Understanding Global Atmospheric Circulation

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TASK:
Make the summary about the global atmospheric circulatión. Send me by e-mail

What is global atmospheric circulation?

What is global atmospheric circulation?

The air across the planet moves in a specific way. This movement starts at the equator, the hottest part of the Earth. Due to high temperatures at the equator, the air rises up into the atmosphere. This creates low pressure (as the air is rising it puts less pressure on the Earth’s surface). As the air rises it becomes colder, causing condensation (forming clouds) that leads to rainfall. This is why tropical rainforests are found along the equator!

Global atmospheric circulation model

When the air reaches the top of the atmosphere it needs somewhere to go. Some of the air travels north and some south of the equator. The air cools and then sinks at around 30° north and south of the equator. As the air is sinking this creates high pressure. As moisture in the sinking air fell at the equator it is dry so few clouds form here. This is why deserts are found along 30° north and south of the equator.
The movement of air between the equator and 30° north and south is known as the Hadley Cell. Air rises again at around 60° north and south and descends again around 90° north and south forming the Ferrel and Polar Cell.
Global atmospheric circulation creates winds across the planet as air moves from areas of high pressure to areas of low pressure. It also leads to areas of high rainfall, like the tropical rainforests, and areas of dry air, like deserts

 Link the original pages

The Difference Between Depressions and Anticyclones

Depressions and Anticyclones

Depressions

Depressions are areas of low pressure,in other words the air is rising. They bring clouds and rain to the United Kingdom. They originate over the Atlantic and then travel with the prevailing winds in a North East direction across the country.
Depressions occur when two air masses meet, one is a warm, moist mass from the tropics, whilst the other is a cool, dense mass from the Pole.
The two air masses meet at a front, and the warm air is forced to rise over the cold air. This is called the warm front. At the same time the cold air circles around behind the warm air and undercuts, lifting the warm air off the ground. This is called the cold front. The two air masses spiral around each other and slowly move across the country.

As a depression moves overhead you can recognise 5 stages in order:

• Initially as the depression approaches you will get a build up of high cirrus and stratus clouds. This is because the warm air that has been forced to rise has begun to condensate.

• The cloud will continue to accumulate overhead as the warm front approaches. The clouds will be lower nimbostratus ones and there will be steady rainfall. The temperature will also fall.

• Once the warm front has passed, you are in the section between the two fronts where the warm air still is touching the ground. This causes the temperature to increase, often from approximately 6 to 12C. There may be some light drizzle, but mostly it is dry during this warm sector.

• The next part is the arrival of the cold front. You would be able to see this clearly coming towards you as it is a steep front with huge high cumulonimbus clouds, bringing with them very heavy rain. The temperatures also fall asthe cold front moves overhead.

• Finally, once the cold front has passed, you get a mixture of sunshine and heavy showers, caused by pockets of rising air in the air that follows the depression.

• The whole process takes between 12 and 24 hours.
  

Anticyclones

Anticyclones are areas of high pressure,where the air is falling towards the ground exerting pressure below it. Although not as common as depressions, anticyclones can remain for days, even weeks.They bring warm, stable conditions, with clear skies and lots of sunshine.
Winds are very light in an anticyclone, as the air moves outwards from the centre in a clockwise motion (in the Northern Hemisphere).
Anticyclones can occur throughout the year and cause different effects when they happen in the winter rather than the summer.
Summer Effects:

• Anticyclones bring hot and sunny weather due to there being clear skies (because of the descending air). The days are long and the sun high in the sky, which gives ample opportunity for the land to be heated up. These conditions could lead to convectional rainfall occurring.

Winter Effects:

• Anticyclones bring clear days in the winter as well. However the sun is low in the sky and the days are short, meaning you get cool, crisp days.

• Clear skies on a winter's night will allow frost to form. The land quickly loses heat during the night, as there is no cloud cover to act as insulation. The rapidly cooling ground cools and condenses any moisture in the air above it, forming droplets of ice when the temperature falls below freezing. This is frost.

• Fogs are also caused by clear winter nights. The ground loses heat. This cools the air above it causing moisture to condensate around dust particles in the layer of air closest to the ground surface. This is fog.

All Year Round Effects:

• The descending air creates dry conditions, as air will warm as it descends, meaning that it can actually hold more water, and so will not need to drop it.( link to this material)

TASK:

a) Describe the type of weather associated with anticyclones.

b) Describe the type of weather associated with depressions.

c) Explain why these types of weather occurs.

d) What is pressure measured in?

e) What type of pressure would you expect to find during an anticyclone.


Answer

Environmental Problems´ Presentation

We will go to make a Power Point about the environmental problems. You can look for ideas  in  your book, pag 64-65 or in these links:

• Link 1
• Link 2  
• Link 3 

These  presentations will make in  groups of 2 people. You send me your work.  Some groups  show us your presentation in class.

Save Environment PPT from VikrantPatankar

Types of rainfall

There are three common types of rainfall, all of which occur in Spain. All have the common theme of air being forced to rise.
As air rises it cools it cannot hold as much moisture as it could when it was warmer. Eventually the rising air reaches a point where it is 100% saturated, in other words it cannot hold any more water. This is called dew point, and it is above this point that condensation occurs.
Condensation is the process by which the water vapour (a gas) held in the air is turned back into water droplets (a liquid), which fall as rain.

Convectional rainfall

Very common in areas where the ground is heated by the hot sun, such as the Tropics. This is why those areas experience heavy rainfalls most afternoons. The United Kingdom does experience some convectional rainfall during the summer, particularly in the South East of the country.
Convectional rainfall occurs when:

• The surface of the earth is heated by the sun.

• The warm surface heats the air above it. Hot air always rises so this newly heated air does so.

• As it rises the air-cools and begins to condensate.

• Further rising and cooling causes a large amount of condensation to occur and rain is formed.

Convection tends to produce towering cumulo-nimbus clouds, which produce heavy rain and possible thunder and lightning.

Frontal rainfall

The United Kingdom experiences a lot of frontal rainfall, as it is associated with the movement of depressions over the country, which are described in more detail elsewhere in this topic.
Frontal rainfall occurs when:

• Two air masses meet, one a warm air mass and one a cold air mass.

• The lighter, less dense, warm air is forced to rise over the denser, cold air.

• This causes the warm air to cool and begin to condense.

• As the warm air is forced to rise further condensation occurs and rain is formed.

• Frontal rain produces a variety of clouds, which bring moderate to heavy rainfall.

Relief rainfall

This is also called orographic rainfall, which is very common in the United Kingdom, especially on the West coast since the prevailing weather comes from that direction.
Relief Rainfall occurs when:

• The prevailing winds pick up moisture from the sea as they travel across it, making the air moist.

• The moist air reaches the coast and is forced to rise over mountains and hills.

• This forces the air to cool and condense, forming clouds.

• The air continues to be forced over the mountains and so it drops its moisture as relief rain.

• Once over the top of the mountain the air will usually drop down the other side, warming as it does so. This means it has a greater ability to carry water moisture and so there is little rain on the far side of the mountain. This area is called the rain shadow. ( link to this material)
  

Activity: Read the texts. Prepare these materials with your words to explain the different three types of rainfall that there are.  You can use this other link. Send me the ideas

What is the jet stream and how does it work?