Atmosphere Contents

6. Secondary Circulation

Conclusions

Secondary circulation is dominated by air masses whose source regions are areas of high pressure. They are grouped into four categories: polar; tropical; continental; and maritime. Different weather extremes occur at different latitudes. Weather fronts form at the boundary between two air masses. Extra-tropical cyclones occur at mid-latitudes as areas of low pressure and tropical cyclones are low-latitude depression systems. Tropical cyclones have local names such as hurricane, typhoon and cyclone. Thunderstorms can develop into hurricanes but only when winds exceed 33 m/s.

Question and answers

1. What are the four categories of air masses?

question 1 answers

2. What is a weather front?

question 2 answers

3. Name the four different types of weather fronts?

question 3 answers

4. Where does the development of extra tropical cyclones occur?

question 4 answers

5. What are the local names for tropical cyclones in i) The Caribbean
and Gulf of Mexico, ii) The West Pacific and China Sea, and
iii) The Indian Sea?

question 5 answers

6. When are hurricanes formed?

question 6 answers

7. What are the main features of a tropical cyclone?

question 7 answers

8. What are the main features of tornadoes?

question 8 answers

Question 1 answer

- Polar
- Tropical
- Continental
- Maritime

Question 2answer

The boundary between two air masses of contrasting temperatures and densities.

Question 3 answer

Stationary, cold, warm and occluded.

Question 4 answer

The development of a cyclone occurs along the polar front (the semi-continuous boundary separating cold polar air and subtropical air).

Question 5 answer

i) 'Hurricane'
ii) 'Typhoon'
iii) 'Cyclone'

Question 6 answer

When the ITCZ is displaced about 5° north or south of the equator. The coriolis force is zero at the equator but displacement away from the equator ensures a rotational component can be achieved about an area of low pressure formed within the ITCZ. They therefore form in oceanic areas with sea surface temperatures > 27°C.

Question 7 answer

- They may last 4 to 5 days with hurricane stage lasting 2 to 3 days, following well-defined paths;
- Travels at 15-25 km per hour;
- Clockwise in the northern hemisphere and counter-clockwise in the southern;
- Approximately 650 km in diameter;
- Sea level air pressure of typically 950 mb;
- Mean surface wind speed > 34 m/s (< 34 m/s = Tropical Storm);
- The storm is fed by the latent heat released from the condensation of water vapour so decay
once over land;
- Can create tidal waves due to the uplift of sea in to the low pressure;
- Rainfall in excess of 1000 mm;
- Generally 80 per year, causing on average 20 000 deaths.

Question 8 answer

- Intense low pressure occurring only over land;
- Diameter between 100 and 600 m;
- Last for an average of a few minutes, travelling only approximately 7km;
- Wind speeds can reach 220 knots but usually reach only about 125knots.
- Form in association with a squall line ( a line of thunderstorms extending hundreds
of kilometres)
- Common over the US in spring and summer;
- the US experiences, on average, 800 tornadoes per year. On average, there is 1
death for every 8 tornadoes;
- 45% of the deaths are associated with mobile homes.