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Thermal processes (Potential temp (Potential temp is a useful quantity for…
Thermal processes
Potential temp
Potential temp= the temp the mass of air would have if moved adiabatically to a reference pressure (1000hPa)
Potential temp is conserved as a mass of air if moved adiabatically, temp is not
Moving a mass of air adiabatically in the vertical will result in a change in temp as pressure changes
Using the ideal gas law & laws of thermodynamics, can show that: θ = T(p/p0)^R/cp Where p= pressure, T= temp and R/Cp= 2/7
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Sea breeze
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By day:
If weather is warm, convective turbulence imposes a strong drag force on air
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Air accelerates down the pressure grad, coriolis force turns it right in NH
By night:
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Sea breeze accelerates, moving more rapidly inland
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Pressure & temp
If the column is warmed, air will expand and density at any level will decrease
The vertical interval between the pressure levels increases so that at any altitude, the pressure in the warmer column is greater than in the cooler
A column of air will have pressure levels P1, P2 etc
Since the total mass of air in the column is constant, the pressure at the surface doesn't change
Lapse rate
Dry adiabatic lapse rate= fall in temp with altitude of dry air resulting from decrease in pressure--> -9.8°C/ km
As condensation releases latent heat, saturated air cools less with altitude than dry air
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Saturated adiabatic lapse rate increases as temp decreases, from 4°C/km fr v warm tropical air to 9°C/km at -40°C
Thickness
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Mid lat low pressure cells have colder air to the rear, as a result the axis of the low slopes towards the colder air
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High pressure cells slope toward the warmest air aloft, the centre of the cell at 3000m may be displaced 10-15° towards equator
Thermal low
Land heats up (solar radiation) and cools down (IR radiation) much more rapidly than ocean -> large diurnal cycle cross-coast temp gradient
A thermal low results from fine, clear, warm weather, so differs from depression associated with cloud and bad weather
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Formation
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Air over land warms and expands upwards to produce a high pressure aloft (thickness has increased with temp but pressure remains constant)
High pressure aloft weakens but is maintained by heating at surface. surface pressure gradient drives flow from sea to land- sea breeze
Horizontally uniform pressure distribution. solar rad warms up land- air near surface is warmed by land, convection mixes warm air upwards --> boundary layer warms
When solar heating stops, pressure driven flows act to equalise pressure- restoring conditions to initial pressure field- if land cools sufficiently at night, the reverse situation can be established
Over large land masses there may be insufficient time over night for sea breeze to reach regions far from coast- weak surface low maintained at night- depends during days following and heat low maintained for days or weeks
Pressure
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The depth of a layer of air increases as its temp increases- thickness can be used as an indicator of mean temp
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