IN THE UNITED STATES, severe weather is defined as a tornado, winds greater than 58 mi per hour (93 km per hour) or hail greater than .75 in (1.9 cm) in diameter. Other countries sometimes include heavy precipitation or a large amount of lightning in their definitions of severe weather. Severe weather is usually considered to come from thunderstorms but other storms, such as blizzards and downslope windstorms, with sufficiently high winds could qualify. The U.S. definition has grouped events that tend to occur under similar meteorological conditions. These conditions are not necessarily the same ones that favor heavy precipitation or large amounts of lightning. The following discussion emphasizes the three elements in the U.S. definition as they are produced by thunderstorms. These features are more likely to come from supercell thunderstorms and are favored by conditions that give rise to these storms. Some comments will also be made about heavy precipitation.
A tornado is a rapidly rotating column of air that extends downward from a cumulonimbus cloud to the ground. Winds in the tornado move upward as well as around the center. Within the cumulonimbus cloud, it predominately forms in and near the updraft. It can range from tens to hundreds of meters wide and is often visible as a funnel-shaped cloud. It is too small for the Coriolis force to influence its motion and therefore may rotate in either direction. Tornadoes are more common over relatively flat terrain where strong horizontal temperature gradients occur. Although they are generally favored by environments with strong vertical wind shear, tornadoes that do not come from supercell storms may occur when the environment contains relatively little wind shear.
Tornadoes occur in several distinct areas of the world but are most common in FLORIDA, the central UNITED STATES, and south-central CANADA. In these places, cyclonic (counterclockwise in Northern Hemisphere) rotation is typical. Although most tornadoes are relatively weak with winds less than 112 mi per hour (180 km per hour), winds in very strong tornadoes may be greater than 300 mi per hour (482 km per hour). The average lifetime is about five minutes, but in some cases, they have lasted several hours. Tornadoes are more frequently associated with supercell thunderstorms but have been known to occur with ordinary cell thunderstorms also.
Before the tornado appears, there will be a lowering of the cloud base in the part of the cloud from which the tornado will appear. This formation is called a wall cloud. A funnel cloud with rapidly rotating winds may be seen to extend down from the wall cloud. When the funnel cloud reaches the Earth's surface, it is considered a tornado. In some cases, the tornado will reach the Earth's surface before there is a visible funnel cloud. The tornado may contain smaller zones of rotation within it called suction vortices. As it reaches the end of its life, the vortex of many tornadoes shrinks in size. The tornado is considered to be in the rope stage but can still be dangerous.
The primary hazard of tornadoes is from their intense winds and the debris driven by these winds. The intensity of tornadoes is measured with the Fujita scale. The scale ranges from F0 to F5. F0 tornadoes have winds less than 72 mi per hour (115 km per hour) and F5 tornadoes have winds greater than 260 mi per hour (418 km per hour). F0 tornadoes cause relatively minor property damage and very few deaths. F5 tornados can damage even steel-reinforced concrete buildings. Although only 2 percent of tornadoes are F4 or F5, they account for about 70 percent of all tornado fatalities. The tornado is a small storm and winds vary greatly within it. Therefore, tornado damage is erratic; one structure may receive substantial damage while another one close by receives little. Tornado warnings have previously relied heavily on human spotters. Today's Doppler radars can often indicate the presence of a tornado before spotters see it.
Thunderstorm winds that do not involve rotation about a vertical axis are called straight-line winds. The strongest straight-line winds are in excess of 134 mi per hour (215 km per hour). These winds originate in the downdraft of the thunderstorm. They include both the microburst (less than 1.8 mi or 4 km in diameter) and macroburst (greater than 1.8 mi or 4 km diameter) types of downburst. Clusters of thunderstorm cells with strong straight line winds are known as derechos.
These winds are formed by an unusually strong downdraft. The downdraft of the thunderstorm forms as precipitation drags on the upward-moving air. Some precipitation may mix with the unsaturated air outside the cloud and evaporate. The latent heat used for this process cools the air and can cause it to become denser than the surrounding air and to sink at an accelerating rate. Once the downdraft leaves the cloud base, additional evaporating may cool it further, resulting in even denser air. The downdraft hits the surface and spreads out horizontally. The leading edge of this downdraft air is called the gust front or the outflow boundary. The gust front may or may not be visible to the observer from a distance.
At one time, when less was known about straight – line winds and severe thunderstorm structure, the damage caused by these winds was often attributed to tornadoes. Indeed these winds cause many of the same types of property damage as tornadoes. Winds of the microburst variety can be extremely hazardous to aviation. As a microburst hits the Earth's surface, winds spread out in all directions. If an aircraft flies through the microburst it will go quickly from a region containing headwinds to one containing tailwinds. This can cause an aircraft near the ground for takeoff or landing to crash. Since the microburst often cannot be seen, the pilots may not know they have encountered it. Microbursts can often be detected by Doppler radar. In addition, many airports have now installed supplementary wind-measuring equipment to help detect microbursts and warn pilots.
Hail is frozen precipitation consisting of concentric layers of ice. Hail forms as the precipitation element interacts with the thunderstorm updraft. It may start to fall and then be carried upward by the updraft. As it travels through the storm it gets coated with liquid water that subsequently freezes on the developing hailstone. This process continues until the hailstone is carried out of the updraft or until it becomes large enough to fall through the updraft. Thus, hail tends to fall out of the part of the thunderstorm between the updraft and the downdraft. Hailstones have been known to grow as large as 5.5 in (14 cm) in diameter. Hail often has layers, some of which are clear ice and some of which are a milky white. If the air is very cold, water freezes quickly on the hailstone and traps air bubbles, producing milky-colored ice. If the air is warmer, water freezes slowly on the hailstone, air escapes, and the ice is clear.
Hail production in thunderstorms is favored when the updrafts are strong and deep. Such conditions are more likely to occur when the instability is high in the atmosphere. Relatively high cloud liquid water contents also favor the development of hail. Hail is encouraged when the updraft is wide and very twisted. Hail kills few people but can cause substantial property damage. Large hail often causes broken windows as well as damage to roofs and walls. Damage to automobiles and aircraft can be especially costly. Livestock may also be killed or injured. Meteorologists cannot count on hail to be unambiguously apparent on Doppler radar. Thus, warnings specific enough to avoid property damage are not common.
In addition to the conditions that favor thunderstorms in general, tornadoes, strong straight-line winds and hail are promoted by strong vertical wind shears, including changes in wind direction with height. Yet the climatological distribution of these events shows that much is not known about the formation processes of these storms. Large parts of the world experience few or no tornadoes—most of Africa, South America, and Asia. The occurrence of hail, however, is much more widely distributed. In many parts of the world, hail occurs more frequently in regions in or near mountain ranges. Florida has large numbers of tornadoes but very little hail. Northern INDIA and PAKISTAN have a lot of hail but few tornadoes. In the United States, strong straight-line winds are preferred in the northern plains as opposed to the southern plains. This distribution differs from those of both hail and tornadoes. Many individual storms will have more than one type of severe weather. Developing a method for determining what type of types of severe weather a particular storm will produce is a goal for storm researchers.
Heavy precipitation does not have a precise definition because the amount of rainfall that would be hazardous varies from one location to another. The primary hazard of heavy precipitation is flash flooding. A location with an abundance of vegetation is able to absorb more rainfall than one with sparse vegetation. A location with steep slopes funneling water to a particular spot is more vulnerable to flooding than a relatively flat region. Thus, precise definitions of heavy precipitation vary from nation to nation and even within the same nation.
Heavy precipitation is more likely to come from multiple cell thunderstorms. Individual ordinary cells don't last long enough. Supercell thunderstorms tend to be less efficient at producing precipitation. Although heavy precipitation is produced under some of the same circumstances as the severe weather types mentioned above, its most ideal environment has weak winds and little vertical wind shear throughout the troposphere. The storms move slowly relative to the ground. They may reform over the same area if a fixed feature, such as a mountain range, is acting as a trigger. Alternatively, new thunderstorm cells may form in a direction nearly opposite to the one that the storms are moving toward. Even though other types of severe weather sometimes occur, heavy precipitation often takes place in situations that are not suitable for the development of other severe weather.