Lesson 4 - Global Circulation
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To do:
- Check the schedule for this week's reading & upcoming assignments
- Read the lecture and assigned reading in the text
- Participate in discussions
- Take the Week 4 Quiz
By the end of this lesson you should be able to:
- Explain the effect of surface temperature on atmospheric pressure
- Evaluate the effect of latitude, altitude, cloud cover, and coastal or inland location on local surface temperature
- Describe the effect of the Coriolis force on high and low atmospheric pressure systems in the Northern and Southern hemisphere
- Relate the Coriolis force to the major (global) wind systems on the plane
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Lithosphere-Atmosphere-Hydrosphere Interactions
Last week we focused on the composition, temperature, function and effect of the atmosphere. We talked about the electromagnetic energy that comes from the sun and is transferred into heat energy when it is absorbed by objects. This week we are going to look at the interaction between the atmosphere, the lithosphere, and the hydrosphere and how sensible and latent heat transfers drive nearly all motion in the atmosphere and hydrosphere.
Turn to figure 6.8 in your text. This graphic illustrates the basic principles in air pressure and their relationship to surface heating. Hot air is at a lower pressure (less dense) than cold air (think about a hot air balloon where you must heat the air inside the balloon to make it rise). The earth is not heated equally, partially because of day and night, partially because of differing amounts of isolation received over the earth (recall seasons from the first lecture), and partially because land and water heat up differently. Therefore, air heated over warmer parts of the earth forms low pressure zones, and air cooled over colder parts of the earth forms high pressure zones. These zones of differing pressure lead to circulation of the atmosphere and ocean (hydrosphere).
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Global Temperature Patterns
If you purchased the Goodes World Atlas, turn to the January and July normal temperature maps. Compare the two and what do you notice immediately? First, in January the northern hemisphere is very cold, and the southern hemisphere is very warm. Next, the overall temperature in July is much warmer than that in January. Both the northern and southern hemisphere are colored yellow, indicating a temperature range of 10 to 21 degrees Celsius. Why is this? You have explored this relationship extensively in Lab 2, so you have probably figured out by now that land and water react differently to the incoming solar radiation (insolation) of the sun.
Most of the earth’s surface ranges between –34 degrees C and 32 degrees C. Temperature is controlled by four factors: latitude, altitude, cloud cover and land-water heating differences.
Let's consider the January Normal Temperature. What are the coldest areas, what are the warmest areas? What about the January Pressure? Where is the pressure the highest? There is a band of very high pressure over eastern Asia. This is known as the Siberian Highs and is the location of some of the consistently highest pressure readings on earth. Next, there is a band of low pressure around the equator: Does it correspond to a warm temperature pattern? The correlation in July is a little less dramatic than January. This is because the earth is overall much warmer in July.
Latitude
Recall from lecture 1 the unequal distribution of isolation over the earth. Areas that are located at higher latitudes (very far north or south) receive much less isolation than areas near to the equator. In fact, it is worth pointing out that the regions that we call the ‘tropics’ (between 23.5N and 23.5S latitude), or regions that are generally considered to be the warmest, are the regions that receive direct sunlight for part of the year. These regions are much warmer than the rest of the earth simply because they receive much more energy than other parts of the globe.
Altitude
The effect of altitude and latitude on temperature is nicely illustrated in figures. A common way of illustrating temperature ranges includes the x-axis (the months of the year) and the y-axis (the temperature). The horizontal lines represent the average temperature for each location, and the vertical bars represent the possible ranges of temperature. For example, in La Paz in June, the temperature can range between –1 and 18 degrees, but the average temperature is 5 degrees.
Recall that the lapse rate for the atmosphere is 6.4 degrees C/per 1000 meters. In addition, as the density of the atmosphere decreases with elevation, its ability to absorb radiant heat is reduced. The result is that at high elevations the temperature difference between day and night is much greater, as heat absorbed during the day is re-radiated at night (and the thin atmosphere does not absorb it well).
Cloud Cover
Low clouds reflect or absorb up to one-quarter of the isolation, and thereby have the effect of cooling our planet. Weather satellites show that at any one time, about 50% of earth is covered by clouds. Think about how daytime temperatures are much lower on a foggy day than on a sunny day. Clouds can also keep air temperatures higher at night by trapping the long wave radiation released by the cooling earth (this is just a magnified version of what the atmosphere already does). Therefore, a low cloud ‘blanket’ regulates earth temperature, keeping it from becoming too hot during the day, or too cold at night. On the other hand, high clouds can have the effect of heating up the earth, and causing the ‘greenhouse effect’.
Ocean and Continent Effects
In January, the majority of the land in the northern hemisphere is very cold, while the ocean is relatively warm. In fact, the warm ocean currents reach all the way up past the arctic circle and keep the United Kingdom and Norway relatively warm, while inland areas at the same latitude (Central Russia, Canada) are very cold. Why is that? Note also the map of ‘Normal annual range in temperature’. The greatest temperature ranges can be found on land, specifically in the center of continents. Why is this?
One part of the answer can be found in the differing abilities of land and water to absorb isolation from the sun. One property of every substance (such as rock or water) is its ability to absorb energy without heating up. Objects that can absorb a lot of energy without heating up very much are said to have a 'high specific heat'’ or 'high heat capacity'. Substances that can only absorb a little energy before they heat up are said to have a 'low heat capacity' or 'low specific heat' (you may recall the concept of specific heat from chemistry class). Heat capacity is the amount of energy needed to raise 1 gram of substance by 1º C. Water has a high heat capacity, while ‘land’ (rocks, vegetation, soil etc.) has a low heat capacity. In fact, the heat capacity of water is nearly four times greater than that of land. Recall our analogy from last week about walking barefoot on a concrete pool deck on a sunny day, then feeling the water, you will have an appreciation for this difference. The pool deck may be very hot to the touch, yet the water is chilly, though both have received the same amount of isolation!
Another reason that the temperature variations on land are much greater than those on water is that the water in the ocean circulates vertically. Fresh water added to the surface of the ocean from rivers, melting glaciers, and precipitation is less dense than the salty ocean water. This drives powerful vertical circulation currents that mix the surface water with cooler water from deep in the ocean.
Look at a series of ocean temperature images from NASA/JPL. The reds and oranges indicate warm colors, the blues and purples indicated cooler colors. What do you notice? There is very little change across the year in the sea surface temperature.
The ocean and continent effects (called ‘oceanality’ and ‘continentality’) are the reason that coastal areas usually have a very mild climate (it does not change significantly year round) while inland areas have drastic swings in their annual temperatures. The ocean acts to regulate the temperature of seaside cities because the wind which blows over the ocean and onto the city is a uniform temperature year-round (because the ocean that it blows over is a uniform temperature). This is why cities that are on the ocean, such as San Francisco are cold and foggy year-round. Here in Los Altos Hills, we are in a valley, protected from the brunt of the ocean breezes by the coast mountains. Therefore, the land around us heats up and cools down more than in San Francisco or Half Moon Bay. San Francisco is at about the same latitude as Salt Lake City, Utah, an inland city. What are the differences in annual temperature between these two cities?
Global Circulation
Global circulation is driven by pressure gradients in the atmosphere, the Coriolis force, and the friction of the atmosphere against the lithosphere. All of these may be summarized in a phenomenon we call ‘wind’. Wind is the horizontal movement of air in response to differences in pressure. Winds are the way that the atmosphere tries to balance the uneven distribution of pressure over the earth’s surface.
Pressure gradients and wind
We talked briefly at the beginning of this lecture about pressure gradients. Isolation differences are the key to the patterns of global circulation. The cold air around the poles of the earth sinks, causing regions of high pressure, while the warm, buoyant air around the equator tends to rise and cause regions of low pressure. We might then expect a gradual increase in pressure from the poles to the equator. However, the earth is composed of many dynamic ‘belts’ of high and low pressure which are far more complicated than the thermally induced low pressures at the equator and high pressure at the poles.
An English Scientist, Richard Hadley, gave this some thought several centuries ago, leading him to postulate a global circulation model where horizontal air movement was coupled with vertical air movement so that he envisioned the atmosphere as consisting of two huge convection cells with air rising at the equator, sinking at the poles and flowing from higher pressure to lower pressure, both at the earths surface and aloft. This has come to be known as the 'Hadley cell'. Here is how a Hadley cell works:
- Intense heating of air in the tropical areas, especially near the equator. As the density of air decreases, large-scale uplift of air between the equator and 5º N & S occurs, creating bands of low pressure
- Uplifted air is then pushed toward the poles.
- Air gradually cools, and sinks between 23.5º and 30º N & S, creating large bands of high pressure at the tropics.
Later work suggested that Hadley's simple model needed significant modification. By the late 1700's, a global circulation model based on a set of wind and pressure belts was in common use. This model presented a set of wind belts and pressure belts with a three cell global model where air flow, pressure belts, and vertical circulation cells all functioned together.
The Coriolis Effect and Wind
The Coriolis effect is the effect of the earth’s rotation on horizontally moving bodies such as the wind and ocean currents. Such bodies tend to be deflected to the right in the northern hemisphere (clockwise) and to the left in the southern hemisphere (counterclockwise). The amount by which the object is deflected depends on its speed and latitude. For a more in-depth explanation of this force and a very effective animated demonstration of its effect, look at the University of Iowa Coriolis page.
The Coriolis force has a different effect on high pressure systems than it does on low pressure systems. You might want to search for an illustration to examine the direction that winds will blow around a low pressure system (called a cyclone) and a high pressure system (called an anticyclone). Would a cyclone in the northern hemisphere blow in the same direction as a cyclone in the southern hemisphere? Why or why not?
The Friction of the Atmosphere Against the Lithosphere
Near the earth’s surface (up to about 1000 meters), frictional drag is very important because it reduces wind speed and causes ocean waves. One way to remember this is that on a very ‘calm’ day at the beach, the waves on the ocean are very flat. On a windy day, there are very large waves.
Atmospheric Patterns of Motion
In an ideal earth (from an atmospheric motion standpoint), there would be a uniform surface and no seasonal change. Recall the Hadley cell from the previous section. The Hadley cell is a model for the more complex pattern of global circulation. Conduct some research and see if you can label the red blanks in the figure below:
The jet stream is a narrow zone of particularly high speed winds (up to 350 - 450 km/hr) located between the tropopause and the stratosphere. The jet stream flows west to east and plays an important role in the weather. You can look at the jet stream and wind velocities over the United States at various altitudes at the Weather Channel web site.
Besides global circulation, there are winds of local importance. The 4 major types of such winds are:
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Land and sea breezes. These result from differential heating/cooling of shore vs. water and a movement of air from high to low pressure.
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Valley and mountain breezes - these are governed by the same principle as land and sea breezes. In the daytime, mountain slopes heat up creating rising air and low pressure. At night slopes cool down quickly creating cold falling air over the high mountains, and a high pressure system which causes wind to blow into the valley.
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Drainage winds (katabatic winds) - air from cold upland areas moves down into lower elevations under the influence of gravity. Off the ice sheets of Greenland and Antarctica, or down the Rhone valley (southern France).
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Foehn/Chinook winds - winds that passed over mountains and descend on the leeward side.
Review learning outcomes.
Please complete the Assignments and Exams section for each lesson before proceeding to the next lesson.
