The Principles of Convection and Advection

It is so important that students of Astronomy and Earth Science understand the principles of convection and advection that a special page has been devoted to ths topic. Convection is a vertical phenomenon that occurs in a beaker of boiling water, the atmospheres of the Earth and some other Solar System objects, the interior of the Earth and many other Solar System objects, the interior of the Sun and other stars. Advection is an horizontal phenomenon that occurs in the atmospheres and oceans of the Earth and some other Solar System objects. Detailed explanations of Convection and Advection can be found by clicking on the words. A more simplified view appears on this page.

Convection

Convection in the atmosphere

Convection in the ocean

Convection in the Earth

Convection in the Sun

Sometimes a single physical process in nature can explain a variety of events. Convection is one such process, and it is a vertical process. It functions because heated fluids, due to their lower density, rise and cooled fluids fall. A heated fluid will rise to the top of a column, radiate heat away and then fall to be re-heated, rise and so on. Gasses, like our atmosphere, are fluids, too. A packet of fluid can become trapped in this cycle. When it does, it becomes part of a convection cell.

Convection cells can form at all scales. They can be millimeters across or larger than Earth. They all work the same way. The convection that students are most likely to have observed is in cumulonimbus clouds or "thunderheads." These towering vertical clouds can be seen to evolve over a few minutes. The tops of the clouds have a sort of cauliflower appearance as warm moist air rises through the center of the cloud. The moisture in the cloud condenses as it cools. The air gives up some of its heat to the cold high altitude air and begins to fall.

As the air falls along the exterior of the cloud, it returns to warmer low altitudes where it can be caught up in the rising column of air in the center of the cloud. This fountain-like cell can form alongside other cells, and a packet can move between cells. Hail forms when water droplets, carried by the strong updrafts, freeze, fall through the cloud and are caught in the updraft again. An additional layer of water freezes around the ice ball each time it makes a trip up through the cloud. Eventually, the hail becomes too heavy to be carried up anymore, so it falls to the ground. Large hailstones, when cut apart, show multiple layers, indicating the number of vertical trips the stone made while it was caught in the convection cell.

Convection also occurs on the Sun. A high resolution white light image of the Sun (seen below) shows a pattern that looks something like rice grains. Very large convection cells cause this granulation. The bright center of each cell is the top of a rising column of hot gas. The dark edges of each grain are the cooled gas beginning its descent to be re-heated. These granules are the size of Earth and larger. They constantly evolve and change.

Thunderheads and granulation are large-scale examples of convection. Fortunately, there are examples of convection that fit into a classroom. An excellent example can be seen in a beaker of hot water with peas. The interior of the beaker is hot. The surface of the water is exposed to cool air. Hot packets of fluid rise out of the interior of the beaker of water to the surface where they give off heat. Now cooled, these packets fall down into the beaker to be re-heated. Left alone, the water will dissipate its heat in this way (and through conduction with the sides of the bowl) and reach room temperature.

The peas highlight the convection cells vividly. As students gaze into their beaker, they will see the rising and descending columns of fluid in the vertical movement of the peas. The cells will evolve and change their positions. If the beaker water is stirred up, students can observe the cells reform. For the teacher, the Law of Watched Pots is important. If the teacher or the student were to do this demonstration at home with a pot of water on the stove, nothing would ever happen. They would watch and watch. The water would never boil, and the peas would never move. This is the Law of "Watched Pots Never Boil." The only way around this is to look away from the pot, and then look back after a few minutes. Typically what happens is the water is now boiling vigorously and spashing all over the stove. You have seen nothing happen. Another method to defeat this law is to used a beaker and a Bunsen Burner ... as seen in the diagram to your left. Since it is a beaker of water and not a pot, the law does not apply and you can witness firsthand the activity described above.

Convection acts as described in the examples above where gravity's effects are present (so that warm, low density fluids can rise and cool, high density fluids can fall). The rising of hot, less dense matter and the sinking of cool and more dense matter creates convection cells that operate in the 4 places imaged at the top of this page. The results are changes in weather, movement of ocean currents, volcanism and earthquakes, and outflow of solar energy. All from the process of convection.

Convection Activity from SOHO

Advection

Sometimes a single physical process in nature can explain a variety of events (read this before?). Advection is one such process, and it is a horizontal process. It functions because heated fluids and gases, due to their lower density, rise and cooled fluids and gases fall. On the level of the Earth's atmosphere, a heated mass of air rises from equatorial latituides and moves northward while a cooler mass of air sinks down from polar latitudes. Two forms of advection are important to understand. The principles are explained in detail in the Weather Unit: Advection of this course, so here is a shortened version.

Cold Advection

Cold advection is the process in which the wind blows from a region of cold air to a region of warmer air. Winds blows from a region of cold air to a region of warmer air, which results in cooling of the warmer region. As the cold advection persists, temperatures in the warmer region will begin to decrease as the colder air moves into the region of warmer air. The winds are the result of the cooler air mass moving into the region of warmer air.

The net result of cold advection is to make a region cooler. Cold advection can lead to sinking motion. Cold advection is occurring in Figure A while Figure B shows a vertical cross section through the region of cold advection. In Figure B, the horizontal lines are isobars and the arrows represent wind vectors. It is important to note that Figure A is along the ground and that Figure B is from the ground up to a higher level in the atmosphere, directly above the region of cold advection.

With the onset of cold advection (Figure A), the isobar in Figure B starts to bend downward since colder air is more dense and occupies less room than warmer air. The bending of the isobar due to cold advection creates a localized area of low pressure ("L" in Figure B"), thus altering the pressure gradient force. As air moves from the regions of high pressure ("H" in Figure B) to the local region of lower pressure, air is pushed downward from above, which is the sinking motion that is caused by cold advection.

Warm Advection

Warm advection is the process in which the wind blows from a region of warm air to a region of cooler air. The winds are the result of the warm air mass moving into a region of cooler air. The following series of three images depicts a very simple example of warm advection. Winds are blowing from a region of warm air to a region of colder air, which results in a warming of the colder region. As the warm advection persists, temperatures in the colder region will begin to increase as the warmer air moves into the region of colder air.

The net result of warm advection is to make a region warmer. The sequence below shows (in a very general sense) how warm advection can produce upward motion. Warm advection is occurring in Figure A while Figure B shows a vertical cross section through the region of warm advection. It is important to realize that Figure A is along the ground and that Figure B is from the ground up to a higher level in the atmosphere, directly over the region of warm advection.

Advection of air masses result in global circulation patterns which cause weather to change at the interface of the different air masses. To learn more about advection, since it applies more importantly to the atmosphere, please look at the Weather page, unless that is where you have come from already :)

The combination of convection and advection on the Earth is a quick lesson in pollution that you are asked to look through next. Move ahead to Pollutants and Transfer.

If you would rather just go back to the Earth-Science Introduction or the Earth Introduction, you may do so now.


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