What happens to hurricanes when they leave the tropics?



Tropical cyclones, including hurricanes, are compact, warm core systems that thrive in environments with light winds in the upper atmosphere and warm seas of at least 26C - conditions often found in the tropics, hence their name. Rains are generated from bands of deep convection, or thunderstorms, rotating around a tight central point - the eye. The strongest winds in hurricanes are found in narrow bands around the eye wall, and drop markedly as you move away from the centre.

However, when such a storm wanders out of the tropics, it begins to go through a process known as extratropical transition. This fancy term basically means the once-tropical cyclone begins to lose its tropical characteristics as it moves over increasingly colder waters, and also encounters stronger winds in the upper atmosphere.

This transition is very much dependent on how a given ex-tropical cyclone interacts with other bigger systems nearby - sometimes they are simply absorbed into larger low pressure systems, providing a bit of added warm, moist air. However, on other occasions cold air begins to get wrapped into the ex-tropical system, and because there is now a difference in airmass across the cyclone, frontal rain begins to develop and a warm / cold front are born. The wind field tends to expand also, from being very strong and compact in a hurricane, to something much larger but less-intense. The strongest winds are usually found away from the low's centre.

Depending on the phasing between this low and the jet stream, this can sometimes lead to fairly intense windstorms in parts of western Europe, including the United Kingdom, should the remnant low be steered in that direction - perhaps even accompanied briefly by a sting jet.

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The image set above depicts Hurricane Ophelia as a Category 2 hurricane versus a power extratropical cyclone 3 days later. Notice the cyclone is much larger in size after extratropical transition, with a bigger (but less-intense) winfield displaced from the low centre. Sea temperatures are also notably colder - spot the upwelling caused by the Hurricane to the southwest of the system during its Category 2 stage. And finally, the difference in structure of the system - tight and compact as a hurricane, but bearing obvious frontal structures in its post-tropical stage.

Click on image to enlarge

Dan Holley  13th October 2017


What causes wildfires in California?

Santa Ana winds, also known as El Diablo winds, are "katabatic" winds, usually formed when high pressure sits over the Great Basin of the United States, and low pressure approaches the coast of California in the Pacific. This will often help to develop an easterly flow of air that moves from the higher elevations of the Sierra Nevada towards lower elevations near the coast of California.

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As this relatively cool, dry air over the Sierra Nevada moves westwards and lowers in elevation it is warmed adiabatically, and without the addition of moisture this lowers the relative humidity of the air, often below 10%. The winds also pass through various valleys and as they do they are "squeezed", thus increasing their their speed. Wind speeds can often reach 40-50mph.

This combination of hot, dry air and strong winds increases the risk and spread of fires dramatically. These winds most commonly occur in Autumn, but can happen other times of year as well.

Chris Bell  11th October 2017