What is a hurricane and how do they form?

Yahoo News UK explains what you need to know about hurricanes.

heavy rain storm with lightning
A hurricane is another name for a tropical cyclone. (Getty)

Hurricane Milton has made landfall in Florida, whipping up deadly tornadoes, destroying homes and knocking out power to nearly three million customers.

Formerly a category five hurricane, Milton was downgraded to a category three when it hit Florida on Wednesday night, before again downgraded to an category two and then category one by the National Hurricane Center (NHC) in the US. Despite the downgrading of the storm, the powerful winds have still caused destruction and reported deaths, and forced more than a million people in coastal areas to evacuate.

The storm made landfall around 8:30pm Florida time (12.30am GMT) on Wednesday as a category 3 hurricane with maximum sustained winds of 120 mph near Siesta Key, the NHC said.

Ahead of Milton’s arrival, US president Joe Biden warned people in evacuation zones to get out or risk death. Milton still poses storm surge danger on the state's Atlantic Coast.

The hurricane became the third-fastest intensifying storm on record in the Atlantic, growing from a category one to a category five in less than 24 hours.

Before its downgrade, Milton was at category five – the highest level possible – with wind speeds of over 160mph.

FILE PHOTO: A satellite image shows Hurricane Milton progressing before its expected landfall in Florida, in the Gulf of Mexico October 8, 2024.    CIRA/NOAA/Handout via REUTERS    THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY./File Photo
A satellite image shows Hurricane Milton progressing towards Florida. (NOAA/Reuters)

A hurricane is another name for a powerful tropical storm that forms over warm ocean waters, characterised by strong winds, heavy rainfall, and a well-defined centre known as the eye. Hurricanes start as clusters of clouds over tropical oceans which come together and rotate to form a tropical depression.

These storms are classified as hurricanes when their wind speeds reach 74mph or higher.

Hurricanes specifically form in the Atlantic or eastern Pacific Oceans and can vary in strength. Their sizes also vary but they can reach between 300 and 500 miles across and up to five or six miles high.

They cause severe weather events such as torrential rain, flooding, and destructive wind gusts. When hurricanes make landfall, they can cause significant damage to coastal and inland areas, resulting in storm surges and landslides.

Hurricanes require high sea surface temperatures – and as a result cannot form around the UK, according to the Met Office. However, the UK can still be affected by them when they move to higher latitudes and produce near-hurricane strength winds, causing severe storms.

According to the National Oceanic and Atmospheric Administration (NOAA), hurricanes are also known as typhoons, severe tropical cyclones, very severe cyclonic storms and tropical cyclones – depending on what part of the world they are formed.

Hurricane Irma seen striking Miami, Florida with 100+ mph winds and destructive storm surge.
Tropical storms are classified as hurricanes when their wind speeds reach 74mph or higher. (Getty)

Hurricanes form under certain conditions – including warm ocean waters, moist air, and specific weather patterns.

The process begins when warm, moist air rises from the ocean, creating low pressure below it. As this air continues to rise, it cools and forms clouds, releasing heat that lowers the pressure even more.

This cycle creates strong winds that flow in from surrounding areas, contributing to the storm’s growth. The Earth’s rotation also causes the storm to spin, which is essential for its formation – known as the Coriolis effect. These conditions usually happen in tropical regions where the ocean is warmest.

Hurricane season in the Atlantic lasts from 1 June to 30 November each year. This period is marked by an increased likelihood of hurricane formation due to warmer ocean waters and atmospheric conditions ideal for hurricanes to form.

The most active months typically occur in August and September, when sea surface temperatures reach their peak, often exceeding 80F, which provides the warm, moist air necessary for storm formation.

In August and September, the atmosphere is also more unstable, meaning there are stronger upward motions of air that help storms grow. This time of year also usually sees a reduction in wind shear – a change in wind speed and direction at different altitudes – which can disrupt developing storms. Low wind shear allows hurricanes to organise and intensify.

As the season moves into October and November, the number of storms usually decreases, but significant hurricanes can still occur.

Oct 1995.
Category 5 hurricanes cause catastrophic damage. (Getty)

A category five hurricane is the most severe classification on the Saffir-Simpson hurricane wind scale. Sustained winds of 157mph or higher must be reached for a hurricane to reach this category.

This level of hurricane can cause catastrophic damage, with a high likelihood of total destruction to buildings, severe flooding, and widespread power outages, according to the National Hurricane Center (NHC). Coastal areas are particularly vulnerable, and such hurricanes can lead to significant loss of life if proper precautions are not taken.

Hurricanes in this category can cause catastrophic damage, including the destruction of buildings and the uprooting of all trees and signs. Power outages can last for weeks, even months, while most of the area will also be uninhabitable for weeks or months.

However, the Saffir-Simpson hurricane wind scale is only based on a hurricane's maximum sustained wind speed. The scale does not take into account other potentially deadly hazards such as storm surge, rainfall flooding, and tornadoes, the NHC said.