1. Objectives 1) Describe the weather conditions associated with different types of fronts. 2) Describe the “life cycle” of a mid-latitude low.

2. Introduction Changes in weather result mostly from the movement of low- pressure systems and their associated frontal systems

3. What is a “Front”? Front: the boundary that separates opposing air masses Most common in mid- latitudes Southward-moving polar air masses and northward-moving tropical air masses often meet

4. What is a “Front”? Cont’d Air masses on either side of a front may differ in temperature and/or humidity Less-dense air mass is forced to rise over the denser air mass Rises high into the troposphere Clouds and precipitation form if there is humidity Usually bring precipitation

5. Kinds of Fronts The weather associated with a particular front depends on the types of air masses involved and the speed at which the front is moving

6. Kinds of Fronts cont’d Sometimes happen between air masses that have the same temperature but different humidity Fronts are usually defined by temperature of the advancing air mass

7. Kinds of Fronts cont’d 4 basic kinds of fronts Cold Warm Occluded Stationary

8. Cold Fronts Cold front: the boundary between an advancing cold air mass and the warmer air mass it is displacing Cold air slides underneath the warm air and forces it upward

9. Cold Fronts cont’d Friction causes the lower part of the cold air mass to lag behind the upper part Have steep slopes

10. Cold Fronts cont’d Weather depends to a large degree on the type of air mass it is displacing Summer: cold cP air + warm, humid mT air = thunderstorms Summer: cold cP air + hot, dry cT air = little to no precipitation

11. Cold Fronts cont’d Winter A cold front may be marked by rain or snow showers The precipitations associated with the front covers only a narrow band of ground b/c of the steep slope Moves quickly—precipitation usually ends shortly after the front passes

12. Warm Fronts Warm front: the boundary between air masses when warm air displaces cold air Slope is more gradual Weather changes associated with the warm front are less dramatic

13. Warm Fronts Cont’d First signs of the approach of a warm front 1—High cirrus clouds 2—Cirrostratus and lower thicker stratiform clouds Thunderstorms occasionally form Not typical Weather warms after front passes

14. Occluded Fronts Cold fronts typically move about twice as fast as warm fronts Occluded front: when a cold front “catches up” to a warm front Warm air that is caught between the 2 colder air masses is forced to rise Air cools causing cloudiness and precipitation

15. Stationary Front Stationary front: a front not moving forward Flooding can occur if a front is stationary too long

16. Life Cycle of a Mid-Latitude Low Fronts are usually connected to mid- latitude low-pressure systems A low-pressure system often begins when cold polar air meets warm tropical air Can grow into an intense storm system

17. Life Cycle of a Mid-Latitude Low Cont’d Usually takes 12-24 hours for a low-pressure system to pass through the 1 st 2 stages 1) Surface air pressure at the polar front drops and a wave forms on the front 2) In the Northern Hemisphere, winds blow counterclockwise around the low 3) After reaching the occluded stage, the low can last for 3+ days a) Still have high winds and rains

18. Upper-Air Flow Upper-air flow is what controls the surface low’s path and intensity Air is constantly spiraling into the low- pressure system

19. Weather Associated with Pressure Systems Weather you experience depends on where the center of the low passes in relation to you If a low passes north of you, a warm front may move over followed by a cold front You may have steady snow or rain if a low passes south

20. Weather Associated with Pressure Systems Cont’d High-pressure areas are associated with fair weather Skies are clear Days may be hot Nights may be cold Winds blow outward in a clockwise direction Little or no wind in the center of the high

21. Weather Associated with Pressure Systems Cont’d Still air takes on the characteristic temperature and humidity of the area Where air masses usually form

22. 20.2 Exit Ticket