Convection Currents

1. Convection Currents SNC2D0

2. Convection • Convection • A method of heat transfer in fluids (liquids and gases) • Liquid/gas particles move in a circular pattern as they go through a cycle of heating and cooling • Convection is the cause of all the wind and ocean currents on Earth • It is driven by the fact that substances undergo thermal expansion • Substances get larger when heated

3. Convection • Thermal expansion • As temperature increases, volume increases • This decreases the density (D = m/V) • Low density fluids float on top of higher density fluids • This is why helium balloons float into the air, He is less dense than air

4. Convection • Therefore, if a fluid is heated, it expands and rises up • This leaves a low pressure region behind, and the surrounding cool fluid rushes in • This cool fluid is then heated • The hot fluid rises to the top and is cooled down

5. Air Currents • Convection occurs in the atmosphere • The sun heats up the land/water • The land/water in turn heats the air above it • This sets up a convection current in the air • The equator gets heated up the most • Hot air rises from the equator, leaving low pressure • Cool air from the North/South rushes in • The hot air that has risen travels towards the poles • This sets up a circular convection cell

6. Air Currents

7. Coriolis Effect • Since the Earth is spinning, wind that would move in a Northern direction ends up moving East/West as well • This effect is called the Coriolis effect • If an object is spinning, and something rises off of that object, it has a curved motion • Coriolis Free Throw

8. Coriolis Effect

9. Prevailing Winds • The Coriolis effect and the convection currents combine to produce prevailing winds • Prevailing winds • Winds that always move in the same direction • Determine climate by moving air (and water) to different places • Trade winds • NE and SE from equator to 30º • Westerlies from 30º to 60º • Polar Easterlies 60º to the poles

10. Prevailing Wind - Climate

11. Local Winds • Wind is also created locally • Often by the uneven heating of air due to land and water • Land heats/cools quicker than water • During the day the land is heated quickly, and heats up the air around it • This air rises and pulls the cool air from above the water towards the land • At night, the land is cool but the water is still warm • The water heats the air, which starts a convection current • Pulls air from the land towards the water

12. Sea/Land Breezes

13. Ocean Currents • Convection currents occur in water in a similar way to how they occur in air • The flow of the water is not identical to the flow of air • Water is heated up near the equator • This water flows on top of the cooler water below it • The heated water flows towards the poles, where it cools • The cool water sinks and flows towards the equator • As water cools near the poles, it gets saltier • Since some of the water freezes, leaving the salt behind • This increases the density of the cold water

15. Thermohaline Circulation • Thermohaline circulation • The circulation of water on the Earth • Driven by heat (thermo) and salinity (haline) • Ocean currents have a huge effect on climate • Warm water heats the air above it • Warm air brings a lot of precipitation with it • Cold water cools the air above it, this also dries it out • The cold air leads to dry climates when it moves onto land • BC and UK vs. Africa and California

16. Homework • Pg 347 # 1 – 7