1. Climate & Weather
Weather Maps
WEATHER MAPS

2. Weather and Climate
Weather is the state of the atmosphere at any one place or time. This includes: humidity, temperature, sunshine hours, cloud cover, precipitation.
Climate is the average or long term weather conditions of a region. It is the result of years of research into data.
1. What is the difference between weather and climate?

3. Barometric pressure
Barometric pressure or atmospheric pressure is the pressure exerted by the weight of the air on the earth’s surface.
Atmospheric pressure is measured in hectopascals (hPa).
Atmospheric pressure is mapped to form isobars.
A line on a Synoptic Chart that joins places of equal barometric or atmospheric pressure is called?
The unit of measurement of atmospheric pressure is termed?
What is the abbreviation for this unit of atmospheric pressure?

4. Isobars
Are lines on a synoptic chart joining places of equal atmospheric pressure.
To estimate atmospheric pressure you need to refer to the two nearest isobars. e.g. if point “A” is situated half way between 1006 hPa and 1008 hPa, “A” would be > 1006 but < 1008 hPa.

5. Barometric Pressure readings
How do they get the land and sea readings?
What do all these dots form when they are linked?

6. Isobars
How do you work out which are high pressure cells and which are low pressure cells?
An isobar is a line on a map joining places of equal atmospheric pressure
High Pressure occurs when the barometric reading is usually above 1010 hectopascals
Low Pressure occurs when the barometric reading is usually below 1010 hectopascals.

7. Synoptic Charts
Synoptic charts or weather maps provide a snapshot of the weather experienced at a particular place at a certain time.
They show information relating to air pressure, air masses and fronts, cloud cover, wind speed, wind direction and rainfall.
This information allows us to make predictions relating to temperature, humidity, ocean conditions and the likely weather for the next few days.
What are Synoptic Charts?
Give six examples of the information shown by Synoptic Charts.
List two groups of people you think that would benefit from using the information contained in Synoptic Charts.

8. Synoptic Charts Synoptic Charts What does the shading refer to?
What is the wind speed and direction?
Identify the type of pressure system over Perth.
What type of weather can Perth expect today?
Why is Adelaide likely to have fine, settled weather?
What is the wind speed and direction at Townsville?
The shading on the map, in the Northern Territory, refers to what?
Estimate the barometric pressure, north east of Hobart – where the pink arrow indicates.
What is this feature?
What is the air pressure?

9. Pressure cells - are defined by the patterns formed by isobars.
Are these high pressure cell s or a low pressure cells?
How can you tell?
As you move your over the isobars between Alice Springs and Brisbane, what happens to the size of the numbers as you move to the centre of this pressure system? (move from 1006 hPa to 998 hPa)
As you move your eyes over the isobars from north – west of Hobart to Perth, what happens to the air pressure readings? (998 hPa to 1012 hPa)

10. High pressure cells (anticyclones)
occur where atmospheric pressure increases towards the centre of the system. High pressure systems are characterised by light winds, clear skies, dry weather and a high diurnal (daily) temperature range.
A ridge is an extension of high pressure from a high pressure system.
List three features of weather associated with High Pressure systems.
The term ‘diurnal means?

11. High pressure cells Why does the air descend?
Why does the air descend in high pressure systems?
2. True or false – air moves from high to low pressure?
Why does the air descend?
Why does the air moves outwards?

12. Typical weather associated with High pressure
light winds
clear skies, dry weather
higher diurnal (daily) temperature range i.e. hot days (sun rays maximum effect)
cool nights (because of lack of cloud cover).
Frosts are likely in winter.
1. What does the word ‘diurnal’ mean?
2. Why are the nights likely to be cool?

13. Clear skies allow the sun’s rays to have maximum effect
Clear skies allow the sun’s rays to have maximum effect. Daytime temperatures are usually high.
At night, clear skies allow heat to be lost causing temperatures to fall. In winter frosts are likely.
Why are daytime temperatures usually high with high pressure systems?
Why are temperatures at night likely to be colder?

14. Low pressure cells (cyclones or depressions)
occur where atmospheric pressure decreases towards the centre of the system.
Low pressure systems are associated with stronger winds, cloudy skies, rain
a lower diurnal temperature range.
A trough is an extension of low pressure from a low pressure system.
True or False – atmospheric pressure decreases towards the centre in low pressure cells.
True or False – Strong winds, cloudy skies and rain are associated with low pressure cells.
True or False – an extension of low pressure is known as a trough.

15. Low pressure cells Why does the air rise?
Ascending air
Why does the air rise?
Why does the air moves inwards?

16. Typical weather associated with Low pressure
strong winds, cloudy skies, rain and a lower diurnal (daily) temperature range i.e. mild temperatures (sun does not have maximum effect because of cloud cover, heat trapped in at night).
Frosts are unlikely.
1. Why are frosts unlikely with low pressure systems?

17. Cloudy skies block some of the sun’s rays
Cloudy skies block some of the sun’s rays. Daytime temperatures are relatively low.
At night, cloudy skies prevent heat loss into space. Clouds reflect heat. In winter frosts are unlikely.
1. What is the effect on the temperature at the earth’s surface, when clouds are in the night sky?

18. Tropical cyclones (T.C.)
Are a very intense low pressure system. They generate very strong winds, produce rough seas and heavy rains
Wide spread damage occurs if they make landfall (contact with land).

19. A cross section of a Tropical Cyclone
What is the relationship between wind speed and the direction of the cyclone?
What happens to wind speed after the cyclone has passed?

20. Wind speed
is determined by the closeness of the isobars. The closer the isobars the stronger the winds.
A steep pressure gradient exists where isobars are close together.
If isobars are far apart a place will generally experience light winds. This is called a slight pressure gradient.
If the isobars are closer together, what happens to wind speed?
If the isobars are further apart, what happens to the speed of the wind?

21. Note the closeness of the isobars
Note the spaced isobars
Note the closeness of the isobars

22. Wind direction
Wind is named according to the direction the wind is coming from, e.g. south = southerly.
Wind direction is largely determined by the location of pressure systems. 
1. What is the golden rule to remember with wind direction?

23. Wind direction
Remember we always refer to where the wind is coming from!
Where has the wind come from? 1 2
With number 1, where is the wind coming from?
In what direction is the wind coming from on this occasion?

24. Wind Direction – Anticyclones (High Pressure cells)
In the southern hemisphere winds blow outwards in an anticlockwise direction from areas of high pressure.
ANTICYCLONE = ANTICLOCKWISE WINDS.
In what way do the winds in the southern hemisphere blow around high pressure systems?
If winds blow anticlockwise in high pressure systems in the southern hemisphere, in what direction do winds blow in low pressure systems in the same hemisphere?

25. Wind blows outwards in an anticlockwise direction in areas of high pressure.
ANTICYCLONE = ANTICLOCKWISE WINDS.

26. Wind Direction – Depressions (Low pressure cells)
In the southern hemisphere winds associated with low pressure systems blow in a clockwise direction towards the centre of the system. 
LOW T. C. = CYCLONE = CLOCKWISE WINDS.
In the southern hemisphere, in what direction do winds in a low pressure system blow?

27. Winds blow towards the centre of a low pressure system and in a clockwise direction
LOW P. C. = CYCLONE = CLOCKWISE WINDS.

28. Wind indicators Wind indicators show wind speed and direction.
The feather indicates wind speed
The shaft indicates the direction from which the wind is coming
The black dot is the place that is experiencing the windy conditions

29. Identifying wind direction
First - Identify the type of pressure system.
Southern Hemisphere
LOW =clockwise
HIGH = anticlockwise.
(the reverse is true in the northern hemisphere)
Wind, generally speaking, moves across the isobars

30. Sea conditions SYMBOLS FOR SEA CONDITIONS: VR = Very Rough R = Rough
M = Moderate
SL = Slight
Sea conditions are closely related to wind speed or strength. The stronger the winds the rougher the seas and vice versa. The best method for working out sea conditions is to look at the closeness of the isobars.
Close isobars = strong winds = rough seas.

31. FRONTS
A front is the leading edge of change. They are the border zone between warm and cold air. There are two types of fronts:
Warm fronts occur when warm air is pushed into a cold air mass. These are very rare in Australia. They result drizzle and light rain over a few days.
What is a front?
Name the two types of fronts – look at the next slide to help you answer this question

32. A warm front
Identify the type of cloud that is bringing rain to the Earth’s surface.
Name the highest cloud type shown on this slide.

33. Cold fronts are far more common in Australia and occur when cold air is pushed into a warm air mass. This forces the warm air to rise causing a number of changes: (a) Decrease in temperature: cold air replaces warm air. (b) Change in wind direction and sometimes speed. (c) Build up of cumulonimbus or rain bearing clouds. (d) Increase in humidity: corresponding to a build up of clouds. (e) Probability of rain.
Make up an acronym to remember these changes.

34. A cold front

35. Temperature and humidity:
are related to the wind. Winds pick up the characteristics of where they form – sea or land – these are called source regions
Remember:
Winds off the land are generally hotter and drier.
Winds off the sea tend to be cooler and wetter (greater moisture = greater humidity).

36. Types of rainfall: Convection

37. Types of rainfall: Orographic
Would the following be a correct definition for Orographic Rain? The rain that results from air being forced to rise over a mountain barrier.
If yes, explain why this definition is correct.
Why is there relatively little rain after the air has been forced to rise over the mountain range?

38. Types of rainfall: Frontal

39. Seasons are determined by the location of various pressure systems.
Remember: 
Summer = Lows over Northern Australia and Highs over Southern Australia.
Winter = Highs over Northern Australia and Lows over Southern Australia.
Over which part of Australia do low pressure systems occur during Summer?
High Pressure systems occur over which part of Australia during Summer?
Low pressure systems occur during which season over Southern Australia?
What type of pressure system occurs during Winter over Northern Australia?

40. Summer weather map Where are the different cells located?
1. How can you prove that this synoptic chart is characteristic of a Summer Weather pattern?
Where are the different cells located?

41. Winter weather map Where are the different cells located?
1. Why is this synoptic chart characteristic of a southern hemisphere winter?
Where are the different cells located?

42. References B. Crowder, The Wonders of Weather, AGPS, Canberra, 1995.
G. Nagle and K. Spencer, Advanced Geography, OUP, Oxford, 1997.
G. Underwood, Geomasters, Brooks Waterloo, Melbourne, 1988.
L. Bryant, Studying Contour and Weather Maps, Jacaranda Press, Milton, 1990.
Special Thanks – Mr N. Shute Knox 2003
Questions / Activities – Mr S. de Botton March 2004