How do heat and water power convection?
So far we have considered energy that flows from the interior of the Earth to its surface. This energy is capable of doing some extraordinary things like causing earthquakes and volcanoes. However, the amount of heat generated from the Earth's interior is nothing compared to the energy that is radiated from the sun and reaches the Earth on a daily basis. Only a fraction of the energy radiated by the sun reaches the Earth and yet that is more than 5,300 times greater than the flow of energy from the interior of the Earth.11 Water has the unique ability to store heat and because it is so plentiful on the earth it is capable of transferring large amounts of heat around the planet. It is the heat stored in water that changes it into water vapor and begins the water or hydrologic cycle. When water evaporates it changes to a gas and absorbs heat. When the water vapor changes back to water through condensation it releases heat. As water freezes to ice, molecules slow down and heat is released. Likewise, when ice melts heat is absorbed and the molecules speed up and move apart from one another. This process of sublimation and deposition is another cycle that continues all over the Earth. All of this keeps the Earth from experiencing extreme ranges in temperature.
How does convection help to create our weather?
The hydrologic cycle is continuously running. As it's heated by the sun, water evaporates from bodies of water and also through transpiration plants add water into the atmosphere. The vapor attaches itself to small particles of dust in the atmosphere and gathers as clouds. As the clouds become heavier the vapor condenses and falls to Earth as rain or snow depending on the temperature of the surrounding air.
As previously stated the surface and near-surface waters of the oceans absorb a large quantity of solar energy. Through tides and winds some of this heat is also distributed into the deeper and dense regions of the ocean. Winds drive the oceans down to a depth of about 325 (100m).12 This helps to bring warmth to the polar regions of the Earth.
The prevailing winds and climate zones are also affected by convection. The earth receives most of the sun's rays along the equator because it hits that area most directly while it just skims the poles. The air around the equator is heated and rises and begins to float toward the poles while at the same time cool air is dragged down toward the equator. This circular motion sets up convection cells above the Earth (similar to those within the Earth's mantle) that distribute warm air around the globe. If not for the rotation of the Earth the rising air from the region of the equator would sink down near the poles. There would be one convection cell from the Equator going to the North Pole and one going from the Equator to the South Pole.
However while this air exchange goes on the Earth is also turning and this pushes the air sideways. The Coriolis Effect, as it is called, makes the air mass move to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The bottom of convection cells drive prevailing winds, and where cells meet leads to the jet streams. For example the Trade Winds are cooling air which is falling and returning to the warmer equator. Instead of reaching the poles the warm air was deflected so far east that it cooled before reaching the subtropic latitude. Thus the Earth's rotation causes the convection process to take the form of three convection cells in each hemisphere. As they fall the Trade Winds are deflected to the right in the Northern Hemisphere and thus push westward.
How does convection help develop thunderstorms, hurricanes, and tornadoes?
A heated rising air mass is another form of convection. When air rises it goes to lower pressure and expands, and the expansion causes its temperature to drop; when the temperature is low enough water vapor in it will condense into liquid water. Thunderstorms begin as the water vapor in the rising air mass condenses and releases more heat which makes the air rise higher, and thus draws in more moist air. Most thunderstorms form in late afternoons during the summer when the temperature of the ground and lower atmosphere are warmest. The early development of a thunderstorm requires that there is a continuous supply of rising warm moist air. When the water drops become too heavy they begin to fall and pull in some cooler air. What follows is a cycle of continuous updrafts and downdrafts which make for the more violent part of the storm. As the rain falls and downdrafts pull in more cool air the heat is cut off and the downdrafts dominate and updrafts dissipate along with the thunderstorm.
To show students the effect that temperature has on air movement have students cut a 2 inch diameter (6 cm) spiral from a piece of tissue paper. Cut a piece of thread and tape it to the center of the spiral. Turn on a desk lamp pointing it upward. Hold the thread and the spiral over the lamp about 4 inches (10cm) above the light. The result is that the tissue spiral will begin to turn. The heat from the light made the air above the light move faster and spread out, and the cooler air rushed in to take the place of the warmer air and so the spiral turns. This movement is called convection.
Hurricanes occur in the North Atlantic and North Pacific oceans. Hurricanes that strike the eastern part of the United States usually begin their development as storms in the western region of Africa or the Caribbean. Hurricanes follow a set of stages in from their formation to their dissipation. Initially a hurricane is a
tropical disturbance
of moist upwellings with a light circulation. The surface low pressure strengthens and a storm begins to develop. The surface air flows into the center which acts like a chimney, pumping, warm, moist air rapidly upward where it cools and condenses water thus releasing great amounts of heat. This re-warmed air strengthens the updrafts ands draws in more warm moist air from below. As the speed increases it is categorized as a
tropical depression
. If the wind speed reaches greater than 39 mi/hr but less than 74 mi/hr it becomes a
tropical storm
and receives a name. The tropical storm will only become a hurricane if the wind speed reaches greater than 74 mi/hr.13
The most feared type of storm in the world is the tornado. Most tornados occur as the result of large (supercell) thunderstorms. Warm moist upwellings of air develop into a thunderstorm. The rotation of the column of air appears to happen when winds at two different altitudes blow at two different speeds causing wind shear. This causes a horizontal rotation of the column of air. If this column of air gets caught in a supercell updraft the updraft tightens the spin (much like a skater spins faster when arms are pulled close to the body) and a funnel cloud is created.14
So we have seen that convection - the rising of a heated fluid and falling of a cooled fluid forms the basis of the dynamic nature of the planet. Hopefully, students will come to see that the planet they live on is in continual motion. Not only are things moving above the surface but a tremendous amount of movement is going on underneath their feet deep below us. It is amazing that such a seemingly simple thing as rising and falling has so much importance for life on our planet.