The wind is simply air in motion, flowing from high atmospheric pressures to low pressures. Moving anything requires a force. Strong winds are due to a strong pressure gradient force. A pressure gradient is how fast pressure changes over distance. So, when pressure changes rapidly over a small distance, the pressure gradient force is large.
Strong winds almost always result from large pressure gradients. Share Flipboard Email. Rachelle Oblack. She specializes in climate and weather. Updated October 27, Featured Video. Cite this Article Format. Oblack, Rachelle. What It's Like to Experience a Hurricane. The Halloween Storm of the Century in Winds and the Pressure Gradient Force.
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The eye can have quite calm winds and cloud-free skies, but this lull is temporary and is followed by destructive winds from another direction. This is because the winds spiral around the eye in a clockwise direction in the southern hemisphere. The effect of this on the ground is that winds on opposite sides of the eye blow in different directions.
Wind damage is mostly caused by the maximum gusts in the cyclone. For this reason, the well-known tropical cyclone severity categories used by the Bureau of Meteorology to communicate warnings are based on maximum gust wind speeds.
Potentially the most dangerous hazard associated with tropical cyclones which make landfall is storm surge. Storm surge has been responsible for more deaths than any other feature of tropical cyclones. Storm surge is a raised dome of water about 60 to 80 kilometres across and typically about two to five metres higher than the normal tide level. It is caused by a combination of strong winds driving water onshore and the lower atmospheric pressure in a tropical cyclone.
In the southern hemisphere, the onshore winds occur to the left of the tropical cyclone's path. In Australia, this is the east side on the north west and north coasts and the south side on the east coast. The largest surge usually extends between 30 and 60 kilometres from the crossing point of the tropical cyclone centre, or eye.
Its magnitude also depends on the local bathymetry of the seafloor and the angle and speed at which the cyclone crosses the coast. If the surge occurs at the same time as a high astronomical tide the area inundated can be extensive, particularly along low-lying coastlines.
Tornadoes are the rarest and most violent of thunderstorm phenomena. A tornado is a violently rotating column of air suspended from large thunderstorm clouds in contact with the ground. It has a funnel shape and can range in width from a few metres to hundreds of metres. The majority of strong and violent tornadoes occur in association with supercell thunderstorms. Tornadoes often occur at the connecting zone between updraught and downdraught regions of a thunderstorm.
Weaker tornadoes may spawn from either multi-cell or single-cell thunderstorms. Tornadoes come in many sizes but are typically in the form of a visible funnel with the narrow end touching the ground.
A cloud of debris commonly encircles the lower portion of the funnel. They usually average 75 metres or less in diameter and travel several kilometres before dissipating, although some are in excess of a kilometre across and can stay on the ground for more than kilometres. Tornadoes typically have a short life span of only a few minutes, but are powerful events which can cause extensive damage.
The six-point enhanced Fujita scale EF0 to EF5 was established to quantify the intensity of a tornado and its danger.
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