AIRMASSES & FRONTS

Question 1

Define Airmass

Answer 1

A large body of air whose physical characteristics (esp temp & humidity) are approx the same, through altitude & over large distances.

Question 2

What are the four source regions

Answer 2

1. (A) Antarctic (very cold air from Antarctic high)
2. (P) Polar (Cold air from southern oceans)
3. (T) Tropical (warm air from sub-tropical high pressure belt)
4. (E) Equatorial (warmer air from zone between trade winds & equatorial trough).

Question 3

What do the four source regions describe
What do we add to indicate moist vs. dry?

Answer 3

the temperature properties of an airmass
Moist: Maritime (m)
Dry: continental (c)

Question 4

What two air masses do we experience in NZ?

Answer 4

mT: Tropical maritime
mP: Polar maritime

Question 5

Describe an mT air mass

Answer 5

Warm & moist. move onto NZ from the north, cool down & become more stable as they move south.
Further cooling results in condensation & cloud formation = low layer cloud/fog with DZ & -RA

Question 6

Describe an mP air mass

Answer 6

Cold and moist. Move onto NZ from the S & SW. Become unstable as they move north & begin to heat up. Parcels of air rise & cool adiabatically, leading to condensation & convective cloud = SHRA, GS & SN

Question 7

What does a front mark?

Answer 7

A boundary between two different airmasses

Question 8

Define frontogenesis

Answer 8

The process of developing a front

Question 9

When will a front weaken

Answer 9

When the contrast between the airmasses is decreased through modification by the underlying surface or by the effects of diffluence

Question 10

Four things that characterise active fronts?

Answer 10

1. Large temp gradient across the front
2. Front is moving at 30kts across the SFC
3. Front is located under an area of upper level DIV
4. The polar frontal jetstream associated with that front has a large N-S component to its flow.

Question 11

What is a cold front

Answer 11

One where cold air undercuts a slower moving mass of warm air. Advancing cold air forces the warmer, less dense air upwards rapidly.
Embedded CBs/TCu are common

Question 12

What would you expect if a cold front is near a developing Low

Answer 12

cloud sheet will be deep & active as a result of upper level DIV, enhancing upward motion.

Question 13

What would you expect if a cold front is far from a developing Low

Answer 13

Usually subsidence (downward motion) aloft (associated with upper level CONV). The cloud band will be narrow & shallow

Question 14

6 characteristics of a cold front?

Answer 14

1. fast moving (15-40kts)
2. Marked temp drop as front goes through
3. Abrupt wind change from N to SW quarters (SH)
4. Sharp pressure rise in colder, heavier airmass following front
5. Cu clouds with RA, GS and SN showers
6. Narrow band of precip, just prior to wind change

Question 15

What is a warm front

Answer 15

Occur when warm air is forced aloft as it rises up and over a slower moving mass of cool air.
Broad area of stratiform type cloud develops ahead of SFC front.
Air rises slower compared to cold front.

Question 16

6 characteristics of a warm front?

Answer 16

1. Slow movement, 5-15kts
2. Temps rise behind the front with clearing wx
3. Wind change from NNE too NW, not as marked
4. Extensive sheet of layer cloud, thickens & lowers 100s of Kms ahead of SFC front
5. RA falling from As and Ns for a long period ahead of wind change
6. Poss low stratus or fog n air below As and Ns due continuous rain

Question 17

What is an occluded front?

Answer 17

A front that has the characteristics of both a cold and a warm front. There are cold occlusions and warm occlusions.
Thermal contrast isn’t as marked, so they are relatively weak, but there still may be a considerable difference in moisture.

Question 18

4 Characteristics of an occluded front

Answer 18

1. Extensive cloud sheet of the warm front type, thickens & lowers as the front appchs
2. Long periods of rain & heavier showers
3. Wind change as with cold front type
4. Temp may drop a little with cooler air behind front

Question 19

Define a stationary front

Answer 19

any front that slows to <5kts G/S

Question 20

Why would a front become slower moving

Answer 20

As a result of running into a blocking H pressure system, or into an area with a slack pressure gradient.

Question 21

Characteristics of a stationary front

Answer 21

Front tends to weaken & lose its upper cloud sheet.
Associated with bands of Cu with scattered middle cloud. Should nothing occur to redevelop the front, it will dissipate.

Question 22

What can the approach of an upper trough or jet do to a stationary front?

Answer 22

Reinvigorate the activity of the front, which in some cases can lead to the development of a new low.

Question 23

Describe the typical associated factors for a southerly flow onto NZ;
- Stability
- Cloud types
- Precip
- Vis reductions
- Turb

Answer 23

• Cold, moist air onto the country. As it moves north, air is warmed from below, airmass is destabilised
• Destabilisation results in convective type clouds (Cu, TCu, and CBs)
• Showers of RA, SN, GS or sleet. may be heavy & frequent (in bursts)
• In Showers from CBs the vis may be particularly poor, in between SH however it will be good
• Mod to Sev Mech turb may exist (southerly flows are strong), particularly in lee of ranges. CBs = SEV turb

Question 24

Describe the typical associated factors for a northerly flow onto NZ;
- Stability
- Cloud types
- Precip
- Vis reductions
- Turb

Answer 24

• Warm, moist air moving south over country, airmass cooled from below, results in low level inversion & stable airmass.
• Even though stable, cloud will still form with any lifting mechanism (e.g. warm front/orographic lifting). Stratiform clouds are common; St, Sc, As, Ns and Cs.
• Any precip must fall from stratiform cloud, thereform will be continuous RA or DZ
• Inversion close to the SFC means vis will slowly decline prior to arrival of rain (due pollutants being trapped). Prior to rain; 15-20km, rain set in; 2000-5000m
• Severity of turb depends on isobar spacing. Usually they are wide, so turb is light-mod in intensity. Likely to be a narrow zone of windshear at the inversion.