Embedded Files
#TestThenCheck
#TestThenCheck
Key concepts for your schema. Use these questions to test your knowledge before scrolling down!
Key concepts for your schema. Use these questions to test your knowledge before scrolling down!
Winds
What are the three factors that affect wind direction? How do they interact?
What are the two factors that affect wind speed? How is one of them related to the isobars?
General circulation of the atmosphere
Can you draw the cross section of the general circulation of the atmosphere (test it out on a scrap piece of paper)? [Do this from understanding, not knowledge!]
What are the three cells and what are their characteristics?
What is the plan view of the general circulation? How do the three factors influencing wind direction produce these winds?
Global pattern of pressure and winds
What is the global pattern? How is it modified? [NB Make the link to continentality from the last section!]
Global pattern of precipitation
It's link time again! What's the link between pressure and precipitation?
What is the global pattern of precipitation?
How is the other factor that modifies this pattern? Can you give any examples?
2.1 Winds - factors affecting wind speed and direction
2.1 Winds - factors affecting wind speed and direction
In the last section, we discovered that there is an imbalance in temperature distribution across the Earth. Let's start by making the link between that fact and the winds that we will be covering in this section.
A. Factors affecting wind direction
A. Factors affecting wind direction
Pressure Gradient Force
Pressure Gradient Force
This force results from the pressure imbalance and acts to redress this by moving air from areas of high pressure to areas of low pressure.
This force results from the pressure imbalance and acts to redress this by moving air from areas of high pressure to areas of low pressure.
This would result in the winds blowing perpendicular to the isobars.
Coriolis Effect
Coriolis Effect
Due to the rotation of the Earth, however, this air movement is deflected:
Due to the rotation of the Earth, however, this air movement is deflected:
• to the right in the northern hemisphere
• to the right in the northern hemisphere
• and to the left in the southern hemisphere.
• and to the left in the southern hemisphere.
This would result in the winds blowing parallel to the isobars.
Friction
Friction
However, friction between the winds and the Earth's surface resist the deflecting influence of the Coriolis Effect.
However, friction between the winds and the Earth's surface resist the deflecting influence of the Coriolis Effect.
This means that the winds actually blow at a slight angle to the isobars, spiralling out from the centre of high pressure and in towards the centre of low pressure.
This means that the winds actually blow at a slight angle to the isobars, spiralling out from the centre of high pressure and in towards the centre of low pressure.
B. Factors affecting wind speed
B. Factors affecting wind speed
i. Pressure Gradient Force
i. Pressure Gradient Force
The main force affecting wind speed is the pressure gradient force. The steeper the pressure gradient, the faster the wind speed will be, while places with more gentle pressure gradients have slower wind speeds.
Pressure gradients can be observed on synoptic weather charts – the closer the isobars, the steeper the pressure gradient.
ii. Friction
ii. Friction
Friction with the earth’s surface which has an influence in the lowest kilometre of the atmosphere. The extent to which friction acts to slow winds is determined by the nature of the surface causing friction. For example, wind blowing over a body of water will experience less friction than wind blowing over rougher land such as a forested area.
Let's have a look at how both these factors work on an actual map. BTW you coud get maps like this as resources in your exam, so make sure you spend time learning how to read them carefully!
Let's have a look at how both these factors work on an actual map. BTW you coud get maps like this as resources in your exam, so make sure you spend time learning how to read them carefully!
Check out the present wind pattern in this beautiful real time global map of the winds. Can you find any depressions in it? Can you see how wind speeds vary over land and sea?
2.2 The general circulation of the atmosphere
2.2 The general circulation of the atmosphere
In general, then, air moves from areas of high pressure to low pressure in the ways outlined above.
In general, then, air moves from areas of high pressure to low pressure in the ways outlined above.
But there is a particular pattern to the way this works itself out on a global scale. And it can be described using what is known as the tri-cellular model.
But there is a particular pattern to the way this works itself out on a global scale. And it can be described using what is known as the tri-cellular model.
A. Cross Section view
A. Cross Section view
Polar cell
Polar cell
The Polar Cell is a thermally direct* cold convection cell operating between 90ÂşN/S and 60ÂşN/S. It is driven by the subsidence of very cold air at the Poles. As this air sinks, it produces high pressure and so air moves away from here to the mid-latitudes to around 60ÂşN/S. Here it meets the warmer Tm air mass which rises over the colder Pm air towards the tropopause, leaving low pressure
From here, it returns to the Poles in the upper atmosphere, completing the convection cell here.
[* Thermally direct means the movement is directly driven by temperature - in this case the sinking of cold air]
Ferrel Cell
Ferrel Cell
The Ferrel Cell lies in between the Hadley Cell and the Polar Cell. It is a thermally indirect cell meaning movement here is not driven primarily due to temperature; in fact, you can envisage the Ferrel cell as like a cog between the other two - it moves because they move.
In theory, the Ferrel Cell is a neat cell similar to the other two. But the reality of air movement in the mid latitudes is different in two main ways.
The mixing at the surface is driven much more by large horizontal eddies of air (when air masses circulate around mid-latitude depressions, for example).
The upper air movement here is not so much dominated by the movement of air equatorwards or Polewards, but rather by what are called jet streams (See below).
Hadley Cell
Hadley Cell
The Hadley Cell between the equator and 30Âş N/S is a thermally direct* warm convection cell driven by the rising of the hot air found in equatorial regions. As the air is heated, it rises upwards and cools, leaving low pressure at the surface.
When the air reaches the tropopause and then it spreads north and south towards the Poles. At around 30Âş N and S, it will start to sink again, producing high pressure at the surface.
Due to the resultant pressure gradient between 30ÂşN/S and the equator, some of this air returns to the equator via the Trade Winds completing the convection cell.
Due to the heat imbalance between the equatorial regions and 40ÂşN/S, some of it moves towards the Poles and is thus an important part of the horizontal heat transfer process.
[* Thermally direct means the movement is directly driven by temperature - in this case the rising of warm air]
The gif starts off looking down on top of the North Pole. Notice how the Polar Front Jet Stream (in red) meanders around the mid latitudes. Then is zooms into North America. Look at the mendering waves in the Jet Stream (known as Rossby Waves). These will be important in the section on the formation of mid-latitude depressions and anticyclones.
Jet Streams
Jet Streams
More recent understanding of the general circulation of the atmosphere has revealed that the tricellular model, and especially the Ferell Cell, is an oversimplification. Instead, the role of fast moving upper air winds known as upper westerlies is recognised as being significant. There are two of these:
Sub-tropical upper westerlies, which forms at the interface bewteen the Hadley and Ferell Cells at around 30 N/S, and circulates around the globe as a westerly wind.
Polar Front upper westerlies is found at the interface between the Ferell and Polar Cells at around 60 N/S. This also circulates around as a westerly wind.
These upper westerlies can travel at speeds of around 100 km/hour. But the fastest moving jet streams within the upper westerlies can be even faster, moving at speeds in excess of 300 km/hour.
These upper air westerlies are crucial in the formation and development of mid-latitude weather systems (depressions and anticyclones), as we will see in a later section of this topic.
The best format for an exam essay on the general circulation
The best format for an exam essay on the general circulation
I would advise this sequence.
It is possible to divide the general circulation of the atmosphere into three main circulation cells.
The first cell is the Hadley Cell, between the equator and 30Âş N/S is a thermally direct* warm convection cell driven by the rising of the hot air found in equatorial regions. As the air is heated, it rises upwards and cools, leaving low pressure at the surface. When the air reaches the tropopause and then it spreads north and south towards the Poles. At around 30Âş N and S, it will start to sink again, producing high pressure at the surface. Due to the resultant pressure gradient between 30ÂşN/S and the equator, some of this air returns to the equator via the Trade Winds completing the convection cell. Due to the heat imbalance between the equatorial regions and 40ÂşN/S, some of it moves towards the Poles and is thus an important part of the horizontal heat transfer process.
The Polar Cell is also a thermally direct* cold convection cell and it operates between 90ÂşN/S and 60ÂşN/S. It is driven by the subsidence of very cold air at the Poles. As this air sinks, it produces high pressure and so air moves away from here to the mid-latitudes to around 60ÂşN/S. Here it meets the warmer Tm air mass which rises over the colder Pm air towards the tropopause, leaving low pressure. From here, it returns to the Poles in the upper atmosphere, completing the convection cell here.
The final cell is the Ferrel Cell and lies in between the Hadley Cell and the Polar Cell. It is a thermally indirect cell meaning movement here is not driven primarily due to temperature; in fact, you can envisage the Ferrel cell as like a cog between the other two - it moves because they move.
In theory, the Ferrel Cell is a neat cell similar to the other two. But the reality of air movement in the mid latitudes is different in two main ways. The mixing at the surface is driven much more by large horizontal eddies of air (when air masses circulate around mid-latitude depressions, for example). The upper air movement here is not so much dominated by the movement of air equatorwards or Polewards, but rather by what are called jet streams.
B. Plan View
B. Plan View
Now it's time to turn to the plan view of the general circulation. And, as we do so, we need to remember the other two factors that determine wind direction!
For the cross section view, all we need to consider is the Pressure Gradient Force, as the air moves from high pressure to low.
But, for the plan view, we need to factor in the Coriolis Effect and Friction.
Here is the final plan view pattern. But, when you're learning this, you would be best to draw it out from understanding than simply to learn it of by heart.
When making this graphic, I used a real time map of winds. Look carefully at the model and the actual winds. Compare and contrast them - to what extent to the actual winds reflect what the model would suggest?
Now, let's do this comparison in a more detailed way.
Text in here
2.3 Global pattern of pressure and winds
2.3 Global pattern of pressure and winds
January
January
In January in the northern hemisphere (where it is winter), the band of higher pressure at 30 degrees extends northwards over the continents, breaking the band of low pressure found at 60 degrees north. At this latitude, low pressure is found over the sea, but not over the land.
The opposite is the case in the southern hemisphere (where it is summer). The low pressure at the equator extends south over the continents, breaking the band of higher pressure at 30 degrees south. At this latitude, the high pressure is found over the sea.
July
July
Based on the principles you've learned in the first map, what patterns and modifications can you see here for the July map?
Why is this the case?
Why is this the case?
These modifications to the alternating bands of high and low pressure are related to something we studied in the last section:
These modifications to the alternating bands of high and low pressure are related to something we studied in the last section:
Continentality
Continentality
Remember, according to this:
land heats up and cools down quickly
sea heats up and cools down slowly.
So let's go back to the maps.
Winter
Winter
Let's go to 60 degrees North.
During the winter, the land cools down very quickly. Especially the large continents of North America and Asia.
Here, temperatures can plummet to - 30 C or below. This very cold air sinks - leading to high pressure over the land.
Conversely, the oceans cool down slowly. This keeps the temperatures here warmer, and so the air can still rise to give low pressure.
Summer
Summer
Now let's go to 30 degrees south.
In the summer, the land heats up quickly. This encourages air to rise over the continents, giving lower pressure there.
Conversely, the sea heats up slowly. The allows the air to continue to fall, reinforcing the high pressure there.
2.4 Global patterns of precipitation
2.4 Global patterns of precipitation
Make the link!
Make the link!
Before we look at the map of precipitation, let's link back to previous material:
Where air is heated, it will rise, giving low pressure at the surface
As air rises it cools adiabatically, condenses, forms cloud and brings rain.
Thus, low pressure is associated with more rain
Where air is cooler, it will fall, giving high pressure at the surface.
As it falls, it warms adiabatically and so condensation does not occur.
Thus, high pressure is associated with less rain.
Remember - in the previous section, the tri-cellular model told us to expect bands of high and low pressure around the Earth. So - as there is a link between pressure and rain - we should also expect bands of more and less rain around the Earth too! Let's see if that's the case.
Remember - in the previous section, the tri-cellular model told us to expect bands of high and low pressure around the Earth. So - as there is a link between pressure and rain - we should also expect bands of more and less rain around the Earth too! Let's see if that's the case.
Bands?
Bands?
Well, mostly yes!
It's wettest at the Equator, where there it LP.
Then there are drier bands at 30 degrees N/S where there is HP.
It's also drier over the Poles where there is HP.
There's a band of rain at 60 degrees south.
60 degrees north is a bit more complex!
There are a few more patterns we can see on the maps though:
At 60 degrees N/S, there is more rain on the western sides of continents than eastern sides.
At 60 degrees N/S, there is more rain on the western sides of continents than eastern sides.
At 30 degrees N/S, there is less rain on western sides.
At 30 degrees N/S, there is less rain on western sides.
Look at the map and make sure you see these for yourself!
Make the link!
Make the link!
This is to do with wind direction. The Global Circulation model tells us to expect:
westerly winds at 60 degrees N/S
easterly winds at 30 degrees N/S
This means that
western coasts at 60 degrees N/S experience onshore winds (which bring mositure laden air that rises, cools and condenses when it reaches land)
western coasts at 30 degrees N/S experience offshore winds (which blow from the land and so don't produce rain).
This Slides presentation will help you see the global patterns in the map.
Key principles:
Key principles:
- where there is low pressure, more rain falls
- where there is low pressure, more rain falls
- where there are onshore winds, more rain falls
- where there are onshore winds, more rain falls
Report abuse