Weather Basics

Weather and Climate basics Charles Darwin Climate Symposium 14 October 2011

Joel Lisonbee NT 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 • Air has mass • Gravity pulls air down AIR 1m

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

•Measured by a barometer

ma

1m

•In millibars (mb) •Hecto Pascals (hPa) 1m

•Sometimes inches of mercury

Side note: air pressure and altitude • If air pressure is weight of air above you • Then less air above you means lower air pressure

Air pressure (cont.)

AIR

AIR

AIR

AIR

Convergence

AIR

AIR

L

H

Divergence

Air Pressure

Pressure lines (isobars) on a weather chart

L

Air flow around Low pressure system

L

Hot Air Rises‌Cold Air Sinks

Atmospheric layers

– Troposphere • 6.5 °C/km

– Dry air parcel • 9.8 °C/km

% of atmosphere above that level

• Temperature change with height

Convection

H

L

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

H

North/South Circulation

•Over Simplified model •Other Complications include: •Spinning planet •Tilted axis •Land masses/oceans •Water vapour

Global Circulation • Forces from Earth’s rotation create: – 6 circulation cells – Bands of high and low pressure – Easterly and westerly winds

L H

H L L

L

H H

East/West circulation

Winds are named for the direction they came from:

H

West wind / westerlies

L

East wind / easterlies

H

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 Australia www.bom.gov.au/climate/change/

www.climatechangeaustralia.gov .au