Some supercells persist for several hours and track over hundreds of miles while producing severe weather. In contrast, ordinary thunderstorms last less than one hour and cover shorter distances.
Meteorologists catagorize supercells as Classic, High Precipitation, and Low Precipitation Supercells. Classic Supercells have a well-defined area of precipitation just northwest of the cloud tower or main updraft. Beneath the main updraft, and a short distance southwest of the precipitation, is a rain-free cloud base. A lowering or rotating wall cloud is evident beneath this base. The Classic Supercell is usually isolated.
High Precipitation Supercells produce more precipitation than Classic or Low Precipitation Supercells. Rain and hail may obscure the rain-free base beneath the main updraft. These types of cells are often embedded in lines or clusters of thunderstorms. The advent of WSR-88D Doppler (also called NEXRAD Radar) has shown that High Precipitation supercells are more prevalent than previously thought. In most of the U.S. High Precipitation storms are now considered the most frequent type of supercell (Vavrek et al., p.6)
Low Precipitation Supercells are usually found on the High Plains of the U.S. where moist air from the Gulf of Mexico meets dry air from the north and west. These produce little rain and are rather small. “Although they look less threatening than other supercells, they can produce severe weather” (Vavrek et al., p.7).
Outflow Phenomena-Downbursts
Information from the WW2010 website, of the Department of Atmospheric Sciences (DAS) at the University of Illinois at Urbanna-Champaign, http://ww2010.atmos.uiuc.edu, describes a downburst as a strong downdraft which includes an outburst of potentially damaging winds on or near the ground. Downdrafts with a diameter greater than 2.5 miles are classified as a macroburst; smaller downdrafts are referred to as microbursts.
As a macroburst or some non-severe gust-front passes overhead, the ragged, concaved-shaped underside of the shelf cloud accompanies the onset of cold outflow winds at the ground. Although some rotation may be visible in these clouds, it is likely to be short-lived and without vertical continuity, precluding a major tornado (DAS, 1999).
Downdrafts, both macrobursts and microburst, are thought to be less significant than tornados. In reality, they are just as lethal, if not more so in some circumstances. Many aircraft crashes in recent years have been caused by either macrobursts or microburst that occurred as pilots were making their critical landing approach (DAS, 1999). On January 3, 2000, a severe storm front that moved through the Ohio Valley uprooting trees, knocking down powerlines, and ripping roofs from structures. National Weather Service investigators determined that the damage was caused by straight line winds from microbursts, some of which had winds clocked in excess of 130 MPH (Weather Channel, 2000, www.weather.com/weather_center).
Tornados
Ebert (1988) wrote that tornados occur in just about any part of the world, but are most numerous in North America, especially in the central and southeastern regions of the U.S.
The total varies from year to year, but it is estimated that about 90 percent of the world’s tornados occur in the U.S.
The development of tornados requires severe weather systems, including violent thunderstorms. These strong thunderstorms may develop as either intensive, single convectional storms, or they may form ahead of well-developed cold fronts of fast-moving mid-latitude cyclones. In either case, the air masses involved must contain high levels of moisture with large amounts of latent energy (Ebert, 1988, p. 81).
Each tornado has its own characteristics; yet most of them form behind the so-called “wall cloud,” that is the usually heavy rain which pours out of the leading sector of the thundercloud or cumulonimbus. “This veil of rain explains why most tornados are obscured during their initial phase of development” (Ebert, p.83).
Since 1971 tornado researchers have used the Fujita-Pearson Tornado Scale, more commonly known as the Fujita Scale, to rate the intensity of a tornado (See Table 7 next page). The scale rates the intensity of the tornado by examining the damaged caused by the tornado after it passes over a man-made structure and by measuring approximate path length and width.
Ebert (1988) describes a hurricane as the most powerful storm in nature. That power is never more evident then when a hurricane makes landfall. A hurricane making landfall does so with tremendous winds and torrential rainfall. Coastal regions are also susceptible to the storm surge, a wall of water being “pushed” ashore ahead of the offshore winds.
Areas inland also experience tornados spawned by the outer thunderstorm bands.
Strong winds are the most common means of destruction associated with hurricanes. Their sometimes continuous barrage can uproot trees, knock over buildings and homes, fling deadly debris around, and sink or ground boats. The intensity of a hurricane is measured by the sustained wind speed found within it. The relative strength of a hurricane is also measured on a scale based on its greatest wind speed. Table 8 on the next page shows the Saffir-Simpson scale, named for the men who invented it, which measures the damage potential of a hurricane based on its greatest wind speed.
Scale #, Category
Central Pressure (inches of mercury)
Wind Speed (MPH)
Storm Surge (Feet)
Observed Damage
1
>=28.94
74-95
4-5
Some damage to trees, and unanchored mobile homes
2
28.50-28.91
96-110
6-8
Major damage to mobile homes, downed trees, damage to roofs
While coastal areas historically bear the brunt of a hurricane’s destructive winds and storm surge, the torrential rains cause both flash and long term flooding. Hurricanes and tropical storms are known to dump as much as a meter (about 3 feet) of rain in just a couple of days (DAS, 1999). Table 9 below shows selected rainfall totals from the landfall of Hurricane Opal.
