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Thunderstorms and Tornadoes

Simulations

An air mass, or ordinary, thunderstorm evolves through three stages over a period of about an hour. Updrafts and warm in- cloud temperatures of the cumulus stage give way to updrafts and downdrafts, precipitation, lightning, and thunder in the mature stage. The dissipating stage is characterized by downdrafts and light precipitation. Regions of intense electric charge develop in a mature thunderstorm through processes not fully understood. Hypotheses include convection currents that carry charge from the ground and the upper atmosphere into different regions of the cloud, and collisions of precipitation particles within the cloud. Lightning is a sudden flow of electricity between two charge centers, discharging them. Thunder is the sound made by the violently heated, expanding air along the lightning's channel. Multicelled, frontal or squall line, and supercell thunderstorms all show significant differences from the basic, air mass thunderstorm model. Interactions among neighboring cells, coupled with the influence of large-scale convergence and divergence patterns, may stimulate dense regions of thunderstorm development called mesoscale convective complexes (MCC) or mesoscale convective systems (MCS). Severe thunderstorms tend to form where shear and/or rotation, dry air aloft, and frontal or orographic lifting supplement the basic requirements of all thunderstorms: warm, moist, and unstable air. Tornadoes form most frequently from supercell storms, in which a rotating air column called a mesocyclone exists. Under some circumstances, the mesocyclone is stretched vertically upward and downward, intensifying as it does so, and extending its influence below the cloud base, where the tornado forms. Tornado winds may reach 300 mph in the most violent cases, causing total destruction of objects in their path. The southern United States is the most tornado-prone region in the world.

Thunderstorm and tornado forecasting are showing substantial improvement, thanks largely to the National Weather Service Doppler Radar network using their Weather Surveillance Radar-88 Doppler (WSR-88D)that became operational in the 1990’s. In some cases, potentially life-threatening storms can be identified more than 20 minutes in advance. However, since the worst parts of these storms can have life-spans as short as a few minutes, warnings with ample lead time cannot always be given. The most dangerous storms are sometimes those that occur late at night, when most people are asleep.

Most summer thunderstorms do not cause damaging winds, hail or tornadoes. Just look around: if destructive storms were really common, would there be many older buildings, trees or other vegetation in your community? It is estimated that even within a typical area covered by a severe thunderstorm or tornado watch, less than 3% of the area is actually affected by damaging conditions.

The short movie here shows the development and movement of clouds as the main part of a thunderstorm approaches the observation point. Notice how fast-moving low clouds are coming toward the camera while higher clouds are moving from left to right. What does this tell you about whether warming or cooling is occurring in the layer between the two cloud decks? If you answered that cooling is occurring because the wind backs with height, you are correct.

As you watch the low clouds advance, you will notice new ones forming near their trailing edge. Why? There is a ridge in the picture, and the air from the other side of the ridge climbs over the ridge. As it climbs, it cools and eventually becomes saturated. At that point, the clouds form. But why don’t the clouds evaporate as the air descends from the ridge crest? You cannot really answer that question from the movie alone. However, it turns out that the slope on the other side of the ridge is much longer than on the side you see. In other words, the air climbs a considerable distance before the clouds form, whereas the descent from the ridge you see is less than 300 feet.

Before the thunderstorm arrived, it was rather tranquil at the location from which the video was shot. Only a slow crescendo of distant thunder and the darkening western sky hinted at the storm’s approach. Even though the early signs may be subtle, that time is the best for moving to a safe location. In addition to the danger of lightning and the prospect of getting soaked, the appearance of fast moving low clouds racing toward you from the base of a thunderstorm tells you that strong gusty winds are on the way. By the time you see these clouds, those winds are just minutes away.

Many people are fooled by the dark low clouds from an approaching thunderstorm. It usually looks brighter right behind the advancing cloud line (which can form a solid roll cloud or wall cloud). It is tempting to think that once that dark line of clouds passes, the storm has "blown over"; i.e; that the worst is over. Quite often, the brighter area has a very uniform or milky look. You cannot see individual clouds in that area the way you easily see the churning cloud elements as the roll cloud approaches. Why? Most of the time, the uniform appearance is a sign of heavy rain. In fact, there is so much rain that it literally hides the individual clouds you could otherwise discern. So, instead of the worst being over, the main part of the thunderstorm is probably still on the way. Each storm is different, but the whole range of thunderstorm products may be found within the cloud area that at first looks relatively benign. There may be window rattling, tree trunk twisting winds, gutter gushing rivulets of flash-flooding rain, plenty of lightning, and hammering, shrub-stripping, crop-flattening hail..maybe even a tornado! However, all of that is hidden. That is why it is so very important to move to the safest place possible before the storm hits. Much of this chapter will deal with safety related issues. Literally, we hope it helps you avoid injury or death. To end this section on a lighter note, the movie’s second half runs in reverse: the clouds retract and dissipate as they back up over the ridge. If only we could press the reverse button when the real thing hits!

Thunderstorms and Tornado Simulation (14110.0K)