ASK THE METEOROLOGIST
“How does a hurricane form?”
Kennedy Elementary School
Hurricanes are the most violent storms on Earth and have a variety of names from typhoon in Japan to willy-willy in Australia, the scientific term for all is a tropical cyclone. Tropical cyclones are like giant engines that use warm, moist air as fuel. A hurricane needs three things to form: sea surface temperatures of 80 degrees or warmer, low vertical wind shear (change in winds speed/direction with height) and high humidity (moisture). The warm, moist air over the ocean rises upward from the surface which creates low pressure at the surface and large cumulonimbus clouds above. The system of clouds and wind spin and grow fueled by the ocean’s heat and moisture. The rising air of the cyclone creates clouds and once it reaches the top of the storm it takes the path of least resistance as sinking air which creates the eye as well as the space between the cloud bands. The eye is an area of localized high pressure within the cyclone or relatively clear skies and quite wind.
When sustained winds of 39mph are reached the storm is called a ‘tropical storm.’ And when winds reach 74mph the storm is officially a hurricane. Once tropical cyclones hit land they tend to weaken because they are no longer being fueled by the warm, moisture Ocean.
Global circulation and sea surface temperatures are the reason that Oregon does not see hurricanes while the east coast and Gulf States do. Global circulation brings warm air/water from the tropics north in the Gulf Stream up the east coast; they tend to weaken the further north they move into cooler water, but as seen in Hurricane Sandy they can still be quite potent given the right dynamics.
On the Pacific coast global circulation causes cold water/air from the arctic to head south towards the tropics, so sea surface temperatures are far too cold for a hurricane to make it as far north as Oregon. Where hurricanes can form is off the coast of Mexico, global circulation takes them further west towards Japan. So the reason we don’t see hurricanes is that global circulation tends to not only send hurricanes west rather than north, but also brings cold air from the north to our coast.