CHAPTER #15
Hurricanes

Hurricane: an intense storm that originates over tropical ocean with maximum sustained winds exceeding 119 km/hr (74 mi/hr). It develops in a uniform mass of very warm and humid air without associating a front or frontal weather.

Air pressure is distributed symmetrically about the hurricane center and a hurricane has about one-third the diameter of an average extratropical cyclone. The central pressure is lower and the horizontal air pressure gradient is much steeper in a hurricane than in an extratropical cyclone.

A mature hurricane is a warm-core cyclone and therefore weakens with altitude, especially above 3 km such that an anticyclonic flow usually exists at altitudes of about 15 km above the hurricane.

Figure 15.3, page #414 (Ahrens)


Hurricanes are often initiated by easterly winds . The wave mooves slowly (10 to 20 knots) westward, bringing fair weather on its western side and slowers on its eastern side. The waves occasionally intensify into tropical cyclone, and is also called tropical wave . the wavlength of a easterly wave is about 2500 km and its position is found in a trough of a streamline pattern.

Figure 15.1, page #412 (Ahrens)

Hurricane formation requires sea-surface temperatures (SST's) greater than 26.5 C (80 F), deep moisture at low levels, light winds throughout troposhere, and convergence at the ground. In addition to easterly wave, triggering mechanims include ITCZ, and weak frontal boundary. Cooler SST's drier air at low levels, strong trade winds inversion, and strong upper level winds inhibit hurricane formation.

Figure 15.7, page #416 (Ahrens)


Hurricane Convection Theory: 1. intensen latent heating heats the lolumn of air near the center of the storm - generates high pressure, divergence aloft, also lowers the pressure at the surface. 2. lower surface pressure increases the pressure gradient at low levels - generates strong surface winds. 3. stronger winds increase friction via choppy seas. 4. stronger convergence into storms center. 5. echange convection - back to 1.

Figure 15.9, page #417 (Ahrens)


Eye of a hurricane: the center of a hurricane characterized by clear skies, subsiding air, and light winds (< 25 km/hr). It generally ranges from 20 to 65 km across, shrinking in diameter as the hurricane intensifies and wind strengthen. At a hurricane's typical forward speed, the eye of a hurricane passes through a given location up to an hour.

Eye wall: a circle of cumulonimbus clouds surrounding the eye of a mature hurricane. It is associated with heavy precipitation and strong winds. The most dangerous and destructive part of a hurricane is near the eye on the side where winds blow on the same direction as the storm's forward motion.

Figure 15.2, page #413 (Ahrens)


For tropical storms and hurricanes to develop, se surface temperatures must be 26.5 C (80 F) or higher through a depth of at least 60 m, and the Coriolis effect must be of significant magnitude (latitude of at least 4 north and south of the equator) to maintain a cyclonic circulation.

Most hurricanes form in the 5 to 20 latitude belt. Major hurricanes are located over i) western tropical North Pacific where it is called typhoon, ii) South Indian Ocean east of Madagascar, iii) Arabian Sea and the Bay of Bengal, iv) eastern Australia, v) Pacific Ocean west of Mexico, and vi) tropical North Atlantic from west coast of Africa to the Caribbean Sea and Gulf of Mexico.

Hurricanes that develop over tropical Atlantic and Pacific are associated with the weather systems within the US. They mostly form in late summer and early autumn.

Figure 15.11, page #420 (Ahrens)


The hazards of hurricanes include strong winds, associated tornadoes, heavy rains, and storm surge. About 90% of hurricane-related fatalities are caused by coastal and inland floodwaters. The eddy circulation near the eye wall is responsible for the most serious hurricane damage.

Once a hurricane makes landfall, it loses its warm-water energy source and experiences greater surface roughness. Hence, its circulation weakens rapidly so that most wind damage is confined to the coast. Heavy rains, on the other hand, often continue well inland and may cause severe flooding. Typically, hurricanes produce rainfall of amount in the range of 13 to 25 cm (5 to 10 inch).


Based on range of central air pressure and wind speed, and potential for storm surge and the property damage, a hurricane is rated 1 (weak) to 5 (very intense) on the Saffir-Simpson scale. Tropical storm is a tropical cyclone having wind speeds of 62 to 117 km/hr (39 to 73 mi/hr), while tropical depression is the developing stage of a tropical storm or a hurricane with wind speeds of 37 to 61 km/hr (23 to 38 mi/h).

Table 15.2, page #426 (Ahrens)


Storm surge: a hurricane-induced rise in sea level that reaches the shoreline ahead of the storm. It results from strong winds and low air pressure. The sea level rise is about 0.5 m for every 50 mb drop in air pressure.

Figure 15.16, page #424 (Ahrens)

The greatest potential for coastal flooding and erosion occurs when a strong surge coincides with high tide and where the tidal range is relatively great. An average tidal range, difference in water level between high and low tides, is less than 1 m along the Gulf of Mexico and several meters along the Eastern coast. A storm surge of 1 to 2 m can be expected with a weak hurricane, whereas the storm surge accompanying an intense hurricane may top 5 meter.

Figures 15.24-15.25, pages #432 (Ahrens)