Fronts And Depressions

Question 1

Describe in terms of air-mass movement, the development of cold fronts.

Answer 1

• 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

Question 3

Describe in terms of air-mass movement, the development of warm, fronts (warm sectors).

Answer 3

• 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

Question 4

Describe in terms of air-mass movement, the development of occluded fronts.

Answer 4

• Cold occlusion = Colder air catching cold air
• Warm occlusion = Cold air catching colder air

Question 5

Describe in terms of air-mass movement, the development of stationary fronts.

Answer 5

• Cold air opposed by warm air
• Weak
• Remains of decaying frontal system
• Less than 5kts

Question 6

Outline the characteristics of mid to high-latitude depressions (‘lows’).

Answer 6

• Move from west to east
• Moist sub-tropical air masses meet cold polar air masses
• Known as polar front depressions

Question 7

Outline the characteristics of sub-tropical depressions.

Answer 7

• More common than tropical cyclones
• Occur when system moves over warm water
• Cold air above increases convection
• 24 - 63kt winds

Question 8

Outline the characteristics of tropical cyclones.

Answer 8

• Move at 10-15kts
• 64+kt winds
• Life cycle of up to 2 weeks

Question 9

Describe the development and the aspects of importance to aviation of lee depressions.

Answer 9

• Lee of mountain sheltered from the wind
• Creates low pressure in the lee
• Bad weather on the leeward side of mountain
• Severe turbulence

Question 10

Describe the development and the aspects of importance to aviation of thermal lows.

Answer 10

• 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

Question 11

Describe the development and the aspects of importance to aviation of depressions crossing a mountain barrier.

Answer 11

• 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

Question 12

Explain what is meant by air-mass modification.

Answer 12

• 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)

Question 13

Describe how stability is associated with either ‘cold air advection’ or ‘warm air advection’.

Answer 13

• K = Cold
• W = Warm
• Cold air advection = cold air moving into warmer areas
• Warm air advection = warm air moving into cold areas

Question 14

Describe how stability is associated with with either ‘cold air advection’ or ‘warm air advection’

Answer 14

• Cold air warms up as it moves to warmer areas
• Increases instability
• Warm air cools down as it moves to colder areas
• Increases stability

Question 15

Describe how cloud types are associated with either ‘cold air advection’ or ‘warm air advection’.

Answer 15

• Cold advection = cumuliform cloud
• Warm advection = low level stratiform cloud or fog

Question 16

Describe how likely precipitation is associated with either ‘cold air advection’ or ‘warm air advection’.

Answer 16

• Cold advection = showers
• Warm advection = drizzle

Question 17

Describe how visibility reductions are associated with either ‘cold air advection’ or ‘warm air advection’.

Answer 17

• Cold advection = visibility good except in showers
• Warm advection = reduced visibility due to drizzle and fog

Question 18

Describe how turbulence is associated with either ‘cold air advection’ or ‘warm air advection’

Answer 18

• Cold advection = moderate to severe turbulence
• Warm advection = Nil or light turbulence

Question 19

Describe the concepts of convergence and divergence.

Answer 19

• (Low) Convergence = air flowing in and ascending at the surface.
• (High) Divergence = air descending at the surface and diverging

Question 20

Explain the vertical motions generated by convergence and divergence near the earth’s surface and immediately beneath the tropopause.

Answer 20

• (Lows) Lower level convergence = upper level divergence
• (Highs) Lower level divergence = upper level convergence

Question 21

Explain the concept of vorticity (rotation or spin) (1)

Answer 21

• 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

Question 22

Explain the concept of vorticity (rotation or spin) (2)

Answer 22

• Absolute vorticity = Relative vorticity + Earths vorticity
• Absolute vorticity is always cyclonic

Question 23

Explain how convergence drives an increase in vorticity conservation of angular momentum.

Answer 23

• 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

Question 24

Explain how convergence drives an increase in vorticity through conservation of angular momentum.

Answer 24

• 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)

Question 25

Outline the effect of vorticity advection on the development of mid-latitude pressure systems.

Answer 25

- 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

Question 26

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.

Answer 26

- 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

Question 27

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.

Answer 27

- 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

Question 28

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.

Answer 28

- The greater the amount of moisture the greater the development of cloud/precipitation. - Larger amounts of latent heat release increasing instability

Question 29

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.

Answer 29

- Unstable = cumulus - Stable = layer type cloud (stratiform)

Question 30

Describe the effect of the slope of the frontal surface on the intensity of fronts, and on the extent of cloud and precipitation.

Answer 30

- Steep sloping = cumulus - Shallow sloping = stratiform

Question 31

Describe the effect of the speed of the front on the intensity of fronts, and on the extent of the cloud and precipitation

Answer 31

- 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

Question 32

Describe the effect of the speed of the front on the intensity of fronts, and on the extent of cloud and precipitation.

Answer 32

Slow front = stable stratiform Fast front = unstable cumulus

Question 33

Describe the effect of the temperature contrast across the front on the intensity of fronts, and on the extent of cloud and precipitation.

Answer 33

- The greater the contrast, the greater the intensity as more air will be able to rise on the warm side

Question 34

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 pressure changes.

Answer 34

- Cold front: Before = decrease At = arrest or fall After = increase - Warm front: Before = decrease At = arrest or fall After = increase

Question 35

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.

Answer 35

- Cold front Before = steady (warm) At = abrupt decrease After = steady (cold) - Warm front Before = steady or slight decrease At = increase After = Little change

Question 36

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.

Answer 36

- 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

Question 37

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.

Answer 37

- 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

Question 38

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.

Answer 38

- 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

Question 39

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.

Answer 39

- 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

Question 40

Explain the concept of an occluded front.

Answer 40

- Cold front overtaking warm front - Depends how cold the air is informs of the warm front - Coldest air will undercut everything else