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MET 2 > Airmasses & Fronts > Flashcards

Airmasses & Fronts Flashcards

1
Q

Define a Front

A

The boundary between two airmasses with differing characteristics (temp/pressure).

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2
Q

Define an air mass

A

A large body of air whose physical characteristics, particularly temp and humidity, are approximately the same over large horizontal distances (100s of KMs)

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3
Q

What are the four Air mass source regions

A

(A): Antarctic (Very cold)
(P): Polar (Cold)
(T): Tropical (Warm)
(E): Equatorial (Warmer/hot)

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4
Q

What does the air mass source region define

A

The temperature properties of the air mass.

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5
Q

What else defines and is assigned to air masses?

A

Whether the air is moist or dry:

m) : Maritime (Moist
(c) : Continental (Dry)

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6
Q

How many subsequent types of airmasses are there?

A
  • Seven: mE, mT, mP, cE, cT, cP, cA

mA cannot exist as there is no maritime area in Antarctica

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7
Q

What are the only two Air masses experienced in NZ

A

mT

mP

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8
Q

How is mT experienced in NZ.

A
  • Tropical maritime air masses are warm and moist.
  • Move over NZ from the North, become cooler and more stable as they move South
  • Due cooling, condensation occurs, low Stratus cloud and fog can form, as well as Drizzle and light rain.
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9
Q

How is mP experience in NZ

A
  • Polar maritime air masses are cold and moist.
  • Move over NZ from the S or SW, becoming warmer and more modified and unstable as they move North.
  • Due warming from SFC, air will rise and cool adiabatically, forming convective Cumulus cloud. Producing Showers of rain, hail or snow.
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10
Q

What are active fronts characterised by? (4)

A
  1. Large temp gradient across the front
  2. Moving at >30kts
  3. Front located under an area of upper level divergence
  4. Polar frontal jetstream associated with the front has a strong N to S flow
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11
Q

Define the process of a cold front

A

A front where Cold air undercuts a slower moving mass of warm air.

  1. Advancing cold air forces warmer, less dense air upwards.
  2. This process is fast, and generates Tcu and Cb
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12
Q

What are the characteristics of a cold front (6)

A
  1. Rapid movement (15 -40kts)
  2. Marked temp drop as front goes through
  3. Abrupt wind change from Northerly 1/4 to SW 1/4
  4. Sharp pressure rise in the colder, heavier air mass following the front.
  5. Cumuliform clouds with SH of rain, hail or snow
  6. Narrow band of precip, usually just prior to wind change.
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13
Q

Define the process of a warm front

A

A front where warm air is forced aloft as it rides up over a slower - moving mass of cooler air.
1. Warmer, less dense air rises over cooler air causing a broad area of cloud to develop ahead of the front. (air rises slower than that of a cold front = Stratiform cloud)

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14
Q

What are the characteristics of a warm front (6)

A
  1. Slow movement (5 - 15kts)
  2. Temp rises behind the front with clearing WX
  3. Wind change from NNE to NW
  4. Extensive sheet of Stratiform cloud that thickens and lowers 100s of kms ahead of the front
  5. Rain falling from As and Ns for a long time ahead of SFC wind change
  6. Possible low stratus and fog below frontal As and Ns.
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15
Q

Define an occluded front and the two different types

A

A front that has both the characteristics of a cold and warm front, and is relatively weaker due less thermal contrast in comparison.

  1. Cold Occlusions
  2. Warm Occlusions
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16
Q

Characteristics of an occluded front (4)

A
  1. Extensive cloud sheet of Stratiform, thickens and lowers as front approaches
  2. Long periods of rain, occ heavy showers
  3. Abrupt wind change from N 1/4 to SW 1/4
  4. Slight drop in temp in cooler air behind the front
17
Q

Define a stationary front

What wx is it associated with?

A 
Any front (cold/warm/occluded), that has weakened and 
 slowed to less than 5kts ground speed.

Associated with loss of original upper cloud sheet, and bands of cumulus and scattered middle cloud

18
Q

Summary of a Southerly flow onto NZ

A
  • Cold mP air moving north, heated from the SFC
  • Air mass becomes increasingly unstable
  • Air begins to rise (convection), forming Cb/Tcu/Cu.
  • Results in SH of rain, hail, snow and sleet.
  • Poor vis at times and often turbulent.
19
Q

Summary of a Northerly flow onto NZ

A
  • Warm mT air moving South, Cooled from the SFC
  • A low level temperature inversion and increased stability are created
  • Any lifting (from widespread ascent/orographic) results in stratiform cloud (St, Sc, As, Cs), with low bases possible.
  • Results in continuous Rain or drizzle and poor vis (due low inversion/RA/DZ) , with light - moderate turbulence (again due to inversion = wind shear)

MET 2 (12 decks)

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