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

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

Coriolois Effect

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.

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.

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

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

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

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

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

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

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

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

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

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

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.

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

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

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