Mid-Latitude and Tropical Cyclones Review

Introduction to Mid-Latitude Cyclones

Mid-latitude cyclones are low-pressure weather systems occurring in the middle latitudes of the Earth, typically between $30^{\circ}$ and $60^{\circ}$ latitude in both the Northern and Southern Hemispheres. These systems are known by various names, including temperate cyclones, extra-tropical cyclones, frontal depressions, or wave cyclones. A mid-latitude cyclone consists of a cold front and a warm front. These systems are specifically associated with the westerlies wind belt and generally move from west to east. In the context of South Africa, cold fronts typically reach the country during winter when pressure belts and wind systems shift slightly northward. This migration is the reason the south-western Cape receives its winter rainfall. The rain is brought by cold fronts moving over the area from west to east, while the warm front is usually bent southwards away from South Africa.

Stages of Development for Mid-Latitude Cyclones

The development of a mid-latitude cyclone involves several distinct phases. The Initial Stage begins when pressure differences exist between warm tropical air and cold polar air, resulting in a pressure gradient around the polar front. A disturbance in the upper atmosphere winds or the presence of mountain ranges causes a wave to form, and a cell of low pressure begins to develop. The Mature Stage occurs as air flows around the low-pressure center, moving clockwise in the Southern Hemisphere and anticlockwise in the Northern Hemisphere. During this stage, the polar front develops into distinct cold and warm fronts, which divide the system into a cold sector and a warm sector. Finally, the Occluded Stage is reached when the faster-moving cold front overtakes the warm front. This replaces the warm air at the surface by pushing it upward, leaving only cold air on the ground.

Weather Conditions and Frontal Occlusions

Weather conditions vary significantly depending on which front is passing over a region. During a Cold Front, temperatures decrease, and while the pressure initially decreases, it increases within the cold sector. Humidity decreases, and cloud cover increases with the appearance of cumulonimbus clouds, leading to heavy rain or snow. Wind direction changes and winds become stronger. In contrast, a Warm Front brings increasing temperatures, decreasing pressure, and increasing humidity. Cloud cover typically consists of nimbostratus clouds, resulting in soft, soaking rainfall.

Frontal occlusions are categorized based on the relative temperatures of the air masses. In a Cold Front Occlusion, the temperature of the air in front of the system is higher than the air behind the system, allowing the cold front to cut into the warm front. In a Warm Front Occlusion, the temperature in front of the system is lower than the air behind it, causing the warm front to cut into the cold front.

Introduction to Tropical Cyclones

Tropical cyclones are intense low-pressure storms that cause significant wind and flood damage. They develop at the Inter-Tropical Convergence Zone (ITCZ) but must form outside $5^{\circ}$ north and south of the equator because the Coriolis force is absent at the equator. These systems develop over oceans where water temperatures are warmer than $28^{\circ}\text{C}$, leading to high evaporation and the creation of hot, humid, unstable air. This air rises to form an intense surface low pressure, which is further intensified by the tropical jet stream in the upper atmosphere. Air is sucked into this low pressure, and the Coriolis force causes winds to spiral toward the center, forming a clear funnel called the eye of the cyclone. Tropical cyclones are named alphabetically according to the season in which they occur and generally move from east to west, driven by the easterlies wind belt.

Life Cycle and Development of Tropical Cyclones

The development of a tropical cyclone occurs in three main stages. The Formative Stage is characterized by very strong updrafts and the intensification of low pressure. While the winds start spiraling, the cyclone is not yet large and lacks a clear eye. The Mature Stage features a very strong pressure gradient with wind speeds reaching hurricane strength and air pressure falling below $950\,hPa$. The eye is well-developed, calm, and clear. In the Southern Hemisphere, the south-western quadrant is the most destructive because the cyclone's winds and the tropical easterly winds coincide. A cyclone in this stage can cover distances of up to $300\,km$ from the eye and moves south-westerly before turning south-east at about $20^{\circ}\text{ South}$. The Dissipating Stage begins when the cyclone moves over land or into cooler sub-tropical areas. Friction over land slows the wind, and decreased evaporation reduces unstable air, causing the pressure to increase and the weather to clear.

Physical Phenomena and Environmental Impacts

Tropical cyclones are associated with several dangerous phenomena. A Storm Surge is a bulge of seawater pushed toward the shore by the force of swirling winds, which, when combined with normal tides, can increase water levels by $4.5\,m$ or more. Inland Flooding results from torrential rains and can persist for several days after winds diminish, destroying agricultural land, cattle, and infrastructure. Flash Floods occur within minutes or hours of excessive rainfall, reaching heights of $10\,m$ or more, capable of rolling boulders and destroying buildings. Urban and Area Flooding are rapid events that turn streets into rivers and basements into dangerous traps. In the eye of the cyclone, conditions are paradoxically calm, with no wind, clouds, or rain, and temperatures are warmer due to adiabatic heating.

Questions and Discussion

Question 1: Identification and Timing Question: Identify the low-pressure system and the season it affects South Africa. What is its movement direction and why? Response: The system is a mid-latitude cyclone (also called a temperate cyclone or frontal depression). It usually affects South Africa during winter. It moves from west to east because it is situated in the westerly wind belt and is driven by those winds.

Question 2: Environmental Impact and Warnings Question: Why is it important for the South African Weather Service to issue warnings for systems like the one on 26 June 2007 (which expected gale-force winds between Plettenberg Bay and East London)? Response: Warnings alert people timeously so they can take precautionary measures, such as evacuations, protecting property, or canceling outdoor and maritime activities. Reports from the public on weather events like snowfalls or hailstorms can also contribute to research and improve future predictions.

Question 3: Global Comparison and Economic Impact Question: Why would the USA suffer more infrastructure damage from a hurricane while Southern Africa would suffer more loss of life? Response: The USA is a developed nation with more expensive and extensive infrastructure to be damaged, but it has highly effective warning and evacuation systems. Southern Africa, containing developing regions, may have less infrastructure but lacks the resources, medical facilities, and communication networks to reach rural populations for early evacuation, leading to higher mortality rates. Factors like poor infrastructure in developing countries also make it difficult to reach affected areas after a disaster, leading to secondary issues like the spread of waterborne diseases (e.g., cholera in Haiti after the earthquake and hurricane).

Question 4: Synoptic and Satellite Analysis Question: How can one distinguish between mid-latitude and tropical cyclones on a map? Response: Mid-latitude cyclones are identified by the presence of cold and warm fronts and are found in the westerly belt. Tropical cyclones, such as Cyclone Fanele, are characterized by a circular shape with a clear eye in the mature stage and move east to west. Global warming is linked to more frequent 'super hurricanes' because increased ocean temperatures result in more evaporation and the release of more latent heat, which deepens the low-pressure system.

Question 5: Agricultural and Regional Effects Question: What are the effects of mid-latitude cyclones on farming in the South-western Cape? Response: These systems provide essential winter rainfall for crops, but they can also cause damage through heavy rain, snow on high grounds, and very cold conditions which impact livestock and crop survival.