Hurricane season in the Atlantic Basin is ending with a whimper, but it was certainly bang-ful at its peak. So what happens when a hurricane or typhoon moves into the temperate zone? Let’s go Around The World.
Where Does Weather Come From?
A reader (Jennifer Young, who writes Decoded Science’s great earthquake articles and lives in Scotland) asked me why the recent Hurricane Ophelia was not considered a tropical cyclone when it affected the British Isles, even though winds exceeded hurricane force.
I will try to explain.
The earth receives only one ten-millionth of the sun’s power output, but all weather is a result of our tiny allotment of this electromagnetic radiation and the geometry of the sun-earth system.
Direct sunlight at the equator heats that part of the earth more than the oblique rays heat the poles. The equator cannot keep getting warmer relative to the poles forever, and the atmosphere and ocean keep the system in equilibrium by transporting equatorial heat poleward.
The atmosphere and ocean accomplish this task in several ways.
1. At mid-levels in the atmosphere, the jet stream, with its undulations, transports cold air equatorward and warm air poleward. In the ocean, the Gulf Stream in the Atlantic Ocean and the Japan Current in the Pacific transport heat poleward.
2. Cold air adjacent to warm air is a higher energy state than warm on top of cold. As cold air moves equatorward from the poles and warm air moves poleward from the equator, the air masses meet in the mid-latitudes. The line of collision is called a front. Middle latitude storms are the mechanism by which the higher energy state is transformed into a lower energy one. In a low pressure area, the cold front on one side wraps around and catches up to the warm front on the other side, until the warm air has been pushed aloft. The front formed when the cold front catches up to the warm front is called an occluded front. The low pressure center creates wind, and as the storm moves poleward, the wind is dissipated into the ground by friction. Heat has been transported poleward.
3. I once asked my thesis adviser at MIT, the smartest meteorologist I ever knew, whether the atmosphere could find alternate ways of keeping the temperature in balance. He said he didn’t know. The system is incredibly complicated, involving seven equations in seven variables that no one can hope to solve analytically. So all we have to go by is empirical observation. That observation tells us that the atmosphere has found a third way to transfer heat poleward: tropical cyclones. These run on a different energy transformation mechanism than the one used by extratropical cyclones.
How A Tropical Cyclone Works
Tropical seas are warm — warm enough for a significant number of water molecules to evaporate into the air. But this evaporation process requires energy to rip the water molecule out of the ocean. This energy is then stored in the water vapor molecule and is known as ‘latent heat.’ When the water vapor molecule condenses, it releases that heat.
The internal process of a tropical cyclone is not so simple, but the principle is: Heat stored in water vapor molecules is released when they condense, and this heat ends up being converted to kinetic energy of wind.
What Kind Of A Storm Is It?
When a tropical cyclone moves away from the equator, it dissipates the kinetic energy of the wind by friction with the ground. This transports heat poleward. But the storm does not necessarily dissipate. It can transition to an extra-tropical cyclone if there are cold and warm air masses in its path. The most famous case is Hurricane Sandy, which, by strict definition, became extratropical before it hit New York City. The National Hurricane Center, therefore, discontinued advisories; many people were caught off guard. NOAA has since changed its practices, and the Hurricane Center continues advisories on what it calls ‘post-tropical cyclones.’ As I’ve said before: It makes no difference what kind of storm you get — wind is wind and rain is rain.
As it moved over the cold northern Atlantic Ocean, Hurricane Ophelia ran out of tropical steam, which normally happens when the water temperature drops below 25° C (77° F). But Ophelia was caught in a powerful jet stream above contrasting air masses. The storm was able to maintain hurricane intensity until it reached Ireland.
Typhoon Hits Tokyo
One of the hallmarks of this northern hemisphere tropical cyclone season has been the propensity of storms to affect population-rich places: Harvey and Houston; Irma and Florida; Maria and Puerto Rico. So it shouldn’t be a surprise that the Pacific Ocean wanted to get into the act. And here came Lan, once a Super-typhoon, but just a minimal typhoon when it scored a direct hit on Tokyo. Seven deaths and some damage was rather a light price for a storm hitting such a population center.
Typhoon Lan Affects Jet Stream
Typhoon Lan also turned into a powerful extratropical cyclone as it moved across the Pacific.
Its effect was dramatic, causing the jet stream to fold up and produce an extreme dip in the US.
A storm that formed on this dip knocked out power to a half million people in New England.
NOAA Forecasts Mostly Warm Winter In US
NOAA’s seasonal forecast for December 2017 through February 2018 indicates a better than even chance of a warm winter over more than half the country. Only the extreme northern Rockies and northwest have a better than even chance of below normal temperatures.
This forecast is based partly on the latest El Niño/La Niña forecast, which indicates a La Niña is likely.
Decoded Science views the La Niña forecast with suspicion, and any forecast for months in advance has a relatively low probability of beating the historical averages.
The weather pattern is in a seasonal transition, and when it settles in, Decoded Science will take a stand.
How’s that for meteorological milksoppery?