At the same time the cyclone’s incredibly low air pressure allows the water to rise higher than normal. In places where the wind is blowing onshore, water is pushed up against the land. Surging watersĪ “normal” storm surge happens when a tropical cyclone reaches shallow coastal waters. If sustained wind speeds reach 178km per hour the storm is classed as a major hurricane.

In the northern hemisphere tropical cyclones rotate anticlockwise and officially become hurricanes once they reach a maximum sustained wind speed of around 120km per hour. When a low-pressure system forms over warm tropical oceans under the right conditions, it can intensify to become a tropical depression, then a tropical storm, and ultimately a tropical cyclone – known as a hurricane in the North Atlantic or a typhoon in the northwest Pacific.Īs this process unfolds, the atmospheric pressure drops ever lower and wind strength increases, because the pressure difference with surrounding areas causes more air to flow towards the storm. Roughly speaking, a 1 hectopascal change in atmospheric pressure (the global average pressure is 1,010hPa) causes the sea level to move by 1cm. This is known as the “ inverse barometer effect”.

Areas of high atmospheric pressure actually push down on the ocean surface, lowering sea level, while low pressure allows the sea to rise slightly. At the same time, Earth’s atmosphere has regions where the air pressure is higher or lower than average, in ever-shifting patterns as weather systems move around.

How does cyme ph dissappear