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Weather hazards

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Key idea Specification content
Global atmospheric circulation helps to determine patterns of weather and climate. General atmospheric circulation model: pressure belts and surface winds.
Tropical storms (hurricanes, cyclones, typhoons) develop as a result of particular physical conditions. Global distribution of tropical storms (hurricanes, cyclones, typhoons).

An understanding of the relationship between tropical storms and general atmospheric circulation. Causes of tropical storms and the sequence of their formation and development.

The structure and features of a tropical storm.

How climate change might affect the distribution, frequency and intensity of tropical storms.

Tropical storms have significant effects on people and the environment.


Primary and secondary effects of tropical storms.

Immediate and long-term responses to tropical storms.

Use a named example of a tropical storm to show its effects and responses.

How monitoring, prediction, protection and planning can reduce the effects of tropical storms.

The UK is affected by a number of weather hazards. An overview of types of weather hazard experienced in the UK.
Extreme weather events in the UK have impacts on human activity. An example of a recent extreme weather event in the UK to illustrate: • causes • social, economic and environmental impacts • how management strategies can reduce risk.

Evidence that weather is becoming more extreme in the UK.

Global atmospheric circulation

Global atmospheric circulation helps explain the location of world climatic zones and the distribution of weather hazards. The most important influence on worldwide variations in climate is latitude.

Latitude influences climate because

of the curvature of the Earth’s surface therefore the equator receives more insolation than higher latitudes. Sun is more concentrated at the equator.


Atmospheric circulation is the large scale movement of air by which heat is distributed on the surface of the Earth. It involves many circular movements called cells.


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Surface winds

Rainfall is high near the equator – hot air rises (low pressure system) = tropical rain forests.

Rainfall is low around the tropics of Capricorn and Cancer – because air descends (high pressure system) resulting in arid conditions.

Surface winds are important in the transfer of heat and moisture from one place to another. Winds on the surface of the Earth are experienced as air moves from high pressure to low pressure belts in the convection cells. On the surface these winds bend because of the Earth spinning, this is called the Coriolis Force

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Tropical storms



Tropical cyclones are amongst the most powerful and destructive meteorological systems on earth. Globally, 80 to 100 develop over tropical oceans each year. Many of these make landfall and can cause considerable damage to property and loss of life.


What is a tropical cyclone?

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A tropical cyclone is the generic term for a low pressure system over tropical or sub-tropical waters, with organised convection (i.e. thunderstorm activity) and winds at low levels circulating either anti-clockwise (in the northern hemisphere) or clockwise (in the southern hemisphere). The whole storm system may be five to six miles high and 300 to 400 miles wide, although sometimes can be even bigger. It typically moves forward at speeds of 10-15 mph, but can travel as fast as 40 mph. At its very early and weak stages it is called a ‘tropical depression’. When the winds reach 39 mph it is called a ‘tropical storm’. If the wind should reach 74 mph or more the tropical storm is called a ‘hurricane’ in the Atlantic and the north-east Pacific or a ‘typhoon’ in the north-west Pacific. In other parts of the world, such as the Indian Ocean and South Pacific the term ‘cyclone’ or ‘tropical cyclone’ is used.

How do tropical cyclones form?

In the tropics there is a broad zone of low pressure which stretches either side of the equator. The winds on the north side of this zone blow from the north-east (the north-east trades) and on the southern side blow from the south-east (south-east trades).

Within this area of low pressure the air is heated over the warm tropical ocean. This air rises in discrete parcels, causing thundery showers to form. These showers usually come and go, but from time to time, they group together into large clusters of thunderstorms. This creates a flow of very warm, moist, rapidly rising air, leading to the development of a centre of low pressure, or depression, at the surface.


There are various trigger mechanisms required to transform these cloud clusters into a tropical cyclone. These trigger mechanisms depend on several conditions being ‘right’ at the same time. The most influential factors are:

a source of warm, moist air derived from tropical oceans with sea surface temperatures normally in the region of, or in excess, of 27 °C;
winds near the ocean surface blowing from different directions converging and causing air to rise and storm clouds to form;
winds which do not vary greatly with height – known as low wind shear. This allows the storm clouds to rise vertically to high levels;
sufficient distance from the equator to provide spin or twist.
The Coriolis force caused by the rotation of the Earth helps the spin of this column of rising air. The development of the surface depression causes an increase in the strength of the trade winds. The spiralling winds accelerate inwards and upwards, releasing heat and moisture as they do so.

As the depression strengthens it becomes a tropical storm and then a hurricane or typhoon. A mature hurricane or typhoon takes the form of a cylinder of deep thundercloud around a centre that is relatively free from clouds. There is a relatively small area of intense horizontal winds at the surface, often well over 100 m.p.h., while air rises strongly above, maintaining the deep cumulonimbus clouds.

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Further aloft at about six miles, the cloud tops are carried outwards to give thick layer clouds due to the outward-spiralling winds leaving the tropical cyclone core. At the centre of the tropical cyclone, air is subsiding, which makes it dry and often cloud free, and there is little or no wind at the surface. This is called the eye of the storm.

How are tropical cyclones ranked?

Although developed in the USA, the Saffir-Simpson hurricane wind scale is used to rank tropical cyclone wind strength in many parts of the world.

Category 1 – sustained wind speeds of 74 to 95 m.p.h
Category 2 – sustained wind speeds of 96 to 110 m.p.h
Category 3 – sustained wind speeds of 111 to 129 m.p.h
Category 4 – sustained wind speeds of 130 to 156 m.p.h
Category 5 – sustained wind speeds greater than 156 m.p.h


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Other phenomena which can be just as damaging than the wind frequently accompany tropical cyclones:

high seas – large waves of up to 15 metres high are caused by the strong winds and are hazardous to shipping;
storm surge – a surge of water of up to several metres can cause extensive flooding and damage in coastal regions;
heavy rain – the tropical cyclone can pick up two billion tons of moisture per day and release it as rain. This also leads to extensive flooding – often well inland from where the tropical cyclone hit the coast;
tornadoes – tropical cyclones sometimes spawn many tornadoes as they hit land which can cause small areas of extreme wind damage

imageStorm surge

Case studies

HIC case study: Hurricane Katrina, New Orleans, USA


Hurricane Katrina struck in August 2005. It tracked over the Gulf of Mexico and devastated most of the coastline from Louisiana to Alabama. It arrived as a category 4 storm with winds of over 140 mph and a storm surge of approximately 6 metres. The city of New Orleans was very badly affected because it is mostly below sea-level and is surrounded by water. The city was protected by defence walls called levees. However, the levees were overwhelmed by the extra water from the storm surge and rainfall, and many collapsed allowing water to flood into the city. About 80% of the city was flooded to depths of up to 6 metres.

The National Hurricane Centre predicted accurately where Hurricane Katrina would make landfall and how strong it would be. This gave people the opportunity to prepare for the storm. The mayor of New Orleans ordered people to evacuate the city. About 80% of the city’s residents did so, but about 20% remained. The majority of these were in the poorest areas of the city (people had little access to transport so couldn’t leave in many cases).

Over 10,000 people sought refuge in the city’s Super dome football stadium. Conditions here deteriorated quickly – food and water soon ran out and the toilet facilities were inadequate. The atmosphere in the stadium was described as ‘very tense and unsafe’.


More than 80% of the city was submerged with floodwater and over 1200 people drowned. Approximately 1 million people were made homeless and thousands of businesses were destroyed. Thousands of jobs were lost and millions of dollars lost in tax income. There was a lot of looting. Criminal gangs roamed the streets, looting homes and businesses and committing other crimes.

Major highways were disrupted and some major road bridges were destroyed. Agricultural production was badly damaged by the tornadoes and also by flooding. Cotton and sugar-cane crops were flattened.

Hurricane Katrina didn’t just impact people in the USA. Many offshore oil facilities were damaged and supplies of oil were reduced. This caused the price of oil to rise on the global markets and the price of petrol in the UK rose as a result.

Estimates suggest that Hurricane Katrina has cost over $300 billion. This makes it one of the costliest hurricanes ever to hit the USA.

LIC case study: Cyclone Sidr, Bangladesh


Cyclone Sidr formed in the central Bay of Bengal and quickly strengthened to reach sustained winds of 160 mph, making it a category 5 storm. The storm eventually made landfall in Bangladesh on November 15, 2007. It weakened quickly after landfall.

The Joint Typhoon Warning Centre predicted the scale and location of landfall and so people were forewarned. Government officials were recalled from their weekly leave. Ports were closed. There was mass-scale evacuation of the coastal area, much of which is land below sea-level. The image below shows the storm surge prediction for the area. 2 million people were evacuated to emergency shelters. Over 40,000 Red Crescent volunteers were deployed to order residents in the 15 affected provinces into special cyclone and flood shelters (see image below). Relief organisations distributed seven-day emergency disaster kits of food, blankets and clothing for evacuated families.


Coastal districts of Bangladesh faced heavy rainfall as an early impact of the cyclone. The damage in Bangladesh was extensive, including tin shacks flattened, houses and schools blown away and enormous tree damage. The entire cities of Patuakhali, Barguna and Jhalokati District were hit hard by the storm surge of over 5 metres. About a quarter of the world heritage site Sunderbans were damaged. Researchers said mangrove forest Sunderban will take at least 40 years to recover itself from this catastrophe. Much of the capital city of Dhaka was also severely affected, as electricity and water service were cut and significant damage was reported there due to winds and flooding.The local agricultural industry was also devastated, as many rice crops — which have a December harvest — were lost.


Measuring and recording tropical cyclones

Tropical storms are a moving hazard, so they must be tracked and forecasts made of their future progress. That is what meteorologists do.

If they can measure how they are developing, they they can warn people in the predicted path of the storm. This should give people time to prepare such as moving to higher ground (avoid storm surge) or to an emergency shelter. Homes can be made ready by boarding up windows and moving furniture upstairs.

The media which includes TV, radio and the Internet play an important role in keeping the general public updated about the storm and where it is expected to go.

Data meteorologists work on comes from a number of different sources:

Weather Stations
There is a global network of weather stations that track the movement of tropical storms. Some are manned, some are automatic, some monitor the weather all the time and others just at set hours during the day and night. Once all this information about pressure, temperature, humidity, winds and so on is collected and put together, it can be used to predict what will happen to the storm. Will it deepen, with an increase in rainfall and wind speeds or will the storm begin to weaken and fizzle out?

Weather Satellites
These are important for viewing large weather systems on a worldwide scale. They show cloud formation, large weather events such as hurricanes, and other global weather systems. With satellites, forecasters can see weather systems such as tropical storms.
On each satellite, there are 2 types of sensor. One is a visible light sensor called the imager. It works like a camera in space and helps gather information on cloud movements and patterns. This sensory can only be used during daylight hours, since it works by capturing reflected light to create images.
The second sensory is the sounder. It is an infrared sensor that reads temperatures. The higher the temperature of the object, the more energy it emits. This sensory allows satellites to measure the amount of energy radiated by the Earth’s surface, clouds, oceans, air etc. Infrared sensors can be used at night which is helpful for forecasters, considering that the imagers can only pick up data during daylight hours.

Doppler radar is another important meteorological tool. Radar works a little differently from satellite sensors. Instead of reading reflected light or energy, radar measures reflected sound waves. When sound waves are broadcast from a radar mast and come into contact with a moving object, such as a rain cloud, radar will give information about the direction and speed of the object’s movement. By using radar and getting a ‘picture’ of precipitation (e.g water falling to the ground) on the radar screen, meteorologists are able to track a storm’s progress over time.



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Microclimate data can be collected in any location, including the school grounds. This makes it a simple fieldwork investigation to carry out.

The lists below give you an idea of some of the ways that data collected within a microclimate investigation can be used.

  • To investigate microclimatic data at a small scale, within a school grounds, or large scale, for example an urban transect passing from green-belt through a variety of city environments
  • To investigate the abiotic conditions of different parts of a particular ecosystem
  • To investigate diurnal or seasonal changes in the microclimate
  • To investigate the influence of microclimate on something, for example the distribution of a particular species of plant or vegetation cover
  • To compare different locations
  • To investigate the affect of topography on microclimates
  • To investigate the impact of human interference, or features of the built environment on the microclimates of different locations
  • To investigate the most suitable location for something
  • To link with and incorporate into ecosystem investigations
  • To link with other data, for example soil analysis, invertebrate data

Microclimates: Why not try…?

  • The old classic – the most suitable site for…
  • Assessing the possible impacts of a new building on microclimates, for example the building of a new science block
  • Asking a question, for example why is site x so popular with sunbathers? Do the buildings at site y create wind tunnels (known as the venturi effect)?
  • How much does vegetation cover affect microclimatic conditions? Different types, densities or ages of vegetation communities could be investigated
  • How do microclimates affect people’s activities or their perceptions of place? For example questionnaires to investigate how pupils view and use different areas in the school grounds
  • How does proximity to water affect microclimates
  • How large or wide an area of microclimate is affected by buildings
  • During a heat wave – how much more extreme are the microclimates of urban areas
  • Considering hedgerows as microclimates – how do they affect local conditions and what might be the ecological impact of their removal? You could link this with ecosystem data

Wind speed and direction


  • Weather vane
  • Anemometer or Ventimeter
  • Compass
  • Beaufort scale
  • Record sheets


  1. A compass should be used to determine North
  2. Wind direction ‘apparatus’ can be improvised using a home made wind sock or weather vane. The weather vane should be placed in the exact location to be investigated, and the variable, for example height above ground kept constant at each site
  3. The Beaufort scale can be used to give a crude, qualitative judgement of wind speed by observing evidence around the site. Further information about the Beaufort Scale can be found on the Met Office website and a pictorial guide on the Howtoons website
  4. An anemometer can give a more accurate reading, and will work in very low wind conditions, but is expensive. A ventimeter is cheaper but is not as reliable or as accurate in low wind conditions
  5. Readings should be recorded at each location

Considerations: Limitations and validity

  • Very high or low wind speeds can be difficult to measure
  • Wind strength is hard to measure at ground level
  • The Beaufort scale is subjective. You need to consider that the observations are likely to come from around the site, rather than at the exact location. Is it therefore an accurate method for a micro-climate assessment?
  • Taking several readings and finding the average can increase validity of results

Temperature, relative humidity and light levels


  • Thermometer
  • (Whirling) hygrometer/psychrometer
  • Light meter
  • Record sheet


  • Different instruments operate differently and the instructions should be referred to for each
  • The whirling hygrometer or psychrometer should be held above the head for a set period of time, for example one minute and readings are then taken from the wet and dry bulbs. It is the difference between these two readings which informs relative humidity
  • Digital thermometers are reliable and precise but may not be if poorly calibrated or the batteries run low (always check the batteries)
  • Whether using a digital or analogue thermometer, recordings should be taken at the same height above ground at each site, for example one metre
  • Digital light meters are again more accurate, but the same applies as for all digital equipment in terms of calibration and battery life

Considerations: Limitations and validity

  • A sampling method should be decided upon to ascertain the method for data collection, for example the locations to sample, the timings and frequency of recordings
  • Some account should be taken of the fact that recordings will inevitably be taken at different times in different locations – while cloud cover can change from one moment to the next, affecting temperature and light readings
  • Allow for some margin of error in using the instruments – different products vary in accuracy and performance
  • If using an analogue thermometer breakage is a health and safety consideration. Also, readings may be affected by direct sunlight, or hand-heat. Ground temperatures are more extreme, so readings should not be taken directly on the ground
  • Cloud cover at the time of taking measurements could be recorded to help explain anomalies in data. Cloud cover is estimated in Oktas which refers to how many eighths of the sky are covered by cloud, using the following scale:
    • Clear sky
    • 1 okta
    • 2 oktas
    • 3 oktas
    • 4 oktas
    • 5 oktas
    • 6 oktas
    • 7 oktas
    • Overcast

Cloud type could also be recorded, as this affects light intensity.



  • Rain gauge
  • Record sheets


  1. Home made gauges can be used – Ensure the same ones are used to ensure a fair test
  2. Gauges should be set up in the desired locations
  3. Records should be kept of anything at each site which may affect readings, for example shelter from buildings or vegetation cover
  4. Leave gauges for a predetermined period of time at each location
  5. For remote locations, bucket-siphon rain gauges can be used to take measurements which empty themselves daily

Considerations: Limitations and validity

  • Consider the accessibility of the site, is it local or remote. Which is better
  • Time factor, checking every day
  • Practicality of checking all gauges at the same time
  • Affect of vegetation, interception of rain or buildings providing shelter, these should be noted, but may be interesting variables to investigate in their own right
  • Evaporation, some open gauges allow evaporation which will affect readings
  • Rain splash, gauges flush with the ground level may be affected by rain splash, therefore over estimating precipitation
  • Also, extremely heavy rain may cause excess runoff (especially from some surfaces) which may run into flush rain gauges



Rain gauges raised above ground level may underestimate precipitation as rain may be funnelled around the gauge

Measuring, collecting and recording weather data:
During the passage of a tropical storm, local weather stations will record an enormous increase in wind speed and rainfall.
Instrument area is used to measure local weather conditions in calmer, drier conditions-providing primary data.
Care and accuracy important when measuring weather-instrument itself has to be suitable as well as its use accurate.
You Should have an easy to complete record sheet showing date, time and columns for each element of the weather you have instruments for. Eg maximum/minimum temperature and rainfall.
Records should be kept daily and for at least a week. Readings should be taken preferably at same time each day.

Rain Gauge:
It should be placed in open space so it can collect rain water straight from the sky.
Rain is collected in a measuring flask and the measurement can be read easily.
Once reading is noted, the water has to be tipped away daily.

Stevenson Screen:

Instruments used to measure temperature and humidity should be kept inside a Stevenson Screen.
It’s a wooden box used to shade from direct sunlight and radiation so that the instruments inside can measure air temperature.
It’s painted white to reflect sunlight and has vents to allow free flow of air. This makes the readings fair.
Maximum-minimum thermometer housed inside measures the highest and lowest temperature, often within a 24-hour period. –weather data should be standardised.
Readings have to be taken so that they can be compared with those taken at other places and at other times.
After noting temperature readings, the thermometer has to be reset by sliding the magnetic base over the mercury columns.

Cup Anemometer and wind valve:
Wind valve measures wind direction.Cup anemometer is a weather instrument that measures wind speed/strength.
There are 3 to 4 cups mounted on a vertical pole. The cups catch the blowing wind and turn the pole.
Each time the anemometer makes a full rotation, the wind speed is measured by the number of revolutions per minute (RPM).
The number of revolutions is recorded over time and an average is determined.



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