Download Chapter 7 - Weather-Forming Systemsl

CHAPTER 7
Weather-forming systems
In this chapter we examine the more tangible elements of the atmosphere and how they
produce the weather elements which we experience. This makes it easier to relate with
the types of weather observed. Students can record the elements of weather, such as
temperature, wind speed and direction, cloud estimates, amount of precipitation,
depending upon resources. These can then be related to weather maps available on
television, in newspapers, or via the Internet. It can be useful to obtain weather forecasts
and assess their accuracy. Satellite images are widely available too and, if current, can be
used to relate surface weather observations at a single site to the wider weather-forming
systems. The Internet is a vital source of weather information and satellite images.
Useful sites are listed at the end of this chapter section.
Weather-related disasters occur quite frequently around the world. With the information
provided in this and other chapters in the book it may be possible to undertake an indepth study of the atmospheric conditions which gave rise to a disaster and their
consequences. With hindsight, could the disaster have been predicted? How could the
people affected have been warned?
Air masses
•
An air mass can be defined as a large, uniform body of air with no major
horizontal gradients of temperature, wind or humidity.
•
The majority of true air masses develop over the anticyclonic parts of the world.
•
The character of the air mass will depend upon the surface over which it forms.
•
Many systems have been used to classify air masses; all have advantages and
disadvantages.
•
As air masses move away from their source regions they become modified by the
underlying surface over which they pass.
Weather-forming systems of temperate latitudes
•
Anticyclones are areas of relatively high pressure in which the air is
predominantly subsiding. As it does so, it becomes drier and warmer through
adiabatic compression.
•
If anticyclones take a semi-permanent location, the surface beneath will be
predominantly dry or desert-like.
•
Temperatures associated with anticyclones may be either hot or cold, depending
upon the time of year, the source of air and the nature of the surface.
•
Cyclones are areas of relatively low pressure with winds converging and rising as
they approach its centre.
•
Cyclones are often associated with sudden temperature changes (called fronts) as
warm air replaces cold air or vice versa.
•
Cyclones change their character with time as they evolve from a newly formed
low-pressure system to a more diffuse area of lower pressure.
•
The evolution of fronts was first proposed by Bjerknes. Details of the model and
ideas about the origins of the fronts have changed considerably.
Weather-forming systems of the tropics
•
Waves can be found in the tropical easterly flow which cause disturbed weather
over the oceans and may give rise to tropical cyclones.
•
Rotating cyclonic systems can develop from easterly waves or more poleward
parts of the ITCZ. Their intensity and degree of organization varies. Different
names are given for each increase in intensity. The strongest, where wind speeds
reach over 64 knots, are called tropical cyclones, or hurricanes, or typhoons,
depending on location.
•
Tropical cyclones develop only in certain parts of the tropical oceans where sea
surface temperatures are above about 26°. They decay over land areas as their
source of energy, the evaporation of water, is cut off.
Weather prediction
•
Weather prediction is now done by modelling the physical processes within the
atmosphere and using powerful computers to perform billions of calculations.
•
Different methods have to be used, depending upon whether we are trying to
forecast a few days ahead or several months ahead.
•
Ensembles of forecasts are now produced to indicate the likely outcome for a
given starting point. Each forecast starts from the observed state of the ocean,
land and atmosphere. Unfortunately, the initial conditions are not known precisely
and individual forecasts can be very sensitive to small uncertainties in these initial
conditions. This sensitivity is allowed for by making a number of predictions each
with slightly different variations to the starting conditions. The final forecast is
produced by analysing the output from all the ensembles.
CASE STUDY – Tropical Cyclone Sidr (12 – 16 November 2007)
Tropical cyclones or hurricanes are most widely publicized when they affect the United
States, such as Hurricane Katrina described in this chapter. However other parts of the
globe shown in Figure 7.17 can be affected as severely and with equally damaging
consequences. The Bay of Bengal is notorious for the severity of its tropical cyclones,
particularly because they come onshore in the Ganges Delta area of Bangladesh where
there is a very high population density. The rich alluvial soils of the Ganges and
Brahmaputra delta allow several crops of rice to be grown each year so they can support
large numbers of people. Unfortunately, the delta environment means that there is little
high ground to escape storm-induced floodwaters and large numbers of people can drown
following a severe tropical cyclone.
Such an example occurred in November 2007. The storm was first seen on November
9th as an area of enhanced cloud near the Andaman Islands in the Indian Ocean, southwest of Myanmar (Burma). Conditions were favourable for storm development and it
quickly intensified into a deep depression as it moved slowly north-westwards. Between
the 12th and 15th the storm intensified further as it headed slowly northwards towards the
delta (Figure 1)
Figure 1 Cyclone Sidr approaches the Ganges Delta at 1.25 pm local time, November
12th 2007. It was moving northwards at 10 miles per hour with winds near the cyclone
eye blowing at about 150 miles per hour. The eye can be seen to the centre-left of the
image as a darker patch because little cloud is reflecting insolation in this area.
Source: Accessed from http://earthobservatory.nasa.gov/Natural Hazards on 2 January
2008
By the time it made landfall at 1700 UTC on the 15th, Sidr had reached very severe storm
status with peak wind speeds of about 150 mph and a central pressure of 944 hPa. Once
the storm lost its main energy source of evaporation from the tropical sea, it rapidly
weakened but continued to give rainfall over the Assam Hills.
Because of the inevitable uncertainty in predicting the track of the storm a large area of
the delta in both Bangladesh and India could have been affected. The Indian
Meteorological Department issued a cyclone alert for Orissa and West Bengal with plans
being made for evacuation of lowlying areas in Kolkata. In the event, the cyclone passed
to the east of Kolkata. As the area of damaging winds is quite narrow in comparison with
a mid-latitude storm, the impact of the storm on the Indian part of the delta was small
unlike further east in Bangladesh. As the cyclone approached the delta, the site of
landfall could be more clearly established and evacuation plans put in place. About 17
million people live less than 1 metre above mean sea-level so this is a major logistical
problem. About 2 million people were evacuated to emergency shelters, often using
megaphones to warn people and encourage them to prepare for the storm. This contrasts
with the situation in developed countries where communications are so much better with
TV, radio, phones and the internet available to most of the population.
Along the track of the cyclone damage was extensive. Tin shacks were flattened, schools
and houses blown away and many trees uprooted, further affecting transport links. In the
coastal zone and along the tidal estuaries, a surge, reaching up to 5 metres, swept inland.
As the land surface is flat, the area affected by the surge is large. Even the mangrove
forest along the edge of the delta in an area known as the Sundarbans was extensively
damaged; researchers say it may take up to 40 years for the area to recover from this
damage. Inevitably food crops were also damaged by the floods and incursion of salt
water whilst many cattle and poultry were drowned.
Because of the difficulties in warning many of the residents of the delta and a lack of
emergency shelters, deaths were appreciable. Precise figures are unknown but early
estimates range between 5000 and 10,000; this includes the large number of fishermen
caught in the Bay of Bengal who were unaware of warnings. Some farmers may be
reluctant to leave their homes even if there are storm warnings; false alarms may have
been raised before. The survivors then have to be provided for as most will have lost their
food supplies and crops; drinking water and power will be a problem for some time until
repairs can proceed. It is not easy to put figures to the cost of damage but early estimates
give a value of $450 million; a vast sum for a poor country. Aid is often promised from
other sources but does not always materialise.
Although cyclone Sidr was classed as very severe, its impact was not as great as the
cyclones of 1991 when 139,000 people are estimated to have died and the cyclone of
1970 when 300,000 deaths occurred. Much has improved over this period of time so that
plans for evacuation, emergency shelters and better communications mean that more
people can get away from the affected areas. But as we saw with hurricane Katrina in
New Orleans, even the richest and best prepared countries of the world can be devastated
by tropical storms. To compound matters, global warming has led to higher sea surface
temperatures in the Bay of Bengal which would be expected to increase the frequency
and intensity of storms generated over the Bay with even more severe consequences.
Essay and discussion questions
1 Describe the air masses that affect your area. What types of weather are associated
with each? How much variability is there within each air mass?
2 What range of weather conditions might occur beneath an anticyclone in the midlatitude westerly zone?
3 How does flow in the upper westerlies influence the formation of surface low-pressure
systems?
4 Why are fronts an important feature of weather formation in a temperate cyclone?
5 Using weather radar, satellite images and pressure maps from the internet, examine the
relationships between low pressure systems, fronts and rainfall distribution.
6 Why do tropical cyclones form only in certain parts of the world?
7 Choosing one tropical cyclone as an example, follow its track, its changes of intensity
and its impact. Websites will be the main source of information.
8 What efforts are being made to improve weather forecasts?
Further reading
O’Hare, G. and Sweeney, J. (1986) The Atmospheric System, Edinburgh: Oliver and
Boyd. Elementary and clear description of weather systems at the synoptic scale.
Yarnal, B. (1994) Synoptic climatology in environmental analysis, Chichester: Wiley &
Sons. An introduction to the methods of synoptic climatology — the relationship
between the atmospheric circulation and the surface environment — and shows the vital
importance of this approach in the understanding of environmental systems. Intermediate
to advanced level.
Carlson, T.N. (1991) Mid-latitude Weather Systems, London: Harper Collins. A book
concerned with all aspects of mid-latitude weather systems. It includes aspects of the
development of the systems as well as the
weather associated with them. Intermediate to advanced level.
Barry, R.G. and Carleton, A.M (2001). Synoptic and dynamic climatology. Routledge,
London and New York. Another more advanced text providing a modern viewpoint of
synoptic and dynamic climatology. Demonstrates the senior author’s long experience in
this field.
Website addresses
http://www.ncdc.noaa.gov/oa/ncdc.html
A vast source of information from the US National Climate data centre.
Contains much data, some of which can be downloaded together with current information
about the state of the atmosphere in terms of ENSO and the variety of other
teleconnections and hurricane activity. There is the provision of climate information,
mainly for US but some world maps of temperature and precipitation, etc.
http://www.metoffice.gov.uk/weather/europe/surface_pressure.html
The UK MetOffice produce a series of surface pressure maps forecasts
for up to four days ahead. Gives a good idea of the nature of temperate
latitude pressure systems and their movement. Try to relate the forecast
pressure patterns with the resulting weather for your area.
http://uk.weather.com/maps/lifestyles/intlaviation
If you want to see satellites images over the last 12 hours, this site allows you to see how
the cloud patterns have changed as a short video. Also contains other climatic
information for UK and Europe.
http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html
If you want any questions answered about tropical cyclones, especially those affecting
North America, this is the website for you. Contains a wide range of questions (and
answers) from methods of formation to impact.
http://www.bom.gov.au/weather/national/charts/
Most examples of pressure systems in the accompanying text are from the northern
hemisphere. This website uses southern hemisphere charts so you can see the difference
in air pressure systems, their movement and associated winds. Also includes much
information about weather and climate of Australia.