Weather and Climate basics

Charles Darwin Climate Symposium14 October 2011

Joel LisonbeeNT Climate Services Centre

Bureau of Meteorology

Weather and Climate basics

Goal: To explain and define basic weather and climate terms

• Weather: “The state of the atmosphere…the short-term (minutes to days) variations in the atmosphere” AMS Glossary of Meteorology

• Climate: “The slowly varying aspects of the atmosphere-hydrosphere-land surface system” AMS Glossary of Meteorology

– Often taken as a 30 year average

Air Pressure

• Force = mass X acceleration, F=ma

• Pressure: net force per unit area, P=F/A

1 m

1 m

ma

• Air has mass

• Gravity pulls air down

• Air Pressure is the force exerted by the column of air directly above you.

•Measured by a barometer

•In millibars (mb)

•Hecto Pascals (hPa)

•Sometimes inches of mercury1 m

AIR

Side note: air pressure and altitude

• If air pressure is weight of air above you

• Then less air above you means lower air pressure

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AIR

AIR

Air pressure (cont.)

AIR

AIR

AIRA

IR

H

DivergenceConvergence

Air Pressure

Pressure lines (isobars) on a weather chart

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Air flow around Low pressure system

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Hot Air Rises…Cold Air Sinks

Atmospheric layers%

of atmosphere above that level

• Temperature change with height

– Troposphere• 6.5 °C/km

– Dry air parcel• 9.8 °C/km

L HH

Sea Surface Temperature (SST): The temperature of the ocean surface

Convection

•Over Simplified model•Other Complications include:

•Spinning planet•Tilted axis•Land masses/oceans•Water vapour

North/South Circulation

Global Circulation

• Forces from Earth’s rotation create: – 6 circulation

cells– Bands of high

and low pressure

– Easterly and westerly winds

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L

L L

H

H

H

H

L HH

East/West circulation

Winds are named for the direction they came from:

West wind / westerlies East wind / easterlies

Walker Circulation

• Walker Circulation: a patterns of rising and sinking motion in the Tropics which creates east/west circulation

climatology average conditions and anomalies

• Climate: “The slowly varying aspects of the atmosphere-hydrosphere-land surface system” AMS Glossary of Meteorology

– Often taken as a 30 year average

• Climate variability: “any variations of the atmosphere/ocean system around a mean state” AMS Glossary of Meteorology

• Climate Change: “Any systematic change in the long-term statistics of climate elements sustained over several decades or longer” AMS Glossary of Meteorology

Mean climate and anomaly

Measured

Expected

Anomalies

Mean Climate and anomaly

Climatology: mean, average, or expected

conditions

Negative Anomaly

Positive Anomaly

Climate Variability

• Example: El Niño/La Niña– Seasonal SST

Changes in the Pacific

– Seasonal to annual effects

– Drought or Flood?• Australia or Pacific

Islands

Climate Change: Trends in past climate

Climate Change in Australiawww.climatechangeaustralia.gov.auwww.bom.gov.au/climate/change/