Fronts and Depressions

Question 1

What is cold advection?
What are the two different types of cold air mass?
What happens to the lower layers?

Answer 1

• Cold air moving to warmer regions.
• Polar maritime (PmK) and Continental polar (PcK).
• Lower layers warm-up.

Question 2

What happens to the ELR when cold advection occurs?

Answer 2

• ELR becomes steep.

- Instability - vertical current penetrates - CU may form. How much depends upon the surface that is traversed.

Question 3

What will happen initially when cold advection occurs?

Answer 3

• Low level air warms and absorbs some moisture.
• This produces instability and development of a few oktas of SC or CU clouds.
• As air enters low latitudes, its stability will intensify.

Question 4

What will happen to the cold air as it it advects into lower latitudes with warmer oceans?

Answer 4

Large CU and CB formations with associated shower activity take place.

Question 5

What happens as the cold air advects inland over Australia?

What is the weather like when this situation occurs?

Answer 5

• Coastal regions will continue to be affected by the unstable, moist air.
• Weather is cold with frequent showers, visibility good except in showers, if the air is very unstable, thunderstorms with hail showers are possible.
• Because the air mass moves inland, it is away from water sources, the air remains unstable but clouds rapidly decrease. Essentially, the air mass dries out.

Question 6

What happens as the cold air mass advects to NZ?

Answer 6

• Affected by the same instability factors.
• Because of the topography and lack of continental area, the predominant influences of the northbound air stream are felt on the WEST COASTS of both islands, while the EAST COASTS are sheltered.

Question 7

Where does warm air originate from typically?

What are the two types of air masses?

Answer 7

• Originates from subtropical subsidence regions

- Tropical Continental (TcW) and Tropical Maritime (TmW).

Question 8

What happens if the warm air mass moves towards the equator?

Answer 8

• Gets caught up in Easterly winds.

- Results in massive cumuliform and stratiform cloud occurring at the ITCZ.

Question 9

What happens if the warm air mass leaves the subtropics and moves further down to a higher latitude?

Answer 9

• Travel over cooler surfaces will cause lower layers to cool.
• ELR shallows - stability enhanced.
• If originating air is moist - lower levels reach saturation very quickly as temperature reduces.
• Low-level cloud –> fog if the transport speed is slow, if not, then low stratus cloud, poor visibility and drizzle.

Question 10

What happens to the RH when a warm air mass advects to NZ?

Answer 10

Increases

Question 11

What causes further moisture to occur when warm advection occurs in NZ?

Answer 11

Ocean evaporation. This is due to the warm air mass likely originating from AUS and then travelling to NZ and picking moisture up on the way.

Question 12

What type of cloud development will occur as the warm air mass reaches NZ?
Where are clouds restricted to?

Answer 12

• Layer type cloud with a low ceiling is a possibility.
• Possible fog.
• Below inversions.

Question 13

What will an increase in stability as a result of warm advection cause?
What happens to the ELR?

Answer 13

• Minimises convection

- ELR is shallow, accompanied with inversions.

Question 14

Where are clouds restricted to with a warm advection in NZ?

Answer 14

• Below the inversion layer.

Question 15

What happens as the warm air mass reaches land in NZ?

Answer 15

Forced ascent (mechanical lifting) produces clouds with bases possibly to sea level.

Question 16

With regards to fog and drizzle with a warm air mass, where in NZ is it typically found?

Answer 16

• Northland
• Auckland
• Bay of Plenty
• Marlborough Sounds
• Nelson

Question 17

Why do winds at progressively higher altitudes in mid-latitudes become westerly?

Answer 17

• Caused by unequal heating of the Earth.
• This produces a latitudinal temperature gradient which causes pressure surface to slope poleward.
• The higher you go in the different temperatures, the larger the pressure difference.
• Pressure would be reduced quicker in the cold dense column of air whereas in the warm column, the opposite will occur.
• This means, at the same altitude of each column, there will be a higher pressure in the warm column compared to the cold column.
• This produces a poleward directed pressure gradient.
• This poleward movement of air will then be subject to the Coriolis force, deflecting it to the right in the Northern Hemisphere, and left in the Southern Hemisphere.
• This creates the westerly wind.
• The greater the temperature difference, the greater the pressure gradient, and the stronger the westerly winds will be.
• This is why they’re stronger in winter.