Lesson: Atmospheric Dynamics

Transcription

1 Lesson: Atmospheric Dynamics By Keith Meldahl Corresponding to Chapter 8: Atmospheric Circulation Our atmosphere moves (circulates) because of uneven solar heating of the earth s surface, combined with the earth s rotation. The jet stream shown in this figure is one example of air movement resulting from these processes.

2 Summary of Important Concepts The circulation of the atmosphere controls winds and weather, moderates surface temperatures, creates waves, and drives ocean surface currents. The earth absorbs more solar energy near the equator than near the poles. This uneven solar heating causes the atmosphere to circulate by convection. Each hemisphere consists of three large, roughly circular vertical paths of moving air called cells. Going from the equator toward the poles, these are the Hadley Cells, Ferrel Cells, and Polar Cells. Air RISES near the equator and near 60 degrees latitude north and south. These areas of rising air have HIGH precipitation (rain and/or snow). Air SINKS near 30 degrees latitude north and south. These areas of sinking air have LOW precipitation. Earth s rotation causes the Coriolis effect, which deflects moving air into right-curving paths in the Northern hemisphere and leftcurving paths in the Southern hemisphere. The vertical cells of circulating air, combined with the Coriolis effect, produce the prevailing surface winds.

3 Summary of Important Concepts, continued Each hemisphere has three large regions of prevailing surface winds: 1. The trade winds (or easterlies) blow from east to west in the region between the equator and 30 degrees latitude in both hemispheres. 2. The westerlies blow from west to east in the region between 30 and 60 degrees latitude in both hemispheres. 3. The polar easterlies blow from east to west in the region between 60 degrees latitude and the poles in both hemispheres. Prevailing wind patterns can be influenced by sea breezes and land breezes, and by monsoons. Most large storms are cyclonic systems; that is, they consist of large spinning masses of air spiraling into an area of low pressure. Tropical cyclones (hurricanes) are spinning air masses that develop within the warm, humid air over tropical oceans. Extratropical cyclones are spinning air masses that develop at the boundaries between warm and cold air masses away from the equator.

4 Composition and Properties of the Atmosphere Important properties of the atmosphere. The lower atmosphere consists mostly of nitrogen gas (N2; about 78%) and oxygen gas (O2; about 21%) Water vapor (H2O molecules) can make up as much as 4% of the volume of the atmosphere. The density of air is influenced by temperature and water content. Warmer air is less dense than cool air, and will thus tend to rise upward. Humid air is less dense than dry air, and will thus tend to rise upward. (Note: this last point may seem strange, but water molecules (H2O) are lighter than oxygen molecules (O2) or nitrogen molecules (N2), so air containing lots of water vapor is lighter!)

5 Composition and Properties of the Atmosphere Note that this table shows the composition of dry air. Air can contain up to 4% water vapor.

6 Composition and Properties of the Atmosphere Rising air cools as it expands. Cooler air can hold less water, so water vapor condenses into clouds. Therefore, areas of rising air tend to have high precipitation (rain and/or snow). In contrast, sinking air warms as it compresses. Warmer air can hold more water vapor, so the vapor does not condense into clouds. Therefore, areas of sinking air tend to have low precipitation (rain and/or snow).

7 Uneven Solar Heating and Latitude The main reason the atmosphere moves (circulates) is because the earth gets uneven amounts of heat from the Sun. The equator gets more solar radiation than the poles. Why is this? Near the equator the Sun hits from straight overhead, concentrating its energy in a small area. Near the poles the Sun hits at a low angle, so the same amount of energy is spread out over a larger area.

8 Uneven Solar Heating and Latitude The result of this uneven heating is that the equator areas have net heat gain, and the polar areas have net heat loss. Since heat tends to move from areas with high amounts to areas with less heat, the result of this imbalance is that the atmosphere and the oceans both circulate, acting to even out the imbalance of heat!!

9 Uneven Solar Heating & Atmospheric Circulation TWO factors govern the global circulation of air: 1. Uneven solar heating -- warm air near the equator rises; cold air near the poles sinks. 2. The Coriolis effect -- the spin of the earth causes moving air to change its direction. As air warms, expands, and rises at the equator, it moves toward the pole, but instead of traveling in a straight path, the air is deflected eastward. In the Northern Hemisphere the air follows right-curving paths. In the Southern Hemisphere the air follows left-curving paths..

10 Atmospheric Circulation Cells

11 Atmospheric Circulation Cells Uneven solar heating of the earth, combined with the Coriolis effect, creates three large atmospheric circulation cells in each hemisphere. Hadley cells occur on either side of the equator. They are formed from air rising at the equator and sinking at about 30 degrees latitude north and south (see previous slide). Ferrel cells occur at the mid-latitudes. They are formed from air sinking at about 30 degrees latitude north and south and rising at about 60 degrees latitude north and south (see previous slide). Polar cells occur near the poles. They are formed from air rising at about 60 degrees latitude north and south and sinking at the poles (see previous slide).

12 Atmospheric Circulation Cells Areas of the three great circulation cells where air either rises straight upward or sinks straight downward tend to have weak and variable surface winds. Two areas of weak surface winds are particularly apparent: The doldrums are the areas of weak surface winds along the equator where Hadley cells air rises upward. The horse latitudes are areas of weak surface winds at about 30 degrees latitude north and south of the equator, where Hadley cell and Ferrel cell air sink downward. These areas were the nemesis of ancient sailors, whose sailing ships could be stranded for weeks by calm winds. Often to lighten their loads ship captains would order cargo thrown overboard -- and heavy horses were often the first things to go! Hence the name horse latitudes. Sorry, you horse lovers out there.

13 Prevailing Winds As air moves in the three great circulation cells, it s direction changes due to the Coriolis effect. In the Northern Hemisphere the air follows right-curving paths. In the Southern Hemisphere the air follows left-curving paths. The result is that each circulation cell corresponds to a single direction of prevailing surface wind. The trade winds (or easterlies) blow from east to west in the region between the equator and 30 degrees latitude in both hemispheres. The trade winds are surface winds of the Hadley cells. The westerlies blow from west to east in the region between 30 and 60 degrees latitude in both hemispheres. The westerlies are surface winds of the Ferrel cells. The polar easterlies blow from east to west in the region between 60 degrees latitude and the poles in both hemispheres. The polar easterlies are surface winds of the Polar cells.

14 Prevailing Winds The orange parts of the arrows show the prevailing surface trade winds and westeries. The third set of prevailing winds the polar easterlies are not well illustrated here.

15 Sea Breezes & Land Breezes The prevailing surface winds discussed above may be changed by local conditions. One example is the sea breeze / land breeze situation. Sea breeze: during the day, the land will heat up more than the ocean (because water has higher heat capacity than land). The warm air over the land rises, and the cool air over the ocean moves in to replace it. This creates a wind blowing from the ocean toward the land, and is called a sea breeze. Land breeze: during the night, the land will cool down more than the ocean. The cool air over the land sinks, and the relatively warmer air over the ocean rises, creating a wind blowing from the land toward the ocean - - a land breeze. These are illustrated on the following slide:

16 Top: Sea breeze created during the day by cool air sinking over the ocean and warm air rising over the land. Bottom: Land breeze created during the night by cool air sinking over the land and warm air rising over the ocean.

17 Storms Storms are regional atmospheric disturbances characterized by high winds and often high precipitation. Most major storms are cyclones, meaning that they are spinning masses of air caused caused by air spiraling into an area of low atmospheric pressure. TROPICAL CYCLONES occur in tropical regions. These storms are known as hurricanes to most of us, but go by other names in other areas of the world. The ultimate cause of hurricanes is the massive amount of heat that is released by the condensation of huge volumes of water vapor. The air over warm tropical oceans gets very humid, and as that vapor condenses, it releases heat. The heat causes the air to begin rising, which causes more water vapor to condense, releasing more heat. This creates a runaway effect of massive amounts of rising air. Air gets sucked into the area to replace the rising air, creating extreme winds!

18 Storms The diagram of a hurricane illustrates how air rises as heat is released by condensation of water vapor. The rising air is replaced by air rushing in from the sides. This inrushing air creates the high winds of a hurricane. Hurricanes are characterized by heavy rain and extreme winds. Both processes result from one cause: the condensation of huge amounts of water vapor!!

19 Storms The orange shaded areas show regions of warm tropical oceans over which hurricanes form. The arrows show the paths of hurricanes once they form. Notice that these storms follow rightcurving paths in the Northern hemisphere and left-curving paths in the Southern hemisphere -- the Coriolis effect in action!

20 The Reason for the Seasons Changes in seasons are caused by the variations of incoming solar energy as Earth makes its annual rotation around the Sun. The 23 1/2 o tilt of the Earth s axis means the northern hemisphere gets less solar energy in the winter, and more in the summer.