EAE 1 Weather Flashcards
Where is weather located?
All the weather is in the troposphere
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How deep is the troposhere?
The troposphere varies between 8 km deep at the poles and 16 km deep in the tropics
For comparison the radius of the Earth is 6400 km.
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What are the key featues of the troposphere?
- It is mainly heated from the surface as little solar radiation is absorbed by the atmosphere.
- The temperature decreases with height. (Why doesn’t the warm air rise?)
- Most of the mass and virtually all clouds are found in the troposphere.
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What is the tropopause?
The tropopause is the boundary between the troposphere and the stratosphere.
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How does pressure work?
- Fluids (gases and liquids) comprise molecules that jiggle around and bounce off one another.
- The mean kinetic energy (the energy of their motion) of the molecules is proportional to their temperature.
- When you touch a fluid, the molecules strike you and bounce off you.
- Each strike exerts a small force on you, the sum of which results in a net force per unit area. This is the fluid pressure.
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What are the units for pressure?
Pressure is a force per area and
Dimensions:
* Newton per metre squared: Nm⁻² or N/m² * Pascal: Pa
Note: 1 Pa = 1 N/m²
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What are the units for pressure?
100 Pa = 1 hPa = 1 mb
100 pascal
=
1 hectpascal
=
1 millibar
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How do you increase pressure?
For a given box:
Add more molecules (i.e. more mass)
The number of molecules striking the wall per second increases, thereby increasing the pressure
Heat the box
This increases the temperature and hence the mean speed. The molecules strike the walls harder, thereby increasing the pressure.
Reduce the volume
Reduces the surface area for impacts
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How does the pressure change with height?
- Pressure decreases rapidly with height.
- The pressure decreases with height because the mass above a given level decreases with height.
- The molecules in the atmosphere are attracted to the Earth by gravity. Therefore the number of molecules per volume increases towards the surface. Thus the pressure increases towards the surface.
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What is the mean surface pressure?
The mean surface pressure is 1013.25 mb.
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What is an isobar?
A line of constant pressure
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What is ‘pressure gradient force’?
Change in pressure per change in distance
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Which way do winds blow?
Outside of the tropics, the wind blows mostly parallel to the isobars with low pressure to the right in the Southern Hemisphere.
Note: In the Northern Hemisphere low pressure is on the left.
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What do Wynoptic charts show?
Synoptic charts show the pressure at sea level and are good large-scale summaries of the current weather.
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What are the main structures on synoptic charts?
- Highs (anticyclones)
- Lows (extratropical or tropical cyclones)
- Ridges
- Troughs
- Fronts
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Climatology of Mid-Latitude Cyclones
What are storm tracks?
The maxima in the number of cyclones are called the storm tracks.
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What drives weather?
Differential heating drives the weather.
Latitudinal variations in the annual incoming solar radiation (insolation) density and distance to the surface
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What produces fronts?
Fronts are produced by stirring and are inextricably connected to midlatitude cyclones
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What is the impact of the mid-latitude fronts?
- Most of the significant weather in the mid-latitudes is connected to these cyclones.
- Midlatitude cyclones and their associated fronts are the main cloud and rain producing systems at these latitudes.
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What is this symbol?
Cold Front
Cold air mass displaces a warm air mass.
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What is this symbol?
Warm front
Warm air mass displaces a cold air mass.
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What is this symbol?
Stationary Front
The cold and warm air masses remain relatively stationary.
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What is this symbol?
Occluded Front
Two cold air masses in an extratropical cyclone
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Describe occluded fronts
An occluded front separates the air behind the cold front from the air ahead of the warm front.
The formation of an occluded front is common late in the life of midlatitude cyclones.
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What is the structure of Cold Fronts?
- Cold air undercutting a warm air mass.
- Strong convergence and vertical motion sometimes leads to deep convective clouds.
- Strong temperature changes.
- Often precipitation and a change in wind direction.
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What is the structure of Warm Fronts?
- Warm air overriding cold air.
- Diffuse front leads to large expanse of stratiform rain (floods).
- Often not analysed over Australia as the storm track lies far to the south.
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How do rainfall patterns vary?
The pattern of rainfall in the mid-latitudes is qualitatively different from the tropics.
- In the tropics the patterns are more circular
- In the mid-latitudes the patterns reflect the shape of the mid-latitude cyclones and fronts.
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How do the poles compare to the tropics?
- Cyclones are most numerous around the Antarctic.
- The cyclones mostly have attendant fronts.
- Broad anticyclones are found in the subtropics.
- The pressure gradients are largest near the pole and weakest in the tropics.
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What is the climatology of Cold Fronts?
Cold fronts are located predominantly slightly equatorward of the storm tracks.
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Key points
What do midlatitude cyclones do to air masses?
Midlatitude cyclones transport warm air poleward and cold air equatorward.
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Key points
How do fronts form?
- Fronts form as part of midlatitude cyclones as the cyclone redistributes the temperature.
- However, midlatitude cyclones form on temperature gradients, which can be thought of as weak fronts.
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Where do Cyclones predominantly form?
Cyclones form predominantly in the
- North Atlantic
- Northwestern Pacific
- High latitudes of the Southern Ocean
These regions are called the storm tracks.
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What is the meaning of ‘cyclonic’?
- Rotation in the same sense as the rotation of the Earth is said to be cyclonic.
- In the Northern Hemisphere, cyclonic rotation is anticlockwise.
- In the Southern Hemisphere, cyclonic rotation is clockwise.
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What is the important aspect of rotation for weather?
- Only the locally vertical component of the rotation is important for weather systems.
- There is no vertical component of the Earth’s rotation at the equator.
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What is the geostrophic balance?
The balance of forces between the coriolis force and the pressure gradient force.
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What is a tropical cyclone?
- A tropical cyclone is a cyclone that forms over the tropical oceans and is driven principally by heating from the ocean.
- Tropical cyclones (TCs) are also known as hurricanes (in the Atlantic and eastern North Pacific) and typhoons (in the western North Pacific).
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How are tropical and mid-latitude cyclones different?
- Tropical Cyclones have eyes and warm cores (meaning that the warmest air is near the centre of the storm).
- Tropical Cyclones do not have fronts.
- The winds strongest near surface in Tropical Cyclones
c.f. In a mid-latitude cyclone the strongest wind are in the upper part of the troposphere (i.e. the jet stream).
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What is the Australian classification of a Tropical Depression?
Has a maximum wind speed of 17 m/s or less
Measured at 10m averaged over 10 minutes.
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What is the Australian classification of a Tropical Storm?
Has a maximum wind speed in the range of 18 m/s to 32 m/s
Measured at 10m averaged over 10 minutes.
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What is the Australian classification of a Tropical Cyclone?
Has a maximum wind speed exceeding 32 m/s
Measured at 10m averaged over 10 minutes.
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Where do tropical cyclones form?
- Around 80 tropical cyclones form each year.
- Tropical cyclones usually form in the summer and early autumn.
- Tropical cyclones do not form close to the equator (because the vertical component of the Earth’s rotation is zero there).
- TCs form over the warmest oceans and dissipate over colder waters or when they make landfall.
- Virtually all TCs form over water warmer than 26.5oC
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What is Tropical Cyclone spin up?
- Close to the surface friction retards the tangential flow, and the pressure gradient force draws air towards the centre (just like the tea-leaf problem).
- The inflow (almost) conserves angular momentum and spins up the vortex at low levels.
- The inflowing air is moistened by evaporation from the warm sea surface.
- The inward spiralling air ascends in the eye wall, producing convection, thereby drawing more air towards the centre.
- This leaves the eye wall and flows outwards, conserving angular momentum and spinning down the vortex.
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Which way do Tropical Cyclones move?
Tropical Cyclones generally move westward and poleward.
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