Section 11.2 – Properties of the Atmosphere

1. Section 11.2 – Properties of the Atmosphere 8th Grade Earth and Space Science Class Notes

2. Temperature • Temperature – measure of the average kinetic energy of the particles in a material • Higher the temperature = faster particle movement

3. Air Pressure • The Earth’s atmosphere exerts a downward force due to the weight of the atmosphere. • Air pressure – pressure exerted on a surface by the weight of the atmosphere • Units – N/m2 or mb (1 mb = 100 N/m2) • The higher you go in the atmosphere the less air pressure.

4. Density of Air • Density of air decreases with increasing altitude because the molecules are further apart. • Near sea level density is 1.2 kg/m3

5. Pressure-Temperature-Density Relationship • In the atmosphere, the temperature, pressure, and density of air are related.

6. Air Pressure and Temperature • Warmer air = higher pressure • Particles move faster and exert a greater force

7. Air Pressure and Density • At the same temperature, air with higher density exerts more pressure than air with lower density. • More gas particles means more collisions.

8. Temperature and Density • At the same pressure, warmer air is less dense than cooler air. • Heat causes particles to move faster and spread apart.

9. Temperature Inversion • Increase in temperature with height in an atmospheric layer

10. Cause and Effects of Temperature Inversion • Land is not radiating thermal energy to lower layers of atmosphere • Example – cold, clear winter night when air is calm • Leads to fog, low-level clouds, and trapping pollutants close to the Earth’s surface

11. Wind • Wind – movement of air; caused by cold, dense air rushing towards warm, less dense air

12. Wind and Pressure Difference • Air moves from regions of higher density to areas of lower density • Air pressure increases as density increases, so air moves from areas of high pressure to low pressure as well

13. Wind Speed and Altitude • Wind moves slower near the Earth’s surface due to friction • Wind speed is measured in: • Miles per hour (mph) • Kilometers per hour (km/h) • At sea – in knots (1 knot = 1.85 km/h)

14. Humidity • Humidity – amount of water vapor in the atmosphere at a given location • Two ways to express – relative humidity and dew point

15. Relative Humidity • Water molecules are constantly evaporating into the atmosphere and condensing to form clouds and precipitation. • If rate of evaporation > rate of condensation then the amount of water vapor increases • Relative humidity – amount of water vapor needed for a volume of air to reach saturation • Saturation – amount of water vapor in the air is at its max • Relative humidity is expressed as a percentage (100% when fully saturated)

16. Dew Point • Temperature to which air must be cooled at a constant pressure to reach saturation • If dew point is close to the air temperature then the relative humidity is high

17. Latent Heat • As water vapor in the air condenses thermal energy is released as latent heat.

18. Condensation Level • A process in which temperature changes without the addition or removal of thermal energy from a system is called an adiabatic process. • Adiabatic heating occurs when air is compressed, and adiabatic cooling occurs when air expands.

19. Condensation Level • At equilibrium, evaporation and condensation occur at equal rates, so the amount of water in the liquid form remains constant. • Condensation occurs at the lifted condensation level (LCL). Air above the LCL is saturated and thus cools more slowly than air below the LCL.