23rd October 2015. The ‘strongest ever’ hurricane recorded in the Western hemisphere is about to make a “potentially catastrophic” landfall on the western coast of Mexico. This is hurricane Patricia. At that time, the super storm is a Category 5. Weather scientists predict 20 inches of rain and 200 miles per hour winds…
Every year, dozens of large storms occur within the Tropical regions of our planet. A tropical storm is an intense low pressure weather system, that can last for days to weeks and spread devastation when it reaches populated land.
Despite the fact that in the UK we only really get to hear about tropical storms affecting the USA, it is worth noting that the highest number of storms does not generally occur in the Atlantic, close to the USA, but in the North Pacific, affecting many countries such as the Philippines and Japan. The most affected region of South East Asia receives an average of 26 storms per year. The least affected area, India, has an average of 2 tropical storms per year.
How do hurricanes and tropical storms form?
A Major Dip in Atmospheric Pressure
Hurricanes are some of the most awesome and violent storms of the Earth’s atmosphere. They form along the Equator over warm ocean waters. The term ‘hurricane’ is used only for the largest storms that form over the Atlantic Ocean or eastern Pacific Ocean. Wherever they occur, the generic and scientific term for these storms, is tropical cyclone.
Other names are given depending on where in the world they are born: typhoons, cyclones, severe tropical cyclones, or severe cyclonic storms. Whatever they are called, the same forces and conditions are at work in forming these giant storms, any of which can cause damage or devastation when they hit land where people live.
Tropical cyclones could be compared to engines that require warm, moist air as fuel.
The first thing that is needed to make a tropical cyclone is warm ocean water. Tropical cyclones form only in tropical regions where the temperature of ocean waters rise above 27 °C (around 80 degrees Farenheit), for at least the top 50 metres (about 165 feet) below the surface.
The second thing that is needed for a tropical cyclone to develop, is wind.
The Beaufort Scale – Quantifying the Wind
The Beaufort Wind Force Scale is an empirical measure that relates wind speed to observed conditions, whether at sea or on land:
|Beaufort Number||Description||Wind Speed||Waves Height||On Land||At Sea|
|0||Calm||< 1.1 km/h|
(< 0.7 mph)
< 0.6 knot
|0 m||Calm. Smoke rises vertically.||Flat|
|1||Light air||1.1-1.5 km/h|
|0-0.2 m||Smoke drift indicates wind direction. Leaves and wind vanes are stationary.||Ripples without crests.|
|2||Light breeze||5.5-11.9 km/h|
$ ($3.4-7.4 mph$ )$ 3-6.4 knot
|0.2-0.5 m||Wind felt on exposed skin. Leaves rustle. Wind vanes start to move.||Small wavelets. Crests of glassy appearance, not breaking.|
|3||Gentle breeze||11.9-19.7 km/h|
|0.5-1 m||Leaves and small twigs constantly moving. Light flags extended.||Large wavelets. Crests begin to break. Scattered whitecaps.|
|4||Moderate breeze||19.7-28.7 km/h|
|1-2 m||Dust and loose paper raised. Small branches begin to move.||Small waves with breaking crests. Fairly frequent whitecaps.|
|5||Fresh breeze||28.7-38.8 km/h|
|2-3 m||Branches of moderate size move. Small trees in leaf begin to sway.||Moderate waves of some length. Many whitecaps. Small amounts of spray.|
|6||Strong breeze||38.8-49.9 km/h|
|3-4 m||Large branches in motion. Whistling heard in overhead wires. Umbrella use becomes difficult. Empty plastic bins tip over.||Long waves begin to form. White foam crests are very frequent. Some airborne spray is present.|
|7||High wind, moderate gale, near gale||49.9-61.8 km/h|
|4-5.5 m||Whole trees in motion. Effort needed to walk against the wind.||Sea heaps up. Some foam from breaking waves is blown into streaks along wind direction. Moderate amounts of airborne spray.|
|8||Gale, fresh gale||61.8-74.6 km/h|
|5.5-7.5 m||Some twigs broken from trees. Cars veer on road. Progress on foot is seriously impeded.||Moderately high waves with breaking crests forming spindrift. Well-marked streaks of foam are blown along wind direction. Considerable airborne spray.|
|9||Strong/severe gale||74.6-88.1 km/h|
|7-10 m||Some branches break off trees, and some small trees blow over.||High waves whose crests sometimes roll over. Dense foam is blown along wind direction. Large amounts of airborne spray may begin to reduce visibility.|
|10||Storm, whole gale||88.1-102.4 km/h|
|9-12.5 m||Trees are broken off or uprooted. Structural damage is likely.||Very high waves with overhanging crests. Large patches of foam from wave crests give the sea a white appearance. Considerable tumbling of waves with heavy impact. Large amount of airborne spray reduce visibility.|
|11||Violent storm||102.4-117.4 km/h|
|11.5-16 m||Widespread vegetation and structural damage likely.||Exceptionally high waves. Very large patches of foam, driven before the wind, cover much of the sea surface. Very large amounts of airborne spray severely reduce visibility.|
|12||Hurricane force||>=117.4 km/h|
>= 63.4 knot
|>=14 m||Severe widespread damage to vegetation and structures. Debris and unsecured objects are hurled about.||Huge waves. Sea is completely white with foam and spray. Air is filled with driving spray, greatly reducing visibility.|
In the case of the hurricanes that form in the Atlantic Ocean, the wind blowing westward across the Atlantic from Africa provides the necessary ingredient. As the wind passes over the ocean’s surface, water evaporates (it turns into water vapour) and rises. As it rises, the water vapour cools, and condenses back into large water droplets, forming large cumulonimbus clouds. These clouds are just the beginning…
The Lifecycle of a Tropical Cyclone
Meteorologists have divided the development of a tropical cyclone into four stages: from a tropical disturbance, or a tropical depression, to a tropical storm, and a fully-fledged tropical cyclone:
When the water vapour from the warm ocean condenses to form clouds, it releases its heat to the air. The warmed air rises and is pulled into the column of clouds. Evaporation and condensation continue, building the cloud columns higher and larger.
A pattern develops, with the wind circulating around a centre (much like water going down a drain). As the moving column of air encounters more clouds, it becomes a cluster of thunderstorm clouds, called a tropical disturbance.
As the thunderstorm grows higher and larger, the air at the top of the cloud column cools and becomes unstable. As the heat energy is released from the cooling water vapour, the air at the top of the clouds becomes warmer, making the air pressure higher and causing winds to move outward, away from the high pressure area.
This movement and warming causes pressures to drop at the surface. Then, air at the surface moves toward the lower pressure area, rises, and creates more thunderstorms. Winds in the storm cloud column spin faster and faster, whipping around in a circular motion. When the winds reach between about 40 and 60 km/h (25 and 38 mph, anything from a strong breeze to moderate gales), the storm is called a tropical depression.
When the wind speeds reach over 60 km/h (39 mph), the tropical depression becomes a tropical storm. This is also the very point when the storm gets a name! The winds blow faster and begin twisting and turning around the eye, or calm centre, of the storm.
Tropical cycloneWhen the wind speeds reach 120 km/h (74 mph), the storm is officially a tropical cyclone. The storm is at least 50,000 feet high and around 125 miles across. The eye is around 5 to 30 miles wide. The trade winds (which blow from east to west) push the tropical cyclone towards the West – towards the Caribbean, the Gulf of Mexico, or the south-eastern coast of the U.S.
The winds and the low air pressure also cause a huge mound of ocean water to pile up near the eye of the tropical cyclone, which can cause monster storm surges when all this water reaches land.
The central pressure of hurricane Patricia was initially estimated to be about 880 millibars. And as we know, the lower the pressure, the stronger the winds are expected to be.
Tropical cyclones usually weaken when they hit land, because they are no longer being fed by the energy from the warm ocean waters. However, they often move far inland, dumping many inches of rain and causing lots of wind damage before they die out completely.
23rd October 2015 at 6:15 PM CDT. Hurricane Patricia officially makes landfall 88 kilometres (55 miles) west northwest of Manzanillo, Mexico. At that point in time, the mega-storm lost some of its peak, record intensity, but it still hit with 265 km/h (165 mph) winds and a pressure of 920 mb.
Hurricane Patricia weakened rapidly over Mexico. Tropical storms are defined by their wind speeds and the potential damage they can cause. The initial Category 5 hurricane was downgraded to a Category 2 tropical storm on the Saffir-Simpson Hurricane Wind scale:
|Category||Sustained Wind Speeds||Central Pressure||Expected Damage|
|5||>= 252 km/h|
(>= 157 mph)
|< 920 mbar||Catastrophic|
Extremely dangerous winds
|980-994 mbar||Some Very dangerous winds|
|Tropical Storm||63-118 km/s|
|> 995 mbar|
|Tropical Depression||< 62 km/h||(38 mph)|
Patricia was the first Category 5 storm to make landfall in North America since Dean in 2007. But earlier in the day, it had peaked with 879 mb pressure and 320 km/h (200 mph) winds. Gusts were up to 390 km/h (245 mph).
Hurricane Patricia began as a powerful storm, and very much on the scale of Typhoon Haiyan, which made such a devastating landfall on the Philippines in 2013.
We can credit El Niño warmed waters for fuelling this beast. Patricia was not a large storm, as hurricane force winds extended only (35 miles) from its centre. But it was epic in its timeline and its intensity.
Hurricane Patricia ranks as the strongest ever on record in the Northern hemisphere, as well as the second strongest on record globally.