GEOS2111 Chap.3 Weather, climate change and how the climate is measured
Weather, climate change and how the climate is measured
Module 2 opens on the atmosphere because most of the hazards that follow are atmospheric or are mediated by it. The first distinction is the one students most often blur. Weather is the current state of the atmosphere, a short-term object measured over minutes to months.
Climate is a long term statistical description of atmospheric conditions averaged over a specified period, and the word specified is load-bearing, because a climate statement without a baseline period is not a claim about anything.
Meteorology is defined here as the study of how the atmosphere behaves physically and chemically, narrowed in practice to the movements that bear on human life, and the elements measured are few: air temperature, humidity, wind direction and speed, cloud, precipitation, pressure and visibility.
What makes forecasting hard is the interaction between them, which is why forecasting is probabilistic and why products are issued as watches, advisories and warnings. The week then teaches a mechanical and highly examinable skill: reading a synoptic chart.
An isobar joins points of equal pressure; standard sea level pressure is about 1013 hPa; a high is anticyclonic with air sinking, warming and drying, and a low is cyclonic with air rising, cooling and condensing; wind is strongest where isobars merge; and fronts and troughs are boundaries between air masses with their own characteristic weather.
Above that sits the general circulation: three cells per hemisphere producing four pressure bands and three surface wind belts, turned by the Coriolis force, opposed by the pressure gradient force and slowed by friction, and migrating seasonally with solar heating and the axial tilt.
The week closes on variability and change: the modes that swing Australian weather, above all the El Nino Southern Oscillation measured by the Southern Oscillation Index and the Indian Ocean Dipole; the model families and emissions scenarios used to project change; the assessment report findings the unit teaches as fact claims; and extreme event attribution, which answers a probability question rather than a causation one.
What this chapter covers
- 01
Weather against climate, and why a baseline period is part of the claim
- 02
What meteorologists actually measure, and why forecasting is probabilistic
- 03
Reading a synoptic chart: isobars, highs, lows, fronts and troughs
- 04
The four ways air is lifted, and why lifting is the precondition for rain
- 05
Air masses, and the three cell general circulation with its pressure bands
- 06
Wind belts, the Coriolis force and the seasonal migration of the whole pattern
- 07
Australia's climate drivers, and why a forecast is a combination rather than one mode
- 08
The El Nino Southern Oscillation, its three phases and the index that measures it
- 09
Climate model families, emissions scenarios and what a projection is
- 10
Extreme event attribution: the honest sentence, and the baseline that must travel with a number
Turning a climate driver state into a seasonal hazard outlook, and stating what you cannot say
- +1Give the Pacific mechanism first. El Nino weakens or reverses the trade winds, so warm water spreads east, the thermocline flattens, upwelling is suppressed and the main convection shifts away from Australia, removing one moisture source.
- +1Then the Indian Ocean mechanism. A positive dipole cools the eastern Indian Ocean off Australia at the same time, removing the second moisture source. The combination, rather than either driver alone, is the one associated with the driest and hottest Australian summers.
- +1Convert the mechanism into hazard consequences the council can act on: elevated likelihood of drought conditions, more frequent and more intense heatwave events, and a longer and more severe fire season. Note the compounding: low spring rainfall cures fuels before summer rather than during it.
- +1Name the downstream systems, because that is what a council actually plans. Water supply, agricultural support, health services for heat, and fire preparedness all sit downstream of the same shift, and each has a different lead time.
- +1Add the honesty clause, which carries marks. A driver state shifts probabilities; it does not schedule events. The correct formulation is that the odds of a hot dry summer have risen substantially relative to climatology, not that a severe fire season will occur. Seasonal outlooks are probabilistic for the same reason daily forecasts are.
Key terms
- Weather
- The current state of the atmosphere, a short-term object measured over minutes to months, described by temperature, humidity, wind, cloud, precipitation, pressure and visibility.
- Climate
- A long-term statistical description of atmospheric conditions, averaged over a specified period. The specified period is part of the definition, which is why a climate statement without a baseline is incomplete.
- Isobar
- A contour drawn through places of equal atmospheric pressure on a synoptic chart. Isobar spacing is the most useful thing on the chart, because wind is strongest where isobars merge.
- Anticyclone
- A high pressure system in which air sinks and diverges, warming and drying as it descends, so cloud is suppressed and conditions are settled.
- Cold front
- A boundary at which cooler air replaces warmer air, bringing a narrow band of rain that is sometimes heavy and thundery, followed by clearer, cooler, showery air and an abrupt wind shift.
- Trough
- A long, narrow axis along which pressure sits below that of the air on either side. Like a cold front it separates air masses, usually with moister air on one side and drier on the other.
- Air mass
- A large volume of air roughly uniform in temperature and moisture, able to extend thousands of kilometres and from the ground to the stratosphere. Air masses are classified by temperature and by whether they formed over land or sea.
- Hadley cell
- The low-latitude limb of the general circulation, rising near the equator and sinking near 30 degrees, which is why the equatorial belt is wet and the subtropical belt is dry.
- Coriolis force
- The apparent deflection of moving air caused by the Earth's rotation beneath it. With the pressure gradient force and friction it determines wind direction, and it is why circulation around a low runs clockwise in the southern hemisphere.
- Southern Oscillation Index
- A standardised difference in sea level pressure between two long-running observation stations, used to measure the state of the El Nino Southern Oscillation. Negative values are typical of El Nino episodes.
- Indian Ocean Dipole
- A mode of variability described by the difference in sea surface temperature between the eastern and western tropical Indian Ocean. A positive phase combined with El Nino is associated with hot dry Australian conditions.
- Event attribution
- A method that compares model worlds with and without human influence to estimate how much an event's likelihood or intensity changed. It answers a probability question, so the honest sentence reports a change in likelihood rather than a cause.
Weather, climate change and how the climate is measured FAQ
What is the difference between weather and climate, put precisely?
Weather is the current state of the atmosphere over minutes to months. Climate is a statistical description of atmospheric conditions averaged over a specified period. The distinction is not about duration alone, it is about the kind of object: weather is a state and climate is a distribution. That has two consequences the unit cares about.
First, a single hot day says nothing about climate, and a shift in the distribution says nothing about tomorrow, so arguments that use one to settle the other are category errors in both directions. Second, every climate statement carries a hidden baseline, because an average is always an average over something.
A warming figure quoted without the period it is measured against and the window it is averaged over is not comparable to any other warming figure, and stripping the baseline is the fastest way to write a sentence that is not quite true.
How do I read a synoptic chart under exam conditions?
Four features and one rule. The rule is that wind is strongest where isobars merge, so the crowded side of a system is the dangerous side.
The features are: a high, which is anticyclonic, with air sinking and diverging, so warmer, drier and cloud-suppressed; a low, which is cyclonic, with air rising and converging, so cooler with cloud and rain; a cold front, where cooler air replaces warmer, bringing a narrow and sometimes thundery rain band followed by clearer cool showery air and an abrupt wind change; and a trough, an elongated low-pressure axis that also separates air masses.
Given a marked point on a chart, say which feature it sits near, which side of it, and what that implies for wind and rain. Standard sea level pressure is about 1013 hPa, so a labelled value tells you immediately whether you are looking at a high or a low.
Why are the world's deserts in a belt rather than scattered?
Because they are a consequence of the general circulation rather than of local geography. Air rises at the equator, producing low pressure and heavy rainfall, travels poleward aloft, and sinks near 30 degrees latitude, warming and drying as it descends. Sinking air means high pressure, suppressed cloud and a dry surface, and that is where the great subtropical deserts sit in both hemispheres.
Air rises again near 60 degrees at the polar front, giving another wet band, and sinks at the poles. The same three cells generate the surface wind belts, the trade winds, westerlies and polar easterlies, once the flow is deflected by the Coriolis force.
The whole pattern then migrates north and south seasonally with solar heating and the axial tilt, which is what gives one country a monsoonal north and a winter-rainfall south.
How should I quote the assessment report findings without overreaching?
Attribute them, keep the qualifiers, and never separate a number from its baseline.
The unit teaches several as fact claims: global surface temperature reaching about 1.1 degrees above the 1850 to 1900 baseline over 2011 to 2020; greenhouse gas emissions across 2010 to 2019 running above every earlier decade in the record although the growth rate was lower than the decade before; some changes effectively irreversible on human timescales; and communities that contributed least being disproportionately affected.
The most useful single figure is that across 2010 to 2020, deaths from flood, drought and storm ran about fifteen times higher in highly vulnerable regions than in regions with very low vulnerability, with roughly 3.3 to 3.6 billion people living in highly vulnerable contexts. That is the quantitative form of this unit's whole argument, and it should always be attributed rather than asserted.
What can extreme event attribution actually tell me?
How much more or less likely, or how much more intense, a specific event became because of human influence. The method runs a large ensemble of model worlds with anthropogenic forcing and another with only natural forcings, and compares how often an event of the observed magnitude appears in each.
The output is therefore a change in probability, not a verdict on cause, and the honest sentence says that the event became a stated number of times more likely rather than that climate change caused it.
The unit sets readings on both the method and its reliability, including work arguing that correcting properly for model error changes the attributable risk, so a good answer names the method, gives the probabilistic form of the claim, and acknowledges that the result depends on how well the models represent the event class in question.
Exam move
Treat the synoptic chart as a skill rather than as knowledge, because it is the one mechanical thing in this week and mechanical things are cheap marks. Find charts, cover the forecast, and practise saying what the weather will do at three marked points until it is automatic: which feature, which side, what that implies.
Second, draw the three-cell circulation once from memory with the four pressure bands and three wind belts labelled at their correct latitudes, and check that your wet and dry bands follow from the rising and sinking limbs rather than being memorised separately. If you can derive them you cannot forget them.
Third, build one card per climate driver with three lines: the mechanism in a sentence, what the positive or warm phase does to Australia, and what the negative or cool phase does. Then practise combining two drivers, because the outlooks the unit uses as examples are always combinations.
Fourth, write out the assessment report findings you intend to quote, each with its attribution and its baseline period attached in the same sentence, so that the qualifier travels with the number automatically under exam pressure. Finally, rehearse the attribution formulation until it is a reflex: an event became a stated number of times more likely, not climate change caused this flood.
That single habit protects several answers across Weeks 3, 4 and 5, and it is the kind of precision that separates a competent answer from a careless one.
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