Lucas' Weather Discussions

Last week’s post promised a more detailed explanation of what hurricanes need to form, which answers the question of why they are less common in the southern hemisphere than in the northern hemisphere. First, the ingredients, the availability of most can either limit or enhance the frequency and/or intensity of the storms:

A tropical disturbance: Usually in the form of a tropical wave (just think a weak, disorganized low pressure system) in the Atlantic Basin, elsewhere the disturbance is a remnant region of lower pressure from thunderstorm activity. This is one of the ingredients that is essential to the formation of hurricanes, as these are essentially the seeds from which the storms grow. The reason it is crucial is that, without it, there is no focusing mechanism for the thunderstorms to grow.

Deep convection: Convection is just a fancy meteorological term for thunderstorms, and the ‘deep’ part incites that the thunderstorms are tall, and therefore intense. This early formation ingredient is most beneficial when it is at the center of circulation of the disturbance, as it provides the upward motion that is analogous to the electric starter for an engine; it gets the circulation going at all levels of the atmosphere. A hurricane can still form if the convection isn’t over the center, yet these storms are frequently weaker and take longer to form.

Warm ocean water: Much like the fuel to an engine, this provides the energy for the storm to intensify. This warm water destabilizes the air above it (see post on tornado ingredients for instability explanation), allowing the convection to happen and perpetuate. It is a self-feeding system, as the storm’s intensification expands its surface wind field, thus increasing the rate at which the air gains energy from the water, and intensifying the convection further, and so on. The storm cannot pull this sort of energy out of land, which is why they all weaken when they then make landfall- the fuel source is cut off.

Minimal wind shear: Opposite to tornadoes, hurricanes need a low-shear environment to develop, since the existence of wind shear effectively decapitates the convection and displaces it from the storm’s center, which we now know inhibits the storm’s development.

Time: The last essential ingredient, time is necessary for these storms to form, even in the midst of perfect conditions. Continuing the analogy to an engine, even when warm and brand new, an engine will still turn over a couple times before roaring to life. The same principle applies for hurricanes, which need these ingredients present to some degree for a couple days before the storm’s ‘engine’ can get going.

Above is an image denoting the regions where hurricanes (and the varying regional nomenclature for them) form. From this, one can see that the area in the northern hemisphere where they form is more expansive. This is because the tropical North Atlantic, East and West Pacific basins all have availability of all the ingredients in the recipe to some extent or another every warm season. More telling in the Southern Hemisphere is the location and number of arrows instead of the orange shading- these areas are not nearly as prolific at producing cyclones as the graphic might advertise. This is because these areas are usually lacking in either tropical disturbances, low wind shear, and time- the location of land masses typically disallows any disturbance the time necessary to develop into a cyclone.

Finally, the connection to last week’s post. The main reason why the Mozambique cyclone was so devastating was because it had taken an unusually long time to make landfall- it had stalled in an area of favorable conditions and weak steering currents, allowing it to intensify to anomalous levels before finally making landfall.