Hurricanes

A hurricane is defined as a rotating tropical system with sustained winds exceeding 74 mph.
In the Northern Hemisphere (NH), hurricanes rotate counterclockwise, which is the same direction as low-pressure systems.

The Atlantic hurricane season spans from June 1 to November 30.
Hurricanes typically form at latitudes between 5° and 25°N, avoiding the equator and existing within the trade winds (easterlies).
It is noted that a hurricane is equivalent to a typhoon in the Northwest Pacific region of the world.

Hurricanes develop over warm ocean waters, specifically when sea surface temperatures exceed 28°C (approximately 80°F).
They typically form in regions with minimal vertical wind shear, which means there are generally light winds present.
Moisture from the ocean acts as the fuel that drives the hurricane system, providing the necessary energy for its development.
The focus of these notes will be primarily on Atlantic hurricanes, which are the most studied and documented.

Hurricanes progress through four main stages:
- Tropical Disturbance (Easterly Wave): Initial stage, characterized by an organized area of convergence.
- Tropical Depression: At this stage, the storm displays counterclockwise rotation with wind speeds less than 38 mph.
- Tropical Storm: Here, wind speeds range from 39 to 73 mph, and the storm is given a name.
- Hurricane: Defined by increased wind speeds above 74 mph, categorized using the Saffir-Simpson Hurricane Scale ranging from categories 1 to 5.

Initial Development: A drop in surface pressure leads to low pressure that draws warm, moist air towards the center of the storm.
Coriolis Effect: This force initiates the clockwise movement of the air, later leading to a circular pattern due to centrifugal force.
Latent Heating: As moist air rises and condenses, it releases latent heat, which warms the air column, further decreasing surface pressure and encouraging more air to move inwards through the storm.

Each developmental stage of the hurricane requires continual strengthening and a decrease in central pressure.
Rain within a hurricane does not occur uniformly but rather falls in spiral rain bands, echoing the wind flow patterns of the storm.
The hurricane's eye forms when the Coriolis force balances the pressure gradient force (PGF). The size of the eye does not directly correlate with the storm's intensity, and typically, there is a sinking motion of air within the eye.

Eye: The central part of the hurricane, which features the lowest pressures and calm conditions.
Eye Wall: The wall surrounding the eye, where the strongest winds occur.
Spiral Rain Bands: These are outer bands of thunderstorms that create spiral patterns as they approach the hurricane's center.

The strongest winds are always found in the eye wall while the eye itself offers significantly lower pressure.
Spiral rain bands do not penetrate the eye, but when they spiral inward, eye wall replacement cycles can occur.
Eye sizes can vary, ranging from 1 to 15 km in diameter.

Forecast maps help predict the path of hurricanes, with specific reference to the cone model which illustrates the probable path of the storm center. However, hazardous conditions may extend beyond this cone.
Statistical records show historical hurricane paths, with references made to hurricanes such as Helene (1998), Mitch, Gordon, and others from previous seasons.

Storm surges cause the most significant damage along coastlines during a hurricane's landfall.
The front right quadrant of the hurricane typically experiences the strongest winds and highest potential for wind damage.
Tornado occurrences are common in hurricane conditions, though they can be difficult to detect.
Urban flooding is often severe, particularly with slower-moving hurricanes.

Analysis of hurricane activity shows that certain categories are more frequent based on historical data tracked from 1944 to 2020.
Patterns demonstrate the number of storms during specific months, commonly peaking in September.
Future hurricane seasons prompt continual observations, as categories of named hurricanes are documented year after year, including notable names retired due to significant storm impacts.

Hurricanes contrast with mid-latitude low-pressure systems in several key aspects:
- They form within uniform air masses and lack the presence of frontal boundaries.
- They maintain a vertical symmetry without tilt across heights.
- They do not receive any supportive influences from jet stream flows and rely on latent heating for pressure reduction.
- Hydrostatic balance in hurricanes is primarily governed by the pressure gradient force equating the centrifugal force (PGF = Ce).