Cyclones

Mid-latitude cyclones

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):

• Cool temperatures
• Very low pressure
• Overcast conditions, cumulonimbus clouds
• Thunderstorms

Weather behind the cold front (see point 2 in Figure 1.2.1A above):

• Cold temperatures
• High pressure
• Partly cloudy conditions, cumulus clouds
• Light rain

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.

Tropical cyclones

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:

• temperature of 27°C and more
• Coriolis force
• enough moisture
• less function

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

Table 1.1 compares the characteristics of mid-latitude cyclones and tropical cyclones.

	CHARACTERISTICS
	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: