Fronts And Depressions Flashcards
Describe in terms of air-mass movement, the development of cold fronts.
- Polar front theory = warm air-mass meets cold air-mass and rotates around low
- When cold air undercuts slower moving warm air
- Cold air lifts the less dense warm air front
- Frontal lifting creates cloud
- Formation of cloud releases latent heat increasing instability therefore cumuliform cloud
- 20- 30kts and slope 1:80
Describe in terms of air-mass movement, the development of warm, fronts (warm sectors).
- Warm moist airmass is forced to rise over slower moving cold air
- Rises slower than in cold front as slower moving and more widespread
- Therefore more stable and often stratiform type cloud
- 10 - 15kts and slope 1:150
Describe in terms of air-mass movement, the development of occluded fronts.
- Cold occlusion = Colder air catching cold air
- Warm occlusion = Cold air catching colder air
Describe in terms of air-mass movement, the development of stationary fronts.
- Cold air opposed by warm air
- Weak
- Remains of decaying frontal system
- Less than 5kts
Outline the characteristics of mid to high-latitude depressions (‘lows’).
- Move from west to east
- Moist sub-tropical air masses meet cold polar air masses
- Known as polar front depressions
Outline the characteristics of sub-tropical depressions.
- More common than tropical cyclones
- Occur when system moves over warm water
- Cold air above increases convection
- 24 - 63kt winds
Outline the characteristics of tropical cyclones.
- Move at 10-15kts
- 64+kt winds
- Life cycle of up to 2 weeks
Describe the development and the aspects of importance to aviation of lee depressions.
- Lee of mountain sheltered from the wind
- Creates low pressure in the lee
- Bad weather on the leeward side of mountain
- Severe turbulence
Describe the development and the aspects of importance to aviation of thermal lows.
- Usually develop over the land in summer
- Conduction causes convection which increases instability
- Creates turbulence due to thermals
- Possible thunderstorms and hail if Cb develop
- Reduce density of air
Describe the development and the aspects of importance to aviation of depressions crossing a mountain barrier.
- Surface level stops as it runs up against mountain
- Above mountain the depression continues to move over
- Creates new low level depression on leeward side
- Poor IMC weather conditions on both side of mountain
- Turbulence from rising air and mechanical
Explain what is meant by air-mass modification.
- Continental = land air mass
- Maritime = ocean air mass
- Cold air moving to warm area = cold advection (unstable)
- Warm air moving to cold area = warm advection (stable)
Describe how stability is associated with either ‘cold air advection’ or ‘warm air advection’.
- K = Cold
- W = Warm
- Cold air advection = cold air moving into warmer areas
- Warm air advection = warm air moving into cold areas
Describe how stability is associated with with either ‘cold air advection’ or ‘warm air advection’
- Cold air warms up as it moves to warmer areas
- Increases instability
- Warm air cools down as it moves to colder areas
- Increases stability
Describe how cloud types are associated with either ‘cold air advection’ or ‘warm air advection’.
- Cold advection = cumuliform cloud
- Warm advection = low level stratiform cloud or fog
Describe how likely precipitation is associated with either ‘cold air advection’ or ‘warm air advection’.
- Cold advection = showers
- Warm advection = drizzle
Describe how visibility reductions are associated with either ‘cold air advection’ or ‘warm air advection’.
- Cold advection = visibility good except in showers
- Warm advection = reduced visibility due to drizzle and fog
Describe how turbulence is associated with either ‘cold air advection’ or ‘warm air advection’
- Cold advection = moderate to severe turbulence
- Warm advection = Nil or light turbulence
Describe the concepts of convergence and divergence.
- (Low) Convergence = air flowing in and ascending at the surface.
- (High) Divergence = air descending at the surface and diverging
Explain the vertical motions generated by convergence and divergence near the earth’s surface and immediately beneath the tropopause.
- (Lows) Lower level convergence = upper level divergence
- (Highs) Lower level divergence = upper level convergence
Explain the concept of vorticity (rotation or spin) (1)
- Relative vorticity = Wind velocity pattern aloft
- Caused by shear (horizontal difference in speeds)
- Or curvature (forced change in direction which imparts a spins)
- Can be anticyclonic/cyclonic
- Earths vorticity = generated by the Earths spin
- Always cyclonic
- Stronger than relative vorticity
Explain the concept of vorticity (rotation or spin) (2)
- Absolute vorticity = Relative vorticity + Earths vorticity
- Absolute vorticity is always cyclonic
Explain how convergence drives an increase in vorticity conservation of angular momentum.
- Angular momentum = Air mass x RPM x Radius^2
- Angular momentum is a constant (k)
- Air mass can’t change its size
- RPM and radius can change due to absolute vorticity
Explain how convergence drives an increase in vorticity through conservation of angular momentum.
- If RPM increases radius reduces to keep the angular momentum formula constant
- If radius increases, RPM reduces to keep the formula constant.
- RPM increases if radius reduces (convergence)
- RPM decreases if radius increases (divergence)
Outline the effect of vorticity advection on the development of mid-latitude pressure systems.
- Vorticity advection = horizontal movement of spinning aloft
- If spinning air aloft links up with spinning air at surface it will create either a high or low depending on if its convergence or divergence
- Most highs and lows in in mid latitudes created this way
With respect to depressions of the Southern Hemisphere outside the tropics, describe the development and associated cloud of the mid to high-latitude depression, where upper-level divergence dominates the formation process.
- If upper-level divergence dominates the lows will have very strong upward vertical motion
- Extensive cloud band will form creating cold fronts and warm fronts with mostly unstable
With respect to depressions of the Southern Hemisphere outside the tropics, describe the development and associated sub-tropical depression, where advection of warm moist air and latent heat release dominate the formation process.
- Warm moist air moving towards the poles from tropics/subtropics
- Radius on which it rotates reduces towards the poles
- Creates low level convergence
- Will be enhanced further by release of latent heat
- Unstable cloud
Describe the effect of amount of moisture in the warm rising air on the intensity of fronts, and on the extent of cloud and precipitation.
- The greater the amount of moisture the greater the development of cloud/precipitation.
- Larger amounts of latent heat release increasing instability
Describe the effect of the stability or instability of the rising air on the intensity of fronts, and on the extent of cloud and precipitation.
- Unstable = cumulus
- Stable = layer type cloud (stratiform)
Describe the effect of the slope of the frontal surface on the intensity of fronts, and on the extent of cloud and precipitation.
- Steep sloping = cumulus
- Shallow sloping - stratiform
Describe the effect of the speed of the front on the intensity of fronts, and on the extent of the cloud and precipitation
- The faster the front the narrower it will be but the greater it intensity and activity
- Cold fronts travel at 0.8x speed of wind
- Warm fronts travel at 0.6
- Can be compared to a train ploughing through snow
Describe the effect of the speed of the front on the intensity of fronts, and on the extent of cloud and precipitation.
Slow front = stable stratiform
Fast front = unstable cumulus
Describe the effect of the temperature contrast across the front on the intensity of fronts, and on the extent of cloud and precipitation.
- The greater the contrast, the greater the intensity as more air will be able to rise on the warm side
Describe the sequence of events during the passage of an idealised cold fronts and warm front (or warm sector) in both hemispheres, in terms of pressure changes.
- Cold front:
Before = decrease
At = arrest or fall
After = increase - Warm front:
Before = decrease
At = arrest or fall
After = increase
Describe the sequence of events during the passage of an idealised cold fronts and warm front (or warm sector) in both hemispheres, in terms of temperature changes.
- Cold front
Before = steady (warm)
At = abrupt decrease
After = steady (cold) - Warm front
Before = steady or slight decrease
At = increase
After = Little change
Describe the sequence of events during the passage of an idealised cold front and warm front (or warm sector) in both hemispheres, in terms of cloud.
- Cold front
Before = CS or AS
At = CU,CB, NS
After = Clear, isolated CU, CB - Warm front
Before = CI, CS, AS, NS, ST, SC, CU, CB
At = ST,NS, CU, CB
After. = Low-level cloud may persist
Describe the sequence of events during the passage of an idealised cold front and warm front (or warm sector) in both hemispheres, in terms of precipitation.
- Cold front
Before = none
At = heavy showers/hail
After = isolated showers - Warm front
Before = light turning to persistent heavy
At = drizzle
After = Occasional rain or drizzle
Describe the sequence of events during the passage of an idealised cold front and warm front (or warm sector) in both hemispheres, in terms of visibility.
- Cold front
Before = fair
At = Very poor
After = Very good, reduced in showers - Warm front
Before = Good but becoming poor in rain
At = Very poor
After = fair but poor in drizzle/rain
Describe the sequence of events during the passage of an idealised cold front and warm front (or warm sector) in both hemispheres, in terms of dew point changes.
- Cold front
Before = no change
At= increase
After = Lower than in advance of front - Warm front
Before = slight increase
At = steady
After = higher than in advance of front
Explain the concept of an occluded front.
- Cold front overtaking warm front
- Depends how cold the air is informs of the warm front
- Coldest air will undercut everything else
