Mid- latitude depressions

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.

Lesson idea:

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!

Links:

http://cspgeography.blogspot.co.uk/2010/08/mid-latitude-depressions-depression-is.html
http://www.answers.com/topic/mid-latitude-depression#ixzz27KbPLgFx

http://sparce.evac.ou.edu/q_and_a/air_circulation.htm

Hurricanes

Hurricanes are an important and interesting natural hazard for children to learn. The devastation and severity of hurricanes such as Hurricane Katrina and Irene that have been reported in the news and media often fail to report how the storm is evolved, normally it's just the effects and impacts after the storm occurs.

Tropical storms are intensive, low pressure weather systems known in different parts of the world as hurricanes, cyclones, typhoons and willy- willies. They are summer storms and found in low latitudes.

Hurricanes are occur most in the Pacific Ocean and are most active in the West of the Pacific. The term applied to most storms depends on their location. Only one Hurricane has ever occurred in the South Atlantic, Hurricane Catarina (2004).

Tropical storms have three major effects: very strong winds; torrential rain; storm surges. Together these can cause loss of life, considerable damage and severe economic impacts. 

A map of where hurricanes location.

Hurricanes formation is not fully understood, but it involves the transfer of energy on a huge scale. Very moist, warm air rises rapidly form the surface of the sea. On meeting colder air high in the atmosphere it condenses to form clouds. If the surface of the sea is warm enough, the upward movement sucks in more warm air that evaporates more water and the storm builds. This more this happens the larger the source of energy and the longer the life of the storm. Strong winds are released by the process. These spiral violently upwards, driven by the rotation of the Earth. They are thrown outwards from the centre of the storm. In the centre develops an area of calm- higher temperatures and no cloud- know as the eye. 

When hurricanes travel over land however, they die due to the loss of moisture. 

A map showing where each tropical cyclone has tracked between 1851 and 2007. 

ATLANTIC: Hurricane season in the Atlantic runs from June 1 to November 30. Storms outside of these dates are not unheard of. As you can see from the graph, based on the average of 150 years of storms, activity ramps up in August, and peaks once in early September, then again in October. More statistics are available here. Persons traveling to areas near the Atlantic Basin should exercise caution during the entire Hurricane Season.

EASTERN PACIFIC: The Eastern Pacific basin's hurricane season is from May 15th to November 30th, peaking in late August or early September.

WESTERN PACIFIC: The Western Pacific basin's hurricane season is mostly from July 1 to November 30, peaking in late August or early September, though storms can occur year-round.

SOUTH PACIFIC: The South Pacific basin's hurricane season is from October 15 to May 15, reaching a peak in late February or early March.

INDIAN OCEAN: The Indian basin's hurricane season is from April 1 to December 31 for the northern Indian Ocean, and from October 15 to May 31 in the southern region.

Students would be asked to copy the diagram of the hurricane formation and label it. For higher Year groups I would ask pupils to place on their diagram the relatively high and low wind speeds, a place of calm and the point of most rapid drop of pressure.  

http://www.youtube.com/watch?v=zP4rgvu4xDE

Links: http://www.accuweather.com/en/weather-blogs/hurricanefacts/when-and-where-do-hurricanes-o/31028

Air masses

Air masses bring a variety of weathers. The UK has 6 air masses making it difficult to accurately predict weather unlike in other countries. Fronts form the boundaries of air masses which have differing properties and the most severe weather occurs when dry and cold polar continental fronts collide with warm humid maritime tropical air.

Different air masses can bring weather ranging from warm tropical days to cold and harsh arctic conditions.

The term ‘air mass’ was introduced by meteorologists from Bergen, Norway over 70 years ago. Air mass is a large body of air whose properties- temperature, humidity and lapse rate- are largely homogeneous over an area several hundred kilometres across.

3 factors determine the nature of air masses; source region, age, and modifications as they move away from their source region across the earth’s surface.

Primary classification- (A) Arctic, (P) Polar, or (T) Tropical, and the nature of the source region- (c) continental, (m) maritime.

A large variety of secondary air masses are also described. (E) Equatorial air for example. If a small (k) or (w) is attached to the air mass then this implies that the mass is either warmer or cooler than the surface. If warmer, the air mass is more stable.

Air Masses Prevalent for the British Isles are:

Arctic Air (A)

Continental arctic (cA): Extremely cold, very little moisture. Origin- Arctic Ocean in winter.

Maritime arctic (mA): Same source region but less dry and cold.

Polar Air (P)

Continental Polar (cP): Cold and dry, originating from high latitudes, typically as air flowing out of the polar highs. Brings rattling cold, dry and clear weather in weather and warm pleasant weather in summer.

Maritime Polar (mP): Cool and Moist. Often originating as cP over North America and Asia land mass. It modifies as it heads over Atlantic and Pacific oceans. Heated by relatively warm water masses. This makes the air mass relatively unstable and can result in blustery showers.

Tropical Air (T)

Continental Tropical (cT): Hot and very dry. Origin- arid desert reasons during summer. Least common air mass to affect Britain but can affect record temperatures in the south-east.

Maritime Tropical (mT): Mild and damp in winter, warm and muggy in summer. Origin- the Azores and approaches from the west. Results in overcast skies and prolonged rain.

Lesson starter:

I would ask students to label their maps of the UK with the correct air masses in the right places.
I would give Key Stage 3 students a paragraph and a list of words that they would have to fill the gaps. However, for Key Stage 4 students I would see if they could fill the paragraph in with no words given to them.
http://www.youtube.com/watch?v=hNz6eqzPZFc


Links

http://www.geographylwc.org.uk/A/A2/A2climchange/airmasses.html

http://www.weatheronline.co.uk/reports/wxfacts/Air-masses-and-their-sources.htm

Factors affecting climate

When I researched the topic there was a few key areas that I was aware of, yet had definitely not studied in great detail. I decided to research the factors that affected a regions climate and came up with two key reasons.

Latitude or distance from the equator:

Temperatures drop the further a particular area is away from the equator due to the curvature of the earth. Areas that are close to the poles enables sunlight to have a larger area of atmosphere to pass through and the sun is at a much lower angle in the sky. Therefore,more energy is lost which results in cooler temperatures. Moreover, such large volumes of presence of ice and snow nearer the poles cause a higher albedo which is a measure of how much sunlight a surface reflects therefore means more solar energy is reflected, accumulating to the cold: this is explained in the figure below.

Altitude or height above sea level:

Locations that are situated at higher altitude have cooler temperatures. Temperatures usually decrease  1°C by for every 100 metres in altitude.

Distance from the sea:

Oceans heat up and cool down much slower than land. This means that coastal locations tend to be cooler in summer and warmer in the winter more so than places inland at the same latitude and altitude. One example that students will be able to relate to is that Glasgow is at a very similar latitude to Moscow, however, it is much milder in the winder because it is nearer to the coast than Moscow.

Links:

http://www.bbc.co.uk/schools/gcsebitesize/geography/weather_climate/climate_rev3.shtml