Forecasting the Weather

Why is the weather important?

• Impacts on the timing of operations such as crop spraying or hay cutting

• Can determine seasonal decisions like sowing or lambing

• Climate can also have a long term impact on vegetation and soil resources, which affect farmers financially

Using Historical Climatic Data

Australian rainfall percentages - 6 monthsPercentage of six-monthly mean (1961 - 1990 base period)

3-monthly rainfall totals for Australia

Understanding the weather map

• The weather map is one of the most familiar images in the community.

• Its dominant features are the smooth, curving patterns of sea level isobarsisobars (lines of equal atmospheric pressure), which show the central elements of our weather systems: highshighs, lowslows (including tropical cyclones) and cold frontscold fronts.

• Television and newspapers also often carry forecast weather maps, which indicate how the surface weather patterns are expected to develop.

• MeteorologistsMeteorologists use a wide range of information and techniques to formulate weather forecasts. The weather map does not and cannot show all of these factors. It is a fairly simple representation of past and probable future locations of surface weather systems. Nevertheless, it’s a useful guide to the weather.

Understanding the weather map

• Wind strength is directly proportional to the distance between

isobars; the closer the lines, the stronger the winds.

High Pressure Systems

• The air spirals anti-clockwise in a high pressure system.

• The air associated with a high pressure system sinks down from above and warms as it does so and is very stable.

• A high pressure system (anticyclone), is a system of closed isobars surrounding a region of relatively high pressure. When compared with low pressure systems, highs tend to cover a greater area, move more slowly and have a longer life.

• When the high pressure system is located over land the weather will be typically dry and free of cloud.

Low pressure systems

• The air spirals clockwise in a low pressure system.

• A low pressure system (cyclone) develops where relatively warm air ascends from the Earth's surface.

• These are systems of closed isobars surrounding a region of relatively low pressure. As the rising air cools, clouds will begin to form. The instability of the air will produce clouds with associated rain showers.

• An elongated extension of isobars away from a low

pressure centre is known as a trough of low pressure. This trough usually contains one or more cold fronts.

Air movement

Cold fronts

• A cold front is cold polar air moving towards the equator and undercutting warm tropical air moving poleward.

• The temperature differences across a cold front can be extreme and associated with strong winds. The warm tropical air is forced to rise and become unstable with the development of large clouds.

• Severe weather such as thunderstorms, squall lines and severe turbulence may accompany these cold fronts.

• On a weather chart, a cold front is represented as a line with barbs pointing in the direction of movement of the front, from cold to warmer air

Warm front symbol

ActivityOn this map On this map

identify:identify:• Wind

direction at H and L

• Areas of high and low wind speed

• A cold front

What makes it rain?

• To create rain you need MoistureMoisture and a TriggerTrigger

i.e. Moisture x Trigger = raini.e. Moisture x Trigger = rain(these are independent of each other)(these are independent of each other)

• Evaporation from warmer ocean waters provides the main source of moisture. Cooler air originating from the Southern Ocean will have less moisture available for precipitation, although good falls will still occur in these systems from time to time.

Names of Oceans

Tasman Sea

What are the mechanisms to trigger rainfall?

For rain to fall, moist air needs to cool, resulting in condensation of vapour. This is commonly achieved by something which forces the air to rise.

Air rises because of:• hills and mountains (orographic lift)• irregularities on the earth surface, causing mechanical or

frictional turbulence• air pressure systems colliding, such as cold fronts

undercutting warmer air• low pressure systems, in which there is a general area of

upward movement• Sunset – the heating is removed allowing the air to cool• A trough of low pressure where the air expands,

spreading the heat and cooling slightly.

If there is sufficient moisture and the air rises enough to form dense clouds there could be rain from any of these. The diagram below shows one of the triggers for rain, a classic cold front undercutting and lifting warmer moist air to form rain. This is a common occurrence in southern Australia but rarer in central NSW or further north.

Do clouds mean rain?

• The clouds from which rain fall are just the visible sign of condensation activity.

• All the water in a cloud 1km high would

only be 1mm deep on the ground. • To get rainfall in amounts useful to

agriculture require inflow of moist air fro some time and for the conditions inducing condensation to remain in force.