Week 4 - synoptic charts, weather fronts, weather data, monitoring climate

Intertropical Convergence Zone (ITCZ)

• Belt near the equator where the northeast trade winds from the Northern Hemisphere and the southeast trade winds from the Southern Hemisphere meet (converge)

• The convergence of these winds forces air to rise, creating a zone of low pressure

• Rising air leads to frequent cloud formation and heavy rainfall, making the ITCZ a key driver of tropical climates

Cold front

• Blue spike - represents icicles

• Cold fronts bring a transition from warmer to colder air at a fixed location

• Cold air is more dense

• Area of cold air pushing into warm air

• Cold air will stay close to surface because more dense, wants to hug the surface

• Wedging shape

• Forcing upward of warm air that enables convection to form, clouds to form

• Line of thunderstorms, rain showers

Warm front

• Red semicircles -

• Warm fronts bring a transition from colder to warmer air at a fixed location

• Warm fronts not as frequent in the Southern Hemisphere due to generally weaker temperature contrasts than in Northern Hemisphere

• Require stronger horizontal temperature contrasts to be more well-defined (eg in Northern Hemisphere)

Stationary fronts

• Fronts with no movement

• Winds tend to blow parallel but opposite directions

Occluded fronts

• A frontal boundary associated with cloud, rain and “weather”

• Little temperature contrasts at the surface between two areas of cold air

• The warm air is “occluded” or ‘cut off’ from the cold air at the surface, and pushed upward

Barometer

• Pressure drops with altitude (less air pressing down on you)

• Aneroid barometer is most usual form

• Constant pressure inside cell; expands or contracts depending on outside pressure, then moves dial

Convergence

When air flows toward a central point, causing it to rise (low pressure, clouds, rain)

Divergence

When air flows away from a central point, causing air to sink (high pressure, clear skies)

High pressure

Area where air is sinking and pressing down on the surface; usually brings clear, dry and stable weather

Low pressure

Area where air is rising away from the surface; usually brings clouds, rain and unstable weather

Cyclonic motion

Clockwise motion → Coriolis force results in rotation that is clockwise in low pressure systems, southern hemisphere

Anticyclonic motion

Anti-clockwise → Coriolis force results in anticlockwise rotation in Northern hemisphere - in high pressure systems (hurricane)

Isobars

• Tend to be closer together around low pressure systems, stronger winds

• Tends to be wider around high pressure systems, lighter winds

Stevensons Screens

• Stevenson Screens contain thermometers, barometers and other instruments

• Painted white to reflect as much radiation as possible

• Wind can’t get in and sun can’t get in

Anemometer

• Wind speed measured

• Measured at a standard 10 m height worldwide

Wind vane

Measures wind direction

Radiosondes

• Contain temperature, pressure and humidity sensors

• These are carried aloft via weather balloons

Weather balloons

• Carry radiosondes

• Gives information on upper tropospheric conditions (useful for analysing atmospheric ‘stability’)

Satellites and radar

• Used to measure the weather ‘remotely’ (ie temperature, rainfall or other variable is inferred through the transmission and reflection of radiation)

• Radars transmit microwaves, some of which reflect off precipitation and return to the radar to be measured

Other ways of monitoring

• Early fisherman dropped a bucket into ocean, then measured temp

• Counting tree rings

• Palaeoclimate records

• Cherry blossom blooming in Japan