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Weather - Precipitation

Weather - Precipitation. Precipitation. RECALL… As air goes up, it cools down ! As air cools down, water (vapour) condenses more than it evaporates!. Precipitation. Air can rise for a number of reasons…. 1 – It tries to cross an area of high elevation ( orographic or relief ).

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Weather - Precipitation

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  1. Weather - Precipitation

  2. Precipitation • RECALL… • As air goes up, it cools down! • As air cools down, water (vapour) condenses more than it evaporates!

  3. Precipitation Air can rise for a number of reasons… 1 – It tries to cross an area of high elevation (orographic or relief). 2 – It rises because it has absorbed heat from the earth’s surface (convectional). 3 – It rises because a bulldozer mass of cold air is pushing it up (cyclonic).

  4. Orographic (Relief) Precipitation

  5. Orographic (Relief) Precipitation • Mountains create a barrier, and this barrier creates orographic precipitation. • As air rises, it cools. • The air mass itself expands, while the particles squish together and contract (not able to hold as much moisture). • Condensation occurs, and water droplets are released into the air. • These are still light enough to be carried by the air!

  6. Orographic (Relief) Precipitation • As more vapour condenses, water droplets become larger and heavier! • When they are too heavy to remain aloft, they fall as…

  7. Orographic (Relief) Precipitation • The moisture content of air is measured in • Relative Humidity. • This is just a quick and easy way of saying… • (Amount of moisture in the air / moisture capacity) X 100%.

  8. Orographic (Relief) Precipitation (Textbook p.135)

  9. Convectional Precipitation (Textbook p.136)

  10. Convectional Precipitation • Convectional precipitation occurs when the ground is subject to very intense heating. • The air above the ground heats too, and rises. • As the air rises, it coolsand the water vapour held within now condenses. • This condensed vapour forms clouds in a very high column (sometimes leads to hail). • Eventually, the moisture in these clouds gets heavy enough to fall…

  11. Convectional Precipitation • Air actually grabs the water drops and drags them downwards! • This results in cooler air descending (those nasty drafts and gusts of wind you feel just before a storm). • The rain eventually cools the ground and stops the cycle. • Bonus stuff from convectional precipitation… • Hail (formed when moisture in the cloud reaches high enough altitude); Thunderstorms; Tornadoes.

  12. Convectional Precipitation

  13. Cyclonic (Frontal) Precipitation

  14. Cyclonic (Frontal) Precipitation • Cyclonic precipitationoccurs when warm and coldfronts collide. • This is called a “cyclonic storm”. • It often occurs under the Polar Front jet stream. • The rotation of the earth causes the air to spin in a counter-clockwise direction (giving us our common image of a “spinning” cyclone).

  15. Cyclonic (Frontal) Precipitation

  16. Cyclonic (Frontal) Precipitation • Cyclonic precipitationhas two fronts: • Cold front (the leading edge of the cold air pushing into the warm air mass); • Warm front (the leading edge of the warm air mass on the other side). • Both of these fronts create precipitation. Fun fact: most of the precipitation we receive in Ontario is cyclonic.

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