Convection and Heat Transport Handout

From AstroLab

Roban 14:59, 15 November 2006 (EST)

In previous labs you have learned about some of the factors that can influence the surface temperature of a planet: the luminosity of the star, the distance from the star, and the amount of infrared radiation trapped by the atmosphere (the Greenhouse Effect). These factors are all related to the transportation of heat by radiation. Using that information, you were able to determine an average or "effective" temperature for a planet.

The actual surface temperature of a body often varies widely from the average, as a function of both position and time. On the Moon, for instance, surface temperatures regularly vary from over 400 Kelvin on the day side to less than 100 Kelvin on the night side. In fact, adjoining regions of the surface can show temperature variations almost this large when part of the surface is in shadow and part is illuminated [1]. The most extreme recorded air temperatures on the Earth, on the other hand, only range from about 180K to 330K [2], and the variation from pole to equator (not to mention day to night) is rarely more than 50 degrees Kelvin or so.

What accounts for the difference? The short answer is the atmosphere and oceans. The oceans store and release huge amounts of heat, and the atmosphere traps and circulates heat around the planet. In this lab you will explore convection, the process responsible for distributing heat through the atmosphere and oceans and ultimately producing all of the Earth's weather and ocean currents.

Contents 1 Thinking about convection 1.1 Atmospheric convection 2 Visualizing convection 2.1 Materials 2.2 Procedure 2.3 Analysis 3 Introductory Presention 4 Relative densities of fresh and salt water 4.1 Materials 4.2 Instructions 4.3 Analysis 4.3.1 The Dead Sea 5 Thermohaline circulation presention 6 Convection inhibited by a density gradient 6.1 Materials 6.2 Procedure 6.2.1 Make brine 6.2.2 Create a floating layer of fresh water 6.2.3 Observe differences in convection 6.3 Analysis 6.3.1 The Ocean Conveyor 7 Related Articles

Thinking about convection

1. Is cold water or hot water more dense? How about cold or hot air?
2. Would a blob of a cold fluid rise or sink if it were surrounded by hot fluid? How about a blob of hot fluid surrounded by cold fluid?

Atmospheric convection

1. What parts of the Earth receives the most sunlight (per unit area)? What parts receive the least sunlight?
2. Sketch a cross section of the Earth and its atmosphere and indicate (with arrows) where you think air currents might rise and where they might sink.

Surface winds blow from areas of sinking air (high pressure) to areas of rising air (low pressure). High-altitude winds blow from areas of rising air to areas of sinking air. Explain why winds blow this way. Draw the surface winds on your diagram.

The Earth's weather is driven by this convection in the atmosphere.

Visualizing convection

Materials

1. water (warm works best, ~250mL per group)
2. clear beaker or cup with fairly vertical sides (for observing

convection)

1. food coloring and/or ground spice like paprika
2. ice cube

Procedure

Fill a cup most of the way with water. Let it stand for 2-3 minutes. Sprinkle a healthy amount of the ground spice in the water, then put an ice cube gently in the cup. Now put a single drop of food coloring directly on top of the ice cube. Observe the movement of the water as traced out by the spice flakes and food coloring.

Analysis

1. Draw and describe how the water moves. Where does it move fastest?
2. Explain your observations about the movement of the water.

Introductory Presention

Relative densities of fresh and salt water

Materials

1. fine table salt
2. water
3. clear beaker or cup with fairly vertical sides
4. food coloring
5. spoon (one per group)

Instructions

With your lab group, come up with a method of determining whether fresh or salt water is more dense. Feel free to refine or change your methods as you go along. Record your procedure and observations.

Analysis

1. Is salt water or fresh water more dense?
2. Would a blob of salty water rise or sink if it were surrounded by fresh water? How about a blob of fresh water surrounded by salt water?
3. Can you tell what would happen to a blob of warm salty water surrounded by cold fresh water?
4. Is a layer of fresh water on top of salt water stable? Is a layer of salt water on top of fresh water stable?

The Dead Sea

The Dead Sea used to be stable against convection because the deep layers of the lake are saturated with salt, while the surface layers were replenished with fresh water from the Jordan and other rivers. This kept the surface water from sinking to the bottom even if it was colder than the underlying layers. Why?

As people started to use more and more river water for irrigation, less fresh water was reaching the Dead Sea. In the winter of 1978-1979 the Dead Sea turned over, bringing salt-saturated water from the depths up to the surface. Why did this happen in the winter?

Thermohaline circulation presention

Convection inhibited by a density gradient

Materials

1. fine table salt (you'll need quite a lot)
2. water (warm works best, ~400mL per group)
3. clear beakers or cups with fairly vertical sides (for observing
4. convection, two per group)
5. beakers or flasks (for mixing and pouring, two per group)
6. food coloring
7. ice cubes (two per group)
8. spoon (one per group)

Procedure

Make brine

Put 4 heaping spoonfuls of salt in a beaker and add water up to the 300mL mark. Mix well.

Pour 200mL of the brine into one cup. Pour the other 100mL into the other cup.

Let the cups sit undisturbed until the brine has stopped swirling or circulating (2-3 minutes).

Create a floating layer of fresh water

Put 100mL of fresh water into a clean beaker. In the cup with the smaller amount of brine: hold a spoon horizontally and lower it into the cup until it rests on the surface of the brine. Put the tip of the spoon against the side of the cup. Now very slowly and steadily pour the fresh water into the spoon to minimize the mixing between the fresh and salt water. If you have done it right there should be a clearly-visible layer of fresh water on top of the brine. This step is crucial, so ask a TA if you're not sure if it worked.

Let the cups sit for 2-3 more minutes.

Observe differences in convection

Make sure you have two ice cubes handy, then put a single drop of food coloring in each cup. Do NOT stir. Do you observe any differences between what happens to the drop in each cup?

Now gently set an ice cube in each cup right on top of the spot of food coloring.

Observe the cups for several minutes (until most of the ice is melted) and record any differences between what happens in the two cups. Also note any changes that occur over time.

Once most of the ice is melted in both cups, give each cup one gentle stir with a spoon handle touching the bottom. Record any differences between what happens in each cup.

Analysis

1. Explain any differences you observed between the cups.
2. Explain any differences between what happened in this experiment and what happened in the "Visualizing Convection" activity.
3. Explain any changes that occured as the ice cubes melted.

The Ocean Conveyor

The Gulf Stream and the North Atlantic Current are important ocean currents that transports heat from the equator to the waters around Europe as part of the Ocean Conveyor. In the tropics intense sunlight warms the surface waters of the ocean and causes rapid evaporation that increases its salinity. The Gulf Stream carries this warm salty water along the East coast of North America, then the North Atlantic Current carries it North-Eastward into the waters off Europe. When the current reaches the North Atlantic it begins to cool off and then sinks and forms a river that returns south through the depths of the ocean.

Why would the water from the tropics sink when it reaches the same temperature as the surrounding water (or even before)?

If anything disrupted the Ocean Conveyor, preventing it from transporting equatorial heat to the North, average air temperatures could drop by as much as 5 degrees around the North Atlantic, and Europe, in particular, could have much harsher winters.

As the polar ice cap and arctic glaciers melt, they add fresh water to the surface of the North Atlantic. Can you think of a reason why the Ocean Conveyor might be slowed or disrupted if this water dilutes the water from the tropics? What about if the fresh water forms a cap over the North Atlantic?

Related Articles

"Abrupt Climate Change: Should We Be Worried?" by Robert B. Gagosian, President and Director of the Woods Hole Oceanographic Institution, discusses the possibility that Global Warming could shut down thermohaline circulation in the North Atlantic [3].

"The Source of Europe's Mild Climate" suggesting that shutting down thermohaline circulation might not be as big a disaster as has been suggested [4].