A Hurricane's Energy Comes From

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Hurricane Energetics

Beneath is repeated a section on hurricane germination and dissipation. Please reread the section. You should now better understand the material, particularly the material relating to phase changes of water, stability of the atmosphere, and vertical current of air shear. After that is a new section that describes some of the basic aspects of hurricane strengthening.

Hurricane Formation and Dissipation

The germination of tropical cyclones is still a topic of intense inquiry and is not fully understood, although inquiry has shown that sure factors must be present for cyclones to intensify to hurricane strength.

Hurricanes form over tropical waters where the winds are light, the humidity is high in a deep layer, and the surface water temperature is warm, typically 26.five°C (80°F) or greater, over a vast area. Moreover, the warm surface water must extend down to a depth about 200 m (600 ft) before hurricane formation is possible.

Hurricanes are powered by the latent heat free energy released from condensation. To form and develop they must be supplied with a abiding supply of warm humid air for this procedure. Surface air with plenty free energy to generate a hurricane only exists over oceans with a temperature greater than 26.v°C. Moreover, the atmosphere to a higher place a developing tempest must exist unstable, i.e., the temperature must decrease rapidly with increasing distance. This is condusive to thunderstorm germination in general.

Hurricanes cannot develop on or very most the Equator. In fact they do not form in the region within 4° breadth either side of the equator. Within this region Coriolis force is negligible. Coriolis force is required for the initiation of rotation. In fact it is relatively rare for hurricanes to class within 10° of breadth from the Equator.

Finally, hurricanes will not form if there is significant air current shear, which is a change in wind velocity and/or direction with increasing altitude

Hurricanes grow stronger equally long as the air aloft moves outward away from the storm center more than speedily than the surface air moves in toward the center. They dissipate quickly when they move over colder water or over a large landmass as these weather cut-off the supply of warm, humid air. Hurricanes will as well weaken if they motility into an expanse that has strong vertical current of air shear.

Below is a summary list of v requirements for tropical tempest evolution and intensification.

Warm, humid surface air and an unstable atmosphere
- Latent heat release during condensation is the driving energy for tropical storm intensification.
(a)Warm sea surface waters (Temperature > 26.5°C [80°F]). This is the "fuel" for hurricanes.
(b)Depth of warm water > 60 m (200 ft)
- As cyclone develops, winds churn upwardly h2o, bringing up water from below the ocean surface. If this water is too cold, hurricanes cannot develop.
- The warm water requirement explains why hurricanes are almost mutual in the late summer and early fall ... the time of the year when sea surface temperature is greatest in the oceans where tropical storms originate.
Pre-existing large-scale surface convergence and/or upper-level deviation
- For case, an area of low level convergence is present with easterly waves.
Must be further than iv° Latitude abroad from the Equator
- In gild to go winds to rotate counterclockwise around an surface area of low force per unit area, there must be a Coriolis effect. The Coriolis force at the equator is zero and gets larger as one moves toward the poles.
Absence of stiff vertical air current shear ( Figure L contains a definition and sample picture of vertical current of air shear).
- Air current shear inhibits deep convection, essentially ripping tropical storms autonomously. Generally, tropical storms are best able to strengthen when the ascension warm air about the center of the storm goes straight up. Air current shear can cause the updrafts to bend over, which is not favorable for the storm to strengthen.
- In contrast, vertical current of air shear is necessary for the development of severe thunderstorms.

Merely when all of these conditions are met practise tropical cyclones develop into hurricanes. In addition, once a hurricane has developed, if one or more of these five atmospheric condition go away, the tempest will usually weaken.

The Strengthening of hurricanes via a positive feedback loop

In this section, the energetics of how tropical cyclones intensify to become hurricanes is discussed using a unproblematic process diagram. When conditions are just right, tropical storms intensify via a positive feedback loop. Before looking at the procedure diagram, there are a couple of pieces of background material to review. I concerns the relationship between horizontal convergence and difference of air flow and forced ascension and sinking motion, which is described in this scanned version of dynamical forcing handout. The important concepts are that converging air at surface and diverging air almost the height of the tropopause both forcefulness rise vertical air motion. Additionally, permit's review the human relationship between the air temperature in a vertical column of air the rate of force per unit area decrease with height in that cavalcade (See Figure K). Remember air pressure must decrease as you move upward. The charge per unit at which pressure decreases (for instance pressure level drib per k meters of moving upward) is smaller in warm air (warm core of a hurricane for example) compared with colder air.

Nosotros should at present accept enough background noesis to empathize the effigy below and the basic description of some of the energetics of hurricane strengthening below the effigy

This is a simplified sequence describing some of the energetics involved in hurricane strengthening. The numbers in the diagram above correspond with the steps listed.

Assume a pre-existing surface area of low level convergence. The 'L' represents a region of surface low pressure. The air menses near a surface low is counterclockwise and converging. The converging part is indicated past the arrows just above the ocean surface. Nosotros as well assume that body of water h2o is warmer than fourscore°F to a depth of at to the lowest degree 200 feet and at that place is little vertical wind shear.
As surface air converges, air is forced to rise. Equally air rises upwardly, information technology expands and cools. Once the air is cooled to saturation, clouds brainstorm to course. This condensation releases tremendous amounts of latent energy, warming the air in the column to a higher place the surface low. This forms the warm core of the system.
Equally the column of air above the surface depression warms, the rate at which pressure falls with increasing altitude decreases. Thus, relatively loftier pressure forms in the upper troposphere, labeled with 'H'. (Note that outside of this warm column, pressure falls more apace with increasing distance).
This relatively loftier force per unit area at the top of the troposphere compared to the lower pressure surrounding it, results in strengthening difference (outflow) at the top of the tempest.
When outflow (divergence) of air at the acme of the cavalcade is greater than the arrival (convergence) of air at the lesser of the column, more air is being removed from column (at top) than is added (at lesser). This results in a loss of weight of air in column. Thus, surface force per unit area lowers at the bottom of the cavalcade, since the air pressure at the surface is caused by the weight of air above.
Lower surface pressure means the storm is intensifying as this results in stronger surface winds.
Stronger surface winds bring in more "fuel" (water vapor) to the storm. The lower surface pressure increases convergence and rising motion. In add-on, higher winds increase the net rate of evaporation from the body of water. As winds and waves increase, a lot of sea spray is kicked up into the air. This provides additional expanse for evaporation.
As more water vapor is brought into storm, more latent heat released, further warming column of air above the surface low. This step feeds back to step #3 above, and the storm tin continue to strengthen in this manner through a positive feedback cycle.

This simplified positive feedback wheel cannot continue indefinitely. Strong hurricanes tend to go through cycles of intensification and weakening during their lifetimes. The figure below shows a more than realistic look at beefcake of a hurricane. Every bit opposed to the simple explanation for intensification which only showed a central updraft, real hurricanes have banded structures of rising and sinking motility.