Many people in Kenya have not encountered cyclones and the disasters associated with them.
We only read about them in other parts of the world. The recent occurrence of Cyclone Idai and Cyclone Kenneth, striking Mozambique and Malawi in March and April this year respectively, has raised concerns about the possibilities of cyclones making landfalls in Africa.
There is anxiety about where a cyclone is likely to hit next or in the near future.
But what is a cyclone? A cyclone is a large air mass that rotates around a centre of low atmosphere pressure. In meteorological lingo, cyclones are called depressions or storms.
The majority of cyclones are also called hurricanes, but they have other names given according to areas where they occur.
The names include typhoons in West-north Pacific, Willy-willies in Australia, hurricanes in North and Central America or Baguios in the Philippines.
Therefore, the name cyclone is in reference to those that are formed in the Indian Ocean.
Tropical regions are warm areas due to high amounts of incoming solar radiation (heat). This heat is the source of energy for cyclone formation and the ocean waters is the source of moisture.
For cyclones to form, the surface temperature of the water has to be 27 degrees centigrade or above. This is only possible in tropical oceans.
Warm ocean waters initiate the process of evaporation where water vapour will rise and cool adiabatically, and thereby releasing the heat retained during evaporation (known as latent heat of condensation) to the atmosphere.
The rising warm air creates a low pressure centre at the ocean surface, allowing replacement of the risen air with colder air.
Most cyclones originate on either side of the low pressure zone around the equator called the Intertropical Convergence Zone. This is the zone where southeast and northwest trade winds converge.
We all know that the earth rotates around its axis. A complete rotation lasts 24 hours (a day).
The speed of earth’s spin is greatest at the equator, about 1,670 km/hr and reduces gradually towards the poles.
This spinning of the earth creates an apparent deflection force in the paths of moving objects called Coriolis effect.
Objects (air included) will be deflected to the right in Northern hemisphere and to the left in Southern hemisphere.
Circular low pressure cells and Coriolis effect determine the direction and speed of wind. Perhaps it would be essential to define wind here.
What do we know about air? Air is a mixture of gases mainly nitrogen and oxygen with other gases and particulates like dust, smoke, and sea salt particles.
Atmospheric air is always in motion so, a befitting definition of wind is “movement of air in a hurry”.
Once cyclones are formed they are always mobile, following irregular paths or tracks within the general flow of trade winds.
It is not easy to predict a specific cyclone path in advance, but the general path is highly predictable depending on where they form in the ocean.
The general pattern of cyclone movement is from east to west. There were reports that Cyclone Kenneth would make a landfall at the Kenyan coast but fortunately, this did not happen.
Kenya’s location around the Equator inhibits cyclone occurrence. The country lies within the equatorial zone 5o North and 5o South where Coriolis effect is zero.
This is a zone referred to in meteorological terms as the “doldrums”. Here, air will flow straight into low pressure centres without being deflected.
Most cyclones occur in the tropics and that is why they are mostly referred to as tropical cyclones. Their frequency is higher on East Coast of tropical countries.
They are not common in Africa, but the countries that are prone to cyclone episodes are Mozambique, Malawi, Zimbabwe and of course the adjacent Madagascar Island.
On a global cyclone perspective, countries that experience frequent cyclones are Cuba, Dominican Republic, United States (Florida, Georgia and Texas), Mexico, Japan, Philippines, India and China.
Recall cyclones Katrina in 2005 (Florida, USA), Haiyan in 2013 (Philippines), Irma in 2017 (Cuba), Mangkhut in 2018 (Philippines) and of course Idai and Kenneth this year.
The intensity of cyclone wind speed and associated potential property damage are classified into five categories according to the Saffir-Simpson Hurricane Wind Scale.
Category 1 cyclone is the lowest in terms of damage with wind speeds of 119-153km per hour. Category 5 is the highest with wind speeds over 250km per hour and the damage associated with it is catastrophic.
Most deaths and destruction caused by cyclones occur as a result of high winds and torrential rain. These cause flooding, uprooting of trees, destruction of buildings and disruption of communication networks.
Most cyclone-related fatalities are as a result of drowning. The amount of damage caused by a cyclone depends on its category, physical configuration of landscape together with population size and density.
Cyclones initiate landslides due to intense rainfall. They also damage crops and orchards. Epidemics are common in the affected areas due to poor or destroyed sanitary conditions.
Humanitarian aid is necessary in the affected areas. When a cyclone is expected or strikes, safety measures include avoiding outdoor movement, listening to local radio for warnings and advice and using recommended routes for evacuation.
The life span of a cyclone ranges from three to seven days to four weeks. Once a cyclone moves to the land surface (landfall), the source of heat and moisture (ocean surface) is curtailed and the cyclone dissipates or fizzles out.
Destruction and tragedy are not the only legacies of cyclones. They transfer heat from the tropics towards the poles. They also bring rain to dry areas.
Regions like North West Mexico, North Australia and South-Eastern Asia rely on tropical cyclones for much of their water.
Cyclone-induced rainfall is often a critical source of moisture for agriculture in semi-arid areas which are prone to cyclones.
Dr Ndolo teaches Climatology and Applied Statistics at the University of Nairobi ([email protected] uonbi.ac.ke)