Hadley Cell
George Hadley, an English meteorologist, theorized this first circulation cell in 1735. The Hadley cell is the strongest of the three cells of circulation and is formed as warm air rises above the Equator and starts to flow northward. The northward flow deflects to the right, due to coriolis, becoming an upper-level westerly flow. As this air moves northeastward toward the pole, it cools and a portion of it sinks at about 30°N. This sinking air spreads northward and southward as it nears the surface. The southward moving air again deflects to the right, becoming the northeasterly trade winds.Because of the circulation in the Hadley cell, two pressure belts are created. The first is a belt of semipermanent high pressure that results from the sinking air at 30°. This belt of high pressure is called thesubtropical ridge. The second pressure belt is a trough of low pressure near the Equator. It is called the near equatorial trough.
Polar Cell
This is the northernmost cell of circulation and its mean position is between 60°N and the North Pole. At the pole, cold, dense air descends, causing an area of subsidence and high pressure. As the air sinks, it begins spreading southward. Since the coriolis force is strongest at the poles, the southward moving air deflects sharply to the right. This wind regime is called the surface polar easterlies, although the upper winds are still predominantly from the southwest. Near 60ºN, the southeasterly moving air moving along the surface collides with the weak, northwesterly surface flow that resulted from spreading air at 30°N. This colliding air rises, creating a belt of low pressure near 60°N.Ferrel Cell
The mid-latitude circulation cell between the Polar cell and the Hadley cell is called the Ferrel cell. This cell is named after William Ferrel, a Nashville school teacher who first proposed its existence. Oddly enough, Mr. Ferrel published his observations in a medical journal in 1856.The Ferrel cell circulation is not as easily explained as the Hadley and Polar cells. Unlike the other two cells, where the upper and low-level flows are reversed, a generally westerly flow dominates the Ferrel cell at the surface and aloft. It is believed the cell is a forced phenomena, induced by interaction between the other two cells. The stronger downward vertical motion and surface convergence at 30°N coupled with surface convergence and net upward vertical motion at 60°N induces the circulation of the Ferrel cell. This net circulation pattern is greatly upset by the exchange of polar air moving southward and tropical air moving northward. This best explains why the mid-latitudes experience the widest range of weather types at mid- latitude. This circulation is called a Ferrel cell. The flow of the atmosphere at the mid- latitudes is characterised by the baroclinic instability, which causes high and low pressure systems.
An area of low atmospheric pressure occurring between 30 and 60, shown on a weather map as a circular pattern of isobars with the lowest pressure at the centre. This low is some 1500- 3000km in diameter and is associated with the removal of air at height and the meeting of cold and warm air masses in the lower atmosphere. At the fronts between the air masses, a horizontal wave of warm air is enclosed on either side by cold air.
Although, this is extremely complicated, the concepts explained below are much more simpler with the opportunity for a great lesson plan.
The Bergen
School Model was devised to assist in the understanding of the formation of
Mid-latitude depressions. It comes in 4 stages:
·
Origin and infancy: warm air front meets cold air front, generally warm
coming from the south of the UK and cold from the north.
·
Maturity: the warm air spirals over the cold anticlockwise as the cold
air sinks. Then a warm area exists between the cold and warm fronts.
·
Occlusion: the warm sector disappears as the cold front moves faster.
Cold air is denser, so it forces the warm air upwards. It is harder for the
warm lighter air to move and this causes the cold air to sink. The cold air
moves at about 40/50 mph whereas the warm travel travels at about 20/30 mph.
·
Death: the frontal system dies as the warm air is forced upwards and cools and the cold air remains underneath. The differences in temperatures have levelled out and therefore there is no longer an occluded front.
Death: the frontal system dies as the warm air is forced upwards and cools and the cold air remains underneath. The differences in temperatures have levelled out and therefore there is no longer an occluded front.
A
depression is an area of low pressure accompanied by a front. They commonly
affect the weather in the UK.
There is a
great experiment to help us to understand the differences in air pressure that
can easily be set up in a class room:
·
Blow up a balloon to just beyond the width of a large jar, and tie the
end to keep the air in
·
Place pieces of paper in the jar and set them alight, therefore heating
the air.
·
Blow out the flames and place the balloon on the top of the jar.
Outcome: The balloon will be sucked into the jar, this is called precipitation, generating low pressure and therefore sucking the balloon into the jar, due to a difference in air pressure. This is just an experiment in a class room, which is dramatic can you imagine what it is like when 2 massive air masses, come together creating a mid-latitude depression!
Outcome: The balloon will be sucked into the jar, this is called precipitation, generating low pressure and therefore sucking the balloon into the jar, due to a difference in air pressure. This is just an experiment in a class room, which is dramatic can you imagine what it is like when 2 massive air masses, come together creating a mid-latitude depression!
Links:
http://cspgeography.blogspot.co.uk/2010/08/mid-latitude-depressions-depression-is.html
http://www.answers.com/topic/mid-latitude-depression#ixzz27KbPLgFx
http://www.answers.com/topic/mid-latitude-depression#ixzz27KbPLgFx
No comments:
Post a Comment