Cyclonic Storms
What is a Cyclonic storm?
Cyclonic storms, also known as tropical cyclones, hurricanes, and typhoons, are rotating wind systems that form over warm ocean waters. They are characterized by a central region of low atmospheric pressure and strong winds that rotate in a counterclockwise direction in the Northern Hemisphere and clockwise direction in the Southern Hemisphere.
How is a cyclonic storm formed?
The formation of cyclonic storms is a complex process that requires a number of factors to come together, including:
- Warm ocean waters: Cyclonic storms need warm ocean waters (at least 26°C or 79°F) to form and develop. The warm water provides the energy that drives the storm.
- Low wind shear: Cyclonic storms also need low wind shear, which is the change in wind direction and speed with altitude. High wind shear can disrupt the storm’s circulation and prevent it from developing.
- Atmospheric instability: Cyclonic storms also need atmospheric instability, which is a condition in which the atmosphere is prone to rising air. Rising air can cool and condense, forming clouds and rain.
Once these conditions are met, a cyclonic storm can begin to form. The process typically starts with a small disturbance in the atmosphere, such as a wave on a weather front. As the disturbance grows, it begins to draw in warm, moist air from the surrounding ocean. This air rises and cools, forming clouds and rain. The rising air also releases latent heat, which further energizes the storm.
As the storm continues to grow, it develops a central region of low atmospheric pressure. This low pressure draws in even more air, which causes the winds to strengthen. The storm also develops a characteristic eye, which is a region of calm, clear air in the center of the storm.
Cyclonic storms can reach wind speeds of over 250 kilometers per hour (155 miles per hour). They can also produce heavy rains and storm surges that can cause significant damage.
Here are the stages of cyclone formation:
- Incipient disturbance: This is the early stage of development, when a small disturbance in the atmosphere begins to form.
- Tropical depression: The disturbance develops into a tropical depression when the winds reach speeds of at least 35 kilometers per hour (22 miles per hour).
- Tropical storm: The tropical depression becomes a tropical storm when the winds reach speeds of at least 65 kilometers per hour (40 miles per hour).
- Hurricane: The tropical storm becomes a hurricane when the winds reach speeds of at least 119 kilometers per hour (74 miles per hour).
Cyclonic storms can continue to intensify after they reach hurricane strength. The strongest hurricanes can have wind speeds of over 300 kilometers per hour (186 miles per hour).
Cyclonic storms are a major natural hazard, and they can cause significant damage to life and property. It is important to be aware of the risks posed by cyclones and to have a plan in place if a cyclone is approaching your area.
Rapid intensification of Cyclonic storm
What is rapid intensification of a cyclonic storm?
Cyclonic storms can intensify rapidly, increasing in wind speed and size in a short period of time. This process is known as rapid intensification. The United States National Hurricane Center defines rapid intensification as an increase in the maximum sustained winds of a tropical cyclone of at least 30 knots (35 mph; 55 km/h) in a 24-hour period.
There are a number of factors that can contribute to the intensification of a cyclonic storm, including:
- Warm ocean waters: Cyclonic storms need warm ocean waters to intensify. The warm water provides the energy that drives the storm.
- Low wind shear: Cyclonic storms also need low wind shear to intensify. High wind shear can disrupt the storm’s circulation and prevent it from developing.
- Atmospheric instability: Cyclonic storms also need atmospheric instability to intensify. This is a condition in which the atmosphere is prone to rising air. Rising air can cool and condense, forming clouds and rain.
Other factors that can contribute to rapid intensification include:
- The presence of a mesoscale convective vortex (MCV): An MCV is a small rotating area of thunderstorms within a tropical cyclone. MCVs can help to enhance the cyclone’s circulation and lead to rapid intensification.
- The presence of a brown ocean vortex (BOV): A BOV is a swirling mass of warm water that can form beneath a tropical cyclone. BOVs can provide additional energy to the cyclone, helping it to intensify more quickly. Rapid intensification can be a dangerous process, as it can make it difficult for forecasters to predict the storm’s track and intensity. It can also lead to unexpected and severe impacts on coastal communities.
In recent years, there has been a trend towards more rapid intensification of tropical cyclones. This is thought to be due to a combination of factors, including climate change, which is leading to warmer ocean waters, and natural variability.
Scientists are working to better understand the factors that contribute to rapid intensification and to develop improved forecasting methods. This will help to reduce the risk of loss of life and property from tropical cyclones.
Forecasting of tropical cyclones has improved greatly in recent years.
There are certain areas over the oceans that are particularly favourable for tropical cyclone development, but it is usually certain characteristics in a cluster of thunderclouds that leads forecasters to recognise them as tropical depressions. This is done by people at specialist tropical cyclone forecasting centres around the globe such as the National Hurricane Center in Miami, Florida who are constantly studying satellite images, instruments and other weather data to detect and track them through their life-cycle. Once detected, their track is forecast using a combination of numerical forecasting models, synoptic forecasting and statistical methods, which have been developed from the study of the behaviour of past storms.
Observations from ships at sea are always very useful, although, once the existence of a storm and its forecast track have been broadcast, ships tend to move out of their way! Reinforced aircraft, fitted with instruments, fly through and over tropical cyclones, and weather radar can locate storms within 200 miles of the radar station. In addition, tropical cyclones are tracked by satellites, which provide very useful information both to forecasters and numerical forecast models.
“Official” responsibility for forecasting tropical cyclones in the areas affected lies with the regional centres such as the National Hurricane Center in Miami. However, the Met Office does produce tropical cyclone forecasts from its global model, which are disseminated to the regional centres. They then use these forecasts, along with other forms of guidance, to produce their forecasts. Met Office forecasts of tropical cyclones show considerable skill and are among the best models available to agencies such as the National Hurricane Center.
Measuring tropical cyclones
In order to categorise tropical cyclones around the world, the Saffir-Simpson Hurricane Wind Scale is used defining events by their wind speed and impacts.
Although developed in the USA, tropical cyclones around the world are measured by the Saffir-Simpson Hurricane Wind Scale which originated from 1971 with Herbert Saffir, a civil engineer and Bob Simpson of the US National Hurricane Center.
The Saffir-Simpson Hurricane Wind Scale consists of a five point scale of hurricane intensity and starts at 74 mph. Tropical cyclones with wind speeds up to 38 mph are classified as tropical depressions and those with wind speeds from 39 - 73 mph are classified as tropical storms.
It is important to note that whilst the Saffir-Simpson Hurricane Wind Scale defines wind strengths and their likely impacts, severe impacts from tropical cyclones can also occur due to storm surge and heavy rain which are not necessarily directly related to the strength of the wind in the tropical cyclone.
Saffir-Simpson Hurricane Wind Scale
Category 1
Wind (mph): 74 - 95
Damage: Minimal - No significant structural damage, can uproot trees and cause some flooding in coastal areas.
Category 2
Wind (mph): 96 - 110
Moderate - No major destruction to buildings, can uproot trees and signs. Coastal flooding can occur. Secondary effects can include the shortage of water and electricity.
Category 3
Wind (mph): 111 - 129
Extensive - Structural damage to small buildings and serious coastal flooding to those on low lying land. Evacuation may be needed.
Category 4
Wind (mph): 130-156
Extreme - All signs and trees blown down with extensive damage to roofs. Flat land inland may become flooded. Evacuation probable.
Category 5
Wind (mph): greater than 156
Catastrophic - Buildings destroyed with small buildings being overturned. All trees and signs blown down. Evacuation of up to 10 miles inland