ATS2545 Lecture Notes - Lecture 4: Cumulus Congestus Cloud, Sensible Heat, Evaporator
25 May 2018
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Lecture 4 – Global Hydrology
Global-Scale Hydrology: the Driving Mechanisms
• Solar energy is main evaporator from ocean
o Zone between the tropics has the most solar heat
• Energy partitioning at the Earth’s surface
• Albedo – percentage of solar radiation that is reflected back
o Remainder is absorbed in some way by the surface
• Sensible heat – (H) some partitioned into raising the temperature of soil, rock,
vegetation or water
• Latent heat – (LE) water molecules absorbed that become vapour
o 2.5 x 10^6 J kg-1 (vaporisation)
o Energy is released into the atmosphere as sensible heat (causing
warming) wherever the water vapour condenses
• Air rising from a surface that is both warm and moist contains BOTH sensible
heat and latent heat
o As air rises buoyantly, it loses its sensible heat to its cooler
surroundings
o Condensation of water vapour releases latent heat, which keeps the air
column rising strongly
o Most tropical surfaces give rise to massive convection and towering
cumulus cloud masses
• Convection – vertical movement
• Advection – lateral movement
Global Rainfall – summary
• Highest rainfalls are received within tropics
• Major moisture sources are warm oceans and major tropical forests
Aridity in Australia
• Australia lies beneath sub-tropical high pressure belt
o Experiences warm, clear air and reduced chance of rainfall
• Low elevation of continent reduces the likelihood of uplift required to cool air
• ENSO periodically cools oceans to North of Australia
• Ocean temperature affects how much is evaporated by the sun
Heat and temperature in land and water
• Solar heating delivers up to 400˚W per m^2
• Soil and rock poor heat conductors → very hot (opaque)
o Air adjacent to soil and rock warmed greatly by conduction and
convection
o Warm air rises away from surface, leaving low pressure
o Land surface very warm – 60˚c on bare rock/soil surface
• Sea water has much higher specific heat, reaches a lower temperature for a
given input of heat energy (transparent), cools itself via evaporation
o Summer: sea water 20-30˚c
o Mechanical wave mixing distribute the heat through top 100 meters,
limiting temperature rise of surface waters
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