1. What are Depressions? Depressions are low pressure systems which occur in the mid-latitudes where cold and warm air meet

2. What are Depressions? Cold and warm air masses meet at a front. –Fronts are boundaries between two air masses –Name refers to the air behind the front (i.e. cold front = cold air behind) –The symbols indicate direction of movement. Cold and warm air masses meet at the polar front over the Atlantic Ocean, driven by jet streamjet stream Three stages: –Juvenile –Mature –Occluded

3. Warm wind Cold wind Source: T.Abbott www.sln.org.uk/geography An instability or ‘wave’ on the polar front acts as the initial trigger of the depression, spinning anti-clockwise 1 Juvenile Stage

4. Warm wind Cold wind Source: T.Abbott www.sln.org.uk/geography Warm air pushed over the denser cold air - warm front Cold air undercuts less dense warm air – cold front 2 Mature Stage

5. Warm wind Cold wind Source: T.Abbott www.sln.org.uk/geography Cold fronts move faster than warm fronts - cold front catches up and lifts the warm front away from ground. Warm sector squeezed out and occluded front forms 3 Occluded Stage

6. Cold wind Source: T.Abbott www.sln.org.uk/geography After occlusion, the depression system dissolves.

8. Fold your 3D depression map. Add the warm/cold front symbols to the fronts Using the information on p. 143 of the black book, and in the Geofactsheet, annotate your diagrams on the handout to explain the stages of a depression. Include: –Labels for cold/warm sectors (include density) –Direction of air flow (including rising/undercutting) Challenge: why does this type of air movement lead to low pressure (i.e. reduced air pressure at the surface)? The ‘Life-cycle’ of a Depression

9. What are the Characteristics of a Depression? During the video, annotate the blank rectangle (ground view) on your depression model. Add on: Types of cloud Types of weather Conditions on the ground Challenge: how can we identify which air mass is currently over our patch of ground?

10. What are the Characteristics of a Depression? Warm Sector Cold Front Warm Front Cumulonimbus Nimbostratus High Stratus Cirrus

11. Independent Learning Links: Homework –Complete ‘storms, blizzards, ice storms’ research (pp. 178-179) –Read through and highlight/take notes from Geofactsheet 162 and Geofiles 516 & 530 (http://moodle.bilborough.ac.uk/course/view.php?id=574, topic 2)http://moodle.bilborough.ac.uk/course/view.php?id=574 –Start your weather diary Independent Study –Read through Geo Factfile 146 (moodle). –Depressions: http://www.metlink.org/secondary/key-stage-4/air/http://www.metlink.org/secondary/key-stage-4/air/ –Synoptic charts: https://www.youtube.com/watch?v=wl_FFK_HbjYhttps://www.youtube.com/watch?v=wl_FFK_HbjY –Depressions (technical): https://www.youtube.com/watch?v=DWhM2vbVdFM https://www.youtube.com/watch?v=DWhM2vbVdFM –Jet Streams: https://www.youtube.com/watch?v=huweohIh_Bwhttps://www.youtube.com/watch?v=huweohIh_Bw