ATMOSPHERIC CIRCULATION

Transcription

1 Name ATMOSPHERIC CIRCULATION (adapted from Dr. S. Postawko, U. of Ok.) INTRODUCTION Why does the wind blow? Why do weather systems in the mid-latitudes typically move from west to east? Now that we've explored the various parameters used to describe the state of Earth's atmosphere (temperature, pressure, and moisture), we want to put them all together and see how they determine our weather and climate. Part 1: Global Temperature and Pressure Variations Our atmosphere is an open system - when a region of air is heated, the molecules move faster and move farther apart from one another (that is, we say that the air expands). When air expands, it means the number of molecules in a given volume decreases, that is, the density of the air decreases (density = mass/volume). Therefore the air in that region weighs less - and so it exerts less pressure than an equivalent volume of cold air. In addition, when air in a given region is heated and becomes less dense than air surrounding it, this heated volume of air will rise (think "hot air balloon"). As we saw earlier, sunlight doesn't hit all parts of the Earth equally. This is one of the reasons that there are temperature variations across the planet. Now we also know that when regions of air are at different temperatures, they also will have different pressures. (Open containers work differently than sealed containers!) If we had two open cartons of air that are the same size and at the temperatures given, which would exert the greatest pressure? Why? Assume you have a sealed box, as shown at right. A divider in the box separates part of the box that has air in it from part of the box that has no air in it (i.e., a vacuum). If the divider is removed, will the air move? If so, which direction? Why?

2 In the atmosphere, when pressure is higher in one region than in an adjoining region, air will flow from the area of higher pressure to the area of lower pressure. On the map of California at left, the atmospheric pressure at many different weather stations and buoys have been recorded in millibars. Step 1: Connect the dots of those stations that reported the same pressure (these lines of equal pressure are called "isobars"). Step 2: Using the fact that air will flow from areas of high pressure to areas of low pressure, indicate the wind direction on your map with one or more arrows. Step 3: Winds are named by where they blow from. The winds you just labelled would be called winds. We can get a general idea of the large-scale circulation of Earth's atmosphere in a similar fashion. For simplicity we will assume that the Earth is not spinning nor tilted on its axis, and that the surface of the planet is flat and of uniform composition. The diagram below represents this simplified Earth. General regions of high and low pressure at the surface are indicated. Based on this information, indicate the direction the winds would blow in the latitude "belts" between each of the high and low pressure regions indicated.

3 Because Earth is a sphere and spins on its axis, there is an "apparent" force called the Coriolis force that deflects the flow of air as it tries to travel in any direction across the planetʼs surface. In the Northern Hemisphere the Coriolis force causes winds to turn to the right of their initial direction of motion. For example, air moving in the latitude belt between the Equator and 30 N doesn't flow directly from North to South. Because of the Coriolis force, the air actually moves from Northeast to Southwest, as shown on the diagram below. In the Southern hemisphere, the Coriolis force moves air to the left of its initial direction of motion, as shown in the latitude belt between the Equator and 30 S latitude. Based on this information, draw the wind patterns in the remaining 4 latitude belts, taking the Coriolis force into consideration. The wind patterns in each of the latitude belts are given names, depending on the direction the wind is coming from. Add these names onto your diagram above: > Equator-30 N: Northeast Trade Winds > Equator - 30 S: Southeast Trade Winds > 30 N-60 N: Westerlies > 30 S-60 S: Westerlies > 60 N - North Pole: Polar Easterlies > 60 S - South Pole: Polar Easterlies

4 Explain the statement, The atmosphere socializes humanity, making the entire world a spatially linked society. Illustrate your answer with some examples. So far we have only discussed horizontal motions of air. However, the atmosphere is 3- dimensional, and we know that air not only moves horizontally, but vertically (up and down) as well. Low pressure typically indicates air that is rising, expanding, and cooling. If saturated air at Earth's surface rises and cools, the water vapor in the air condenses to form clouds. On the diagram on the previous page, label those areas where you would expect persistent cloud cover. High pressure typically indicates air that is sinking (moving downward). As air sinks it gets compressed and heats. The warmer air evaporates clouds. On the previous page, label areas that you would expect to be clear most of the time. Does your diagram predict generally clear or cloudy skies for Southern California? Part 2: Influences on Local Wind Patterns Earth's surface certainly isn't uniform in composition - over 70% of Earth's surface is covered by water. Differences in how areas of land and water heat and cool can result in temperature and pressure variations that influence local wind patterns. When you go to the beach on a sunny summer's day, how does the temperature of the sand compare to the temperature of the water? It takes a long time for water to heat up or cool down when compared to land. That is, we say that water has a high specific heat (definition of specific heat, the amount of energy required to raise the temperature of 1 gram of a substance by 1 C). The differences in the temperature of the land and water affect the temperature and pressure of the atmosphere above. And as we now know, differences in temperature and pressure are what cause air to flow.

5 On the diagram below, indicate what you think the relative air pressure would be over land and over water, given the temperature of the land and the water. You can use a two-step approach to determine this: Step 1: First ask yourself if air temperature would be higher over the land or over the water Step 2: Based on your understanding of the relationship between temperature and pressure, draw an L or H to indicate that pressure would be higher over land or over water. Step 3: Now that you have determined the relative air pressures over land and over water, use your knowledge of the general relationship between air pressure and vertical motion of air to indicate with arrows on your diagram if air is moving upward or downward over land and over the water. Step 4: Based on where air is rising and descending, draw in arrows to indicate horizontal flow of air (wind). The circulation that you have just drawn is called a "sea breeze". During the day, as the land area heats up more than the water, air at the surface flows from over the cooler water onto the land. This type of local circulation will dominate over the general circulation of the atmosphere in a given area.