The Weather Guys

Forming over tropical oceans ensures that warm sea-surface temperature (SST) provides a mature hurricane with a means to warm and moisten the air that flows toward the important eye-wall convection. Thus, it is not surprising that hurricanes struggle to develop if the SST is not 79.7 degrees F or warmer. Tropical cyclones also require environments in which the wind speed and direction changes very little with increasing height, in other words, where the vertical wind shear is small.

Certain vast stretches of the tropical ocean have SSTs above the threshold value of 79.7 F and thus qualify as locations where the development of tropical cyclones is favored. However, within such areas it is only when the vertical shear is very low (from the surface to approximately miles above the surface) that hurricanes can form and grow to maturity.

In a given location in the tropics, it is much more likely that the shear condition, not the SST, will vary from one day to the next. There are a number of physical factors that can conspire to render the vertical shear too extreme to allow for hurricane development. One such factor is the presence of the so-called subtropical jet stream that is located between 20 degrees and 30 degrees latitude and about eight miles above the ground in both hemispheres. The subtropical jet stream is an ever-present feature of the general circulation of the tropics and has wind speeds routinely in excess of 130 mph.

Such strong winds well above the surface are more than sufficient to provide a toxic amount of vertical shear to a nascent tropical cyclone. The small number of hurricanes every year testifies to the hostility of the environment to their development.

The organized storms we experience here in Madison in fall and winter are known as mid-latitude cyclones. One of the most notable characteristics of these storms is the presence of strong temperature, humidity and wind contrasts at what are known as fronts. In fact, fronts are such an integral part of the structure of these storms that they are often referred to as frontal cyclones.

Unlike the mid-latitude cyclones, the air temperature is fairly uniform throughout a hurricane. This is the case even though the air sinks and is warmed by compression on the outer edge of the hurricane, while near its eyewall the air rises vigorously and cools by expansion.

Even though some of that cooling is counteracted by the enormous amount of latent heat release that occurs in the eyewall convection as water vapor is condensed into liquid water, one might still reasonably expect the eyewall region to be cooler than the periphery of the storm. This is not the case because the low-level air that is directed toward the center of the storm is greatly modified during that journey — making constant contact with a wildly windswept and very warm ocean surface.

As a consequence of the warm sea-surface temperature (SST), the inflow air is both warmed and moistened as it flows toward the eyewall — evaporation from the ocean being enhanced by the strong winds and disturbed ocean surface. For this reason, very warm, very moist air is the optimal fuel for the sustenance of a hurricane. This explains why warm, tropical ocean basins are where hurricanes form.