Section 12.2 – Weather Systems

1. Section 12.2 – Weather Systems 8th Grade Earth and Space Science Class Notes

2. Global Wind Systems • Cold, dense air sinks towards the surface and moves to the tropics where the cold air forces warm air to rise, cool and flow back to the poles (convection current). • Since the Earth spins, moving air curves to the right in the northern hemisphere and to the left in the southern hemisphere. • This curving is known as the Coriolis effect

3. Coriolois Effect

4. Polar Easterlies • Wind zones between 60 N latitude and the north pole and 60 S latitude and the south pole. • Cold, dense air is deflected in the eastern direction from each pole. • Ends at a polar front which causes an area of stormy weather.

5. Prevailing Westerlies • Located between 30 N and 60 N and 30 S and 60 S. • Surface winds move west toward each pole. • Steady winds that move much of the weather in the US and Canada.

6. Trade Winds • Between 30 N and 30 S • Air sinks, warms, and moves toward the equator in and then rises and moves back toward 30 N and 30 S

7. Trade Winds – Horse Latitudes • Near 30 N and 30 S, sinking air creates an area of high pressure • The Earth’s major deserts are under these high pressure areas

8. Trade Winds – Intertropical Convergence Zone (ITCZ) • Near the equator, air is forced upward and creates an area of low pressure • Drifts north and south as seasons change – directly over equator in Sept. and March • Has bands of cloudiness and thunderstorms and delivers moisture to the tropical rain forests

9. Jet Streams • Narrow band of fast wind – can reach up to 185 km/h • Position can vary with the seasons • Generally located in the region of strongest temperature differences on a line from the equator to a pole

10. Types of Jet Streams • Major jet streams – called the polar streams; separate polar easterlies from prevailing westerlies and move west to east • Minor jet streams – subtropical streams; occur where westerlies meet trade winds

11. Jet Streams and Weather Systems • Storms form along jet streams and create large-scale weather systems • Transport cold surface air to tropics and warm surface air to poles • Weather systems basically follow jet streams

12. Fronts • A narrow region between two air masses of different densities • Four types: • Cold • Warm • Stationary • Occluded

13. Cold Front • When cold, dense air forces warm, less dense air up a steep slope • Causes intense precipitation and thunderstorms • Blue line with triangles • Triangles point to direction of the front’s movement

14. Warm Front • Warm air displaces cold air along a gentle slope • Causes widespread, light precipitation • Indicated by red line with semicircles pointing in the direction of the front’s movement

15. Stationary Front • When air masses meet but neither advances • Usually occurs between two fronts with small temperature and pressure differences • Cause light winds and precipitation • Indicated by evenly spaced, alternating cold and warm front symbols

16. Occluded Front • Rapidly moving cold air mass overtakes a warm front and forces it upward then collides with another cold front • Causes strong winds and heavy precipitation • Indicated by line with alternating purple triangles and semicircles pointing in the direction of its movement.

17. Pressure Systems • Air flows from high pressure to low pressure • Sinking or rising air in combination with the Coriolis effect causes the formation of rotating high and low pressure systems • Air moves in a circular motion around a high or low pressure center

18. Low-Pressure Systems • Air rises and air from outside spirals in • In northern hemisphere, air moves in counterclockwise direction (opposite in southern hemisphere) • Causes cloudy weather and precipitation

19. High-Pressure System • Air sinks and spirals out • In northern hemisphere, air moves clockwise (opposite in southern hemisphere) • Brings fair weather