.

2. What's in Earth's atmosphere? This wonderful protective layer exists around Earth because our planet has just the right balance of size and distance from the sun. Nitrogen gas makes up about 78 percent of Earth’s atmosphere. The second most abundant gas is oxygen, which makes up 21 percent of Earth’s atmosphere.

3. Nitrogen Cycle Nitrogen is important to protein which is found in the body tissues of all living things. Nitrogen is cycled through the soil and into plants and finally when living things die and decay.

4. Why is Earth different? The atmospheres of Venus, Earth, and Mars were formed in similar ways, so we might expect them to contain similar elements. Photosynthesis Photosynthesis changed Earth’s atmosphere.

5. Photosynthesis converts carbon dioxide to oxygen in a process that allows living things to use the sun’s energy.

6. Atmospheric pressure is a measurement of the force due to air molecules at a given altitude. The pressure of the atmosphere decreases as you rise above sea level.

8. At sea level, the weight of the column of air above a person is about 9,800 newtons (2,200 pounds)! This is equal to the weight of a small car. Why aren’t we crushed by this pressure?

9.  pascal (pa) - SI Unit  atmosphere (atm)  pounds per square inch (psi) - English Unit

10. A barometer is an instrument that measures atmospheric pressure.

11. Four Main Layers  Troposphere  Stratosphere  Mesosphere  Thermosphere

12.  Troposphere: where we live. Weather occurs here.  Stratosphere: contains the ozone layer (absorbs sun’s UV radiation)

13. 3.Mesosphere: where the temperature is coldest. Meteors or “shooting stars” burn up here. 4. Thermosphere, where the temperature is highest