In this section, we look at mid-latitude cyclones in more detail. We will focus on the cross-section through a mature mid-latitude cyclone and the weather that occurs as a result of the cold front. This is the most frequently tested section as South Africa is mostly affected by the passage of cold fronts.
Figure 1.2.1A shows a cross-section through a mid-latitude cyclone. You must be able to label and sketch the cross-section from a synoptic view, as shown in Figure 1.2.3A (see page 10, table 1.1).
Learn to redraw Figure 1.2.1A and label it. You must be able to describe the weather that occurs with a mid-latitude cyclone.
As a mid-latitude cyclone moves towards South Africa, it is the cold front that mostly affects our weather (see Figure 1.2.1C on page 8).
Weather in front of the cold front (see point 1 in Figure 1.2.1A above):
Weather behind the cold front (see point 2 in Figure 1.2.1A above):
Note that as a mid-latitude cyclone moves from west to east, we experience the warm air mass in front of the cold front first, then the air behind the cold front. This can be seen in Figure 1.2.1A (above) as you move over from point 1 to 2.
Figure 1.2.1B (below) shows the weather conditions before and after the cold front.
A tropical cyclone is a type of low pressure system which generally forms in the tropics (between 5°C and 30°C North and South). It is accompanied by thunderstorms and a circulation of winds near the Earth’s surface, which is clockwise in the southern hemisphere and counter-clockwise in the northern hemisphere. Tropical cyclones are also known as hurricanes in America; typhoons in China and Japan; and willywillies in Australia. Tropical cyclones are given names alphabetically within the season in which they occured. For example, ‘Alfred’ will denote that it is the first tropical cyclone to occur in that season.
We will now look at tropical cyclones in more detail by focusing on the cross-section through a mature tropical cyclone.
In order for the tropical cyclone to occur there should be:
Figure 1.2.2A shows a crosssection through a tropical cyclone. You must be able to label and sketch the cross-section from a synoptic view as shown in Figure 1.2.3A (see page 10, table 1.1).
Make sure you are able to draw Figure
1.2.2A and label it correctly.
1.2.3 Characteristics of mid-latitude cyclones and tropical cyclones
1. Multiple-choice questions
2. Short answer questions
For example, see Figures 1.2.2A and 1.2.3B.
Table 1.1 compares the characteristics of mid-latitude cyclones and tropical cyclones.
|Mid-latitude cyclone||Tropical cyclone|
|Other names||Frontal depression, temperate cyclone, extra tropical cyclone||Hurricane, typhoon, Willywillies (named alphabetically at the beginning of each season)|
|Formation||40–60o N and S||around 5o N and S|
|Occurrence||30–60o N and S||Over tropical oceans 5–30o N and S|
|Movement||West to east (driven/pushed by Westerlies)||East to west (driven/pushed by Easterlies)|
|Season||All year round in both hemispheres; affects South Africa in winter||Mid- to late summer, early autumn|
|Identifying features||Warm front, warm sector, cold front, cold sector||Stormy weather in the vortex; the eye is a calm, intense low pressure area|
|Weather||Weather associated with a cold front: overcast, low temperatures, strong winds, heavy rain||Warm to hot, violent winds; intense thunderstorm activity|
|Areas affected||Western side of continents in mid-latitudes||Eastern side of continents in tropical latitudes|
|Synoptic representation of the cyclones on a weather map|
You must know the characteristics of each cyclone. In the exam these characteristics are typically asked in one of three ways: