Extreme Weather: Unit 2

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3 Important Weather Elements

There are 3 important elements to the weather and undertanding weather systems.




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What factors determine the temperature of the atmosphere?

The Greenhouse Effect: (GHG: Nitrous Oxide, Water Vapour, CO2, Methane.)


Other factors include: distance from the equator, the amount of concentrated solar energy, the height of land, distance from the sea, amount of cloud cover and ocean currents.

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Factors affecting the atmospheric temperature:

Sun light: As this increases, the temperature of the atmosphere increases.

Radiated Heat: As this increases, the temperture of the atmosphere increases. This is because the the atmosphere is not warmed by the sun directly, but by the heat radiated by the Earth's surface.

Distance from the coast: Coastal regions have a smaller temperature range than inland areas. This is because the land heats and cools quicker than the sea due to the heat capacity of water being much larger than land.

Polar Maritime: The temperature of an air mass is determined by its origin. Winds blowing from the sea tend to be warmer in summer and cooler in winter than winds that blow over land.

Cloud cover: Clouds reflect, absorb and scatter insolation during the day, but also act as a 'blanket' at night. Clear skies during the day will increase air temperature but clear skies during the night will decrease air tempertures.   

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Steps of air pressure:

1) The sun provides the primary energy source for wind on the earth, the sun's rays heat the ground surface.

2) As the ground gets warmer, it begins to transfer heat to the air above. This is known as convection.

3) The air becomes warmer and less dense so it rises, a LOW pressure system is created. Rising air cools as it moves up into the atmosphere.

4) As the air cools it stops rising and begins to move parallel to the eart instead.

5) The air is much cooler and has increased in density and begun to descend towards the earths surface. This creates HIGH pressure.

6) Wind is created on the ground because of the differences in air pressure, wind will always travel from HIGH to LOW air pressure systems.

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Humidity: Water vapour content in the atmosphere.

Condensation from high humidity forms larger clouds which causes precipitation. (convectional rainfall)

Low Pressure brings:

Windy, wet, cloudy, stormy weather conditions.

High Pressure brings:

Dry, arid weather. Hot in summer and cool in winter.

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The General Atmospheric Circulation System

The atmospheric circulation system consists of 3 types of cell, Polar, Ferrel and Hadley.    


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The General Atmospheric Circulation System

POLAR CELL: Located near the North and South pole, low in altitude. Provides no snow, but lots of ice.

FERREL CELL: Located near the UK and Canada, latitudes between 30 and 60 degrees. Provides both rain and dry periods in the North and no rain the the South.

HADLEY CELL: Located around the equator, latitudes between 0 and 30 degrees. Provides dry weather in the North and more intense convestion in the South.

ITCZ: The Inter-tropical convergence zone. Moves throughout seasons and causes flooding and drought in the area that it affects.

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Introductions to Air Masses and Depressions

What are isobars on a synoptic chart? Lines which show the air pressure, the closer together they are the lower the pressure is.

What causes high pressure? The cool air sinks and puts pressure onto the earth surface.

What causes low pressure? The rising of warm air, convection

Where are strong winds found? In areas between High and Low pressure systems.

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Air Masses

POLAR MARITIME: Brings cold and wet weather.

POLAR CONTINENTAL: Brings cold and dry weather.

TROPICAL CONTINENTAL: Brings dry and warm weather.

TROPICAL MARITIME: Brings warm and wet weather.

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Air Masses


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Extreme Weather in the UK: Depressions

The most changeable weather in the British Isles in the short-term is the result in the passage of a depression. A depression can bring strong winds, belts of rain, shifts in wind direction and marked changes in temperatureand sunshine within a few hours.

A depression is an area of LOW atmospheric pressurewitha roughly circular pattern of isobars that occur in the mid-latitudes. Most depressions form along the Polar Front.

The Polar Front Theory:

The  polar front is the boundary between the Polar maritime air from the NW Atlantic which is cold, dense and moist and moves South. And the Tropical maritime air from the SW which is warmer, less dense and moist and moving North.

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Extreme Weather in the UK: Depressions

Depressions follow a life cycle in which 3 main stages can be identified:

1:Embryo Depression

As the 2 bodies of air move towards eachother the warmer, less dense air from the South rises above the colder, dense air from the North. The rising air is removed by strong, upper atmosphere winds (Jet Stream), but as it rises the Earth's rotational spin causes it to twist (anticlockwise in the UK). This twisting votrex produces a wave at the polar front, where there is less air at the Earth's surface, creating low pressure. This increases in size to form a depression. This rotation is known as the Coriolis force.

Two separate parts of the original front have now developed:

The warm front at the leading edge of the depression where warm, less dense air rises over the colder air ahead.

The cold front at the rear of the depression where colder dense air pushes the warmer air ahead.

In between these lies the warm sector-an area of wram moist air. The depression moves largely in a SW to NE direction due to the Jet Stream.

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Extreme Weather in the UK: Depressions

2: A Mature Depression

In the warm sector: Pressure continues to fall as more warm air is forced to rise. The pressure gradient steepens causing an increase in wind speed and strength. Along and just infront of the warm front: The warm air of the warm sector continues to rise along the warm front, it eventually cools to dew point. Condensation occurs and clouds form. Continued uplift and cooling cause precipitation as the clouds become thicker and lower along and infront of the warm front.

As the temperature rises and the uplift of air decreases within the warm sector, there is less chance of precipitation and the low cloud may break up and give some sunshine.

Along the cold front: The cold front moves faster and has a steeper gradient than the warm front. Progressive undercutting by cold air at the rear of the warm sector gives a second episode of precipitation- although with a greater intensity and a shorter duration than the warm front. The air behind the cold front is colder than that in advance of the warm front. Winds often reach their max strength at a the cold front and change to a more northwesterly directionafter its passage.

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Extreme Weather in the UK: Depressions


This occurs when the cold front catches uo with the warm front to form an occlusion or occluded front.

The tropical maritime air will have been squeezed upwards leaving no warm sector at ground level. As the uplift of air is reduced, so too ae the amount of condensation and the pattern of precipitation- there may only be one episode of rain.

Cloud cover begins to decrease, pressure rises and wind speeds decrease as the cold air replaces the uplifted air and infills the depression.

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Simplified Development of a Depression

1.Polar maritime air moves from SW to NE under the influence of the jet stream.

2.The tropical maritime air from the SW, which is warmer, less dense and moist moves North.

3.As the depression matures, the less dense, warm air rises over the cold air ahead. As it rises, it cools and condenses forming clouds and eventually rainfall along and just in front of the warm front.

4.The warm sector is formed between the two fronts, this is an area of warm, moist air where there is less uplift of air and therefore preciitation is light and there may be some cloud breaks and sunshine.

5.The cold front moves faster than the warm front, therefore the war, air is forced to rise rapidly producing more intense rainfall than the warm front and of a shorter duration. The air pressure drops resulting in strong winds.  

6.there is low pressure at the centre of the depression due to rising air. The whole depression moves moves from SW to NE due to the jet stream.

7.An occluded front forms when the cold front catches up with the warm front, all warm air within the warm sector is pushed upwards. Air uplift is reduced and the pressure rises, windspeeds increase and the rain will stop.

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Impacts of Depressions

High winds and gales: Strong wids brought by steep pressure gradient which is brought by extreme contrasts in temperature between the converging polar and tropical air masses. Gales have a huge impact of infrastructure such as power and transport.

Coastal Flooding: (East Anglia 1953 and Towyn, Feb 1990) On-shore gale force winds drawn towards a deep depression, which leads to a storm surge developing as water is 'sucked  up'. This is combined by high spring tides, huge breakers are diven downwards, breaking sea defences and causing serious localised flooding.

Violent Thunderstorms: Associated with violent uplift at the cold front which leads to the towering of cumulonibus clouds. Occasionally violent hailstrorms occur as in London 1968 when hail the size of tennis balls fell.

Heavy rainfall: Depressions which remain static can cause heavy rainfall, as the warm air within them can hold large quantities of moisture. As in 1952 Lynmouth where between 50 and 100mm of rain fell in a 24 hour period.

Snow: The SW blizzard in Feb 1978 wich led to 100-200mm of snow. Can affect transport links.

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What is an anticyclone?

A large mass of subsiding air which produces an area of high pressure on the earths surface. The source of the air is in the upper atmosphere, where humidity is low. As the dry ar descends, it warms, which limits cloud development and produces dry conditions. The isobars are far apart as the pressure gradient is gentle and therefore winds are weak and blow outwards from the centre of high pressure. The winds blow clockwise in the Northern hemisphere. Anticyclones may be 3000km in diameter (larger than depressions) and they move slowly giving days or weeks of settled weather.

Winter weather during an anticyclone:

DAY: Cold temperatures, (below 0 to 5 degrees) as there is little incoming radiation due to low sun angle.

NIGHT: Cloudless nights result in cooler temperatures of below freezing with frosts and the development of radiation fog and temperate inversions.

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Summer weather during an anticyclone:

DAY: Sunny with hot temperatures (25 degrees +) due to the absence of cloud cover.

NIGHT: Warm temperatures (up to 15 degrees) as the warm ground radiates heat.

Why are fogs common in anticyclonic conditions?

Fog is cloud at ground level, which restricts visibility to less than 1km. It consists of tiny water droplets suspended in the atmosphere. There are 2 main types of fog: Fadiation and advenction fog.

Difference between Anticyclones and Depressions

A/C: Rotates clockwise, High pressure, little wind, no rain.

D: Anticlockwise, Low pressure, wind, rain.

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Radiation Fog

Radiation fog usually occurs in the winter, aided by clear skies and calm conditions. The cooling of land overnight by thermal radiation cools the air close to the surface. This reduces the ability of the air to hold moisture, allowing condensation and fog to occur. Radiation fogs usually dissipate soon after sunrise as the ground warms. An exception to this can be in high elevation areas where the sun has little influence in heating the surface.


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Advection Fog

Advection fog occurs when moist air passes over a cool surface and is cooled. A common example of this is when a warm front passes over an area with snow cover. It is also common at sea when moist tropical air moves over cooler waters. If the wind blows in the right direction then sea fog can become transported over coastal land areas.


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Extreme Weather Caused by Anticyclones

High air pressure/air quality: 50% of Earth's population is affected by poor air quality. Severl hundred thousands people die due to poor air quality.

Heatwaves and Forest Fires: Greece, USA forest fires 2000. Canada forest fire 2016, evacuated a whole town.

Smog: UK, ozone levels.

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Differences Between Depressions and Anticyclones

Depressions: Surface pressure, LOW                     Anticyclones:  HIGH

                      Vertical air movement: UP                                        DOWN

                      Wind speed: HIGH                                                    LOW

                      Wind direction: ANTICLOCKWISE                           CLOCKWISE

                      Speed of movement: FAST                                      SLOW

                      Cloud type: CUMULONIMBUS                                 THIN

                      Precipitation: HIGH                                                   LOW

                      Weather: RAIN, FLOOD.                                          HEATWAVE, DROUGHT.

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Hurricanes are violent storms between 200 and 700 kilometers in diameter. They occur in the latitudes of 5 and 20 degrees North and South of the equato. They only occur over warm oceans above 26 degrees and of atleast 70m in depth.

Formation of a hurricane:

1. Warm ocean causes warm, moist air (+75% humidity) to rise (convection) and therefore pressure falls. The rising warm air cools as it gets further into the atmosphere and clouds are formed. (continuous upward convection)

2. The rising air begins to rotate and the hurricane fllows a spiral path due to the coriolis force.

3. Condensation releases latent heat, this warms the air and it rises further, cools, condenses and releses further latent heat forming continuous upward convection currents. Thick vertical cumulonibus clouds grow rapidly forming thunderstorms. This causes a further fall in pressure.

4. Convergence occurs: warm, moist air is drawn into the low pressure system from different directions causing high speed winds at the earts surface. 

5. The eye-between 5 and 50km in diameter, low wind speeds, clear skies and high temperatures. Descending air increaes storm intensity by warming. Formation not fully understood. Upper atmosphere air rotation spreads air outwards and away from the storm.

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Where Do Hurricanes Form?

Over warm ocean where ITCZ sea temperatures exceed 26 degrees: The ITCZ is a low pressure area, where the trade winds meet. The warm oceans provide the energy required to drive the system-cause large scale upward convection of moist air fuelling continuous upward convetion currents and formation of low pressure leading to high wind speeds.

A location of 5-20 degrees N & S of the equator

In the Autumn months: When ocean temperatures are at their highest.

An ocean depth of 70m: To provide sufficient heat energy for continuous upward convection currents.

The move NW in the Northern hemisphere:  Due to the coriolis effect and the curvature of global winds and ocean currents due to the earths rotation on its axis.

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Tornadoes/Case Study

A tornado is a violent windstorm often associated with severe thunderstorms. Windspeeds of 250mph, a narrow tiwisting funnel consisting of vast cumulonimbus clouds.

Tornado Case Study: Birmingham 2005

Details: Flying debris, cars and property damaged, buildings left without rooves, vast devastation, Lasted 4 minutes, travelled 7 miles, 25 second exposure.

Causes: Warm air moving over Birmingham, erratic cloud movement, severe thunderstorms.

Impacts: Destructions of businesses , 100 homes destroyed, domino effect, cost millions, high insurance claims, No deaths, 20 injured, struck fear into residents, winds over 209 km/h, uprooted trees.

Management: Sand bags, vast clean up operation, 100 people spent night in emergency accomodations, areas covered with plastic covers for protection, Reconstruction of infrastructure.

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A drought is a lack or shortage of water for an unusually long period of time involving 50% less rainfall over 3 months.

Causes of drought include:

Constructing a dam on a large river may severely reduce river flow further downstream, Widespread deforestation reduces soils ability to hold water this triggers desertification, climate change, prolonged periods of warm weather.

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Drought Case Studies

Californian Drought: 2007

Where? West America, California.

Primary Hazards: Lack of water, Forest fires.

Secondary Hazards:Destruction of crops and livestock, electricity shortages.

Impacts: Farm job losses, destruction of seafood business, tourism declined, food and energy costs rose, 6 heat related deaths, deserification, no livestock, plants and ecosystems died due to lack of moisture.

Management: Government help, state of emergency released funds to population. 1,018 heat related inspections. Cal-OSHA suggested farms have more shade for workers and livestock, more toilet facilities and water within a short distance.

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Drought Case Studies

East African Drought: 2011

Where? Easterly region of Africa, Ethiopia, Somalia and Kenya.

Causes: No rainfall for 5 years, population increase meant water supplies were extremely low.

Primary Hazards: No food or water, supplies got more scarce=cost of food rose, high demand for supplies.

Secondary Hazards: Loss of livestock and crops, malnutrition.

Impacts: Economic growth fell by 0.3%, power outages, thousands died, 80% suffering from malnutrition, mass migration, famine, degradation of land, desertification.

Management: Foreign aid, supplies from water aid, and Oxfam.

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