… how hurricanes work
Over the last couple of weeks, as we worked through the forecasts for the 2018 Atlantic tropical storm season, it became clear that the people who prepare these forecasts speak a language all their own. In honour of Disaster Preparedness Month and Environmental Awareness Week, the material which follows will try to fill in a few of the blank and blurry spots.
In the Beginning:
To get a hurricane started, an area of low atmospheric pressure must pass over an area of warm sea. Atmospheric Pressure is the weight of the air column pushing down on the earth. It is measured in millibars (mb) and average pressure at sea level is 1013.25 mb, about equal to 14.7 pounds per square inch.
The most common hurricane-starters are Tropical Waves. These are long curving troughs of low pressure that form over central Africa and are carried by the prevailing east-to-west wind, sometimes with a load of clouds and thunderstorms. It is estimated that 60% of all tropical storms and most (85%) of the big ones start with these waves.
Then wave needs to meet a patch of warm sea – about 26oC from the surface to a depth of 50 metres. At this temperature, it’s likely that warm moist air is already rising from the sea surface. If this updraft catches a low pressure centre (no more than 960 mb) in the passing tropical wave, the wave may act like a chimney, making the warm sea air rise faster and start pulling in ,more air from the bottom of the column. This stage of storm formation is called “convection”.
As the sea air rises, it cools and its moisture starts to form clouds. When these get heavy enough they spill out over the top of the column, creating downdrafts on the outside and forcing air in faster at the bottom. If this happens all the way around the low-pressure “chimney”, the developing storm can now be described as having a “closed circulation” with an “eye”.
While all this drama is taking place aloft, the earth continues to turn. It’s rolling more or less west-to-east. The developing storm’s steering wind is pushing it more or less east-to-west. Because the earth is not flat, any point on the surface is traveling faster if it’s near the Equator than if it’s near the Poles.
This creates the Coriolis Force, which shifts objects moving above the earth (including masses of air and water) away from the Equator. The air being pulled in at the bottom of a developing storm isn’t coming in straight, and it’s coming in faster on the left than the right. At some point, the whole mass either falls apart or starts to spin, and in the northern hemisphere it spins counter-clockwise.
Moving Right Along
Every year, there are scores of tropical waves and other disturbances that could, in the right circumstances, become tropical storms. Only about a dozen of them do develop, and half of those die before becoming hurricanes.
Their journey from beginning to end is full of obstacles. They may be blocked or turned by high pressure ridges, or pulled forward by low-pressure troughs. A warm patch of sea may add some strength, but a cold patch cuts off the fuel supply. Hot dry air in the middle atmosphere (from the Sahara dust, usually) can stop the convection process at any point.
Low-level winds may help their forward progress, but wind currents higher in the atmosphere (vertical wind shear) can take their fragile structure apart. And landfall is certain death, though it may be a slow one – no more fuel, solid obstacles and lots of friction to take the strength out of the wind and bring down the rain.
It’s been a long time since Jamaica had a close encounter with a hurricane, but it’s interesting to note that the strongest ones in recent history have all left their marks here. The lowest internal pressure – just 882 mb – was recorded by Hurricane Wilma in 2005, with Gilbert (1988) not far behind at 888. Not surprisingly, these two also have almost the strongest sustained winds, at 295 kmh, but they were narrowly beaten by Hurricane Allen in 1980, at a ripping 305 kmh (190 mph).
Hurricane Wilma heading north, October 2005