Thunderstorm: a mesoscale weather system produced by vigorous convection that reach to great altitudes within the troposphere. It effects a relatively small area and is short-lived. A thunderstorm consists of one or more convection cells accompanied by lightning and thunder and, often, locally heavy rainfall (or snowfall), and gusty surface winds.
A thunderstorm cell typically completes its life cycle in 30 minutes to an hour, but sometimes lightning, thunder, and bursts of heavy rain persists many hours. This is because each thunderstorm cell may be at different stage in its life cycle. A thunderstorm may track in a different direction than the individual cells.
Figure 13.6, page #312 (Moran and Morgan)
The life cycle of a convective cell is divided into three stages: cumulus, mature and dissipating.
Cumulus stage is the initial stage of a thunderstorm in which a cumulus cloud undergoes vertical and horizontal development reaching up to 8 to 10 km (5 to 6 mi.) height as they become a cumulus congestus and cumulonimbus, with a lateral dimension of 10 to 15 km (6 to 9 mi.) over a period of 10 to 15 minutes under favorable atmospheric conditions i.e., warm, humid and unstable air parcel located at surface. Cumulus stage is characterized by updraft throughout the entire system without precipitation, lightning and thunder as well.
Figure 13.2, page #308 (Moran and Morgan)
Mature stage is the middle and the most intense stage of a thunderstorm in which a convective cloud undergoes further vertical development with tops exceeding 18 km (11 mi.) over a period of 15 to 30 minutes with precipitation reaching to ground. Mature stage is characterized by both updrafts and downdrafts with the gusty surface winds, heaviest rainfall, lightning and thunder. Strong winds at high altitudes distort the cloud top into a anvil cloud which has a flat top under extremely stable conditions.
Gust front (mesoscale cold front): the leading edge of a mass of relatively cool air that flow out of base of a thunderstorm and spreads along the ground well in advance of the parent thunderstorm cell. An uplift along the gust front may trigger formation of secondary thunderstorm cell tens of kilometers ahead of the parent cell.
Roll cloud: a low, cylindrically shaped, and elongated cloud that occurs behind the gust front and beneath, but detached from a cumulonimbus cloud. It is seldom accompanied by severe weather.
Shelf cloud: a low, wedge-shaped, and elongated cloud that has a flat base occurring along a gust front and beneath and attached to a cumulonimbus cloud. It sometimes associated with a severe thunderstorms in the presence of strong damaging surface winds.
Dissipating stage is the last stage of a thunderstorm in which precipitation spreads throughout the convective cell over a period of 30 minutes. It is characterized by weak downdrafts throughout the entire system with light-to-moderate rainfall.
Entrainment: the mixing of saturated (cloudy) air with unsaturated air that surrounds the cloud. It is one of the continuous process thoughout the life cycle of a thunderstorm.
Most thunderstorms develop within warm, humid and usually conditionally unstable maritime tropical air as a consequence of uplift i) along fronts, i) on mountain slopes, iii) via convergence of surface winds, or iv) through intense solar heating of the Earth's surface.
Figure 13.8, page #314 (Moran and Morgan)
Outside the tropics, thunderstorms are classified as air mass thunderstorms, frontal thunderstorms, or mesoscale convective complexes, depending on the specific triggering mechanisms.
Air mass thunderstorm develops almost randomly within a mass of maritime tropical air. It is usually relatively weak system that is driven by the intense solar heating mostly during the warmest hours of the day, however, at some locations such as upper Mississippi Valley, air mass thunderstorms are more frequent at night.
Frontal thunderstorm is associated with lifting of air along the surface of a front, usually along or ahead of a cold front. In winter, it sometimes produce snow in northern regions. Frontal thunderstorms are generally more energetic than air mass thunderstorms and may develop at any time of day or night, since a frontal activity persists from hours to days.
Squall line: a line of intense thunderstorms occurring parallel to and ahead of a fast-moving, well-defined cold front. The squall line typically extends 100 to 300 km (60 to 80 mi.) ahead of the front, but it sometimes may extend for over 1000 km (600 mi.), with a huge supercell storms causing severe weather over much its length.
Mesoscale convective complex (MCC) is a nearly circular cluster of many interacting thunderstorms covering an area of many thousands of square kilometers. The size of a MCC can be a thousand times larger than of an individual air mass thunderstorms.
A mesoscale convective complex is not associated with a front and usually develops under conditions of weak synoptic-scale flow. The life time of a MCC is at least 6 hours and often 12 to 24 hours depending on the speed of the system which is generally slow (15 to 30 km/hr) resulting widespread and substantial rainfall.
Mesoscale convective complexes account for a substantial portion of growing season rainfall over the Great Plains and Midwest. They are primarily warm season (March through September) phenomena that generally develop at night. MCCs occur mostly over the eastern two-thirds of the US, where more than 50 may be expected in a single season.
A Mesoscale convective complex also has potential of producing severe weather. For example, NOAA reports 59 MCCs that occurred between February and October of 1985 produced 20 tornadoes and 16 flash floods that claimed 14 lives.
It is estimated that more than 40,000 thunderstorms occur each day throughout the world. Thunderstorms are most common over the continental interiors of tropical latitudes such as steamy Amazon Basin of Brazil, the Congo Basin of equatorial Africa, and the islands of Indonesia that experience thunderstorm activity at least 100 days.
Central Florida, which experiences more 90 thunderstorm days due to sea-breeze circulation, is the most frequent thunderstorm site in North America. The regions east of the Rocky mountains that subject to intense temperature gradient arising from variations in topography, have more than 60 thunderstorm days on average.
Figure 13.9, page #315 (Moran and Morgan)
Forced convection: the convection aided by topographic uplift. It is responsible for high thunderstorm percentage at the regions east of Rocky mountains.
Free convection: the convection triggered by intense solar heating of the Earth's surface. It is responsible for high thunderstorm percentage at Central Florida.
Severe thunderstorm: a thunderstorm that is accompanied by locally damaging surface winds i.e. winds stronger than 93 km/hr (58 mi./hr); frequent lightning; large hail i.e. 1.9 cm (0.75 in.) or larger in diameter; or tornadoes or funnel cloud. When the updraft is tilted as a result of strong vertical shear in the horizontal wind, precipitation in a thunderstorm falls alongside rather than against the updraft. Hence, the updraft maintains its strength and continues to build the cell to great altitudes.
Figure 13.11, page #317 (Moran and Morgan)
Severe thunderstorm cells usually form along a squall line within the cyclone's warm sector, ahead of and parallel to a fast-moving, well-defined cold front. A midlatitude jetstream tilts the updraft, thereby favoring the vertical development of the cell. It also causes dry air to subside from its right-front quadrant over a surface layer of maritime tropical air which is surged particularly at 3 km (9800 ft) altitude by a low-level jet. This produces a layering of that can lead to explosive convection and the development of severe thunderstorms.
Dry line: a boundary between warm, dry and warm, humid air in a southeast sector of a mature midlatitude cyclone. It is likely site of severe thunderstorm formation.
Figure 13.13, page #317 (Moran and Morgan)
Lightning: a flash of light by an electrical discharge of about 100 million volts in response to buildup of an electrical potential between cloud and ground, between clouds, or within a cloud.
On a clear day, the Earth's surface is negatively charged, and the upper troposphere is positively charged. In the presence of a cumulonimbus cloud, a positive changed is developed on the ground directly under the cloud. Within the cloud, a pancake-zone of negative charge region of a few hundred meters thick and several kilometers in diameter forms between the two positive charged regions at the upper and much narrower lower portions of the cloud.
Figure 13.17, page #320 (Moran and Morgan)
Cloud-to-ground lightning, which is usually initiated from mountain tops or tall structures such as antenna towers, consists of a very rapid sequence of events involving stepped leaders, return strokes, and dart leaders, each of which is about 20 to 100 m (66 to 328 ft) long from the cloud base toward the ground.
Stepped leaders are the initial electrical discharges in a lightning, consisting of negative electrical charge that travel from the cloud base to within 100 m of the ground.
Return strokes are the positively charged electrical current that emanates from the ground and meets a downward-moving stepped leader.
Dart leaders are the surges of negative electrical charge that follow the conductive path formed by the initial stepped leaders and return stroke of a lightning bolt.
Typically, a single lightning discharge consists of two to four dart leaders plus return strokes. A positively charged leader emanates from the cloud and initiates a lightning discharge in less than 10% of the cases. More rarely, positive or negative stepped leaders propagate upward from the ground and meet a return stroke surging downward from a cloud.
Electricity flows nearly 50,000 km/sec (31,000 mi/sec); and hence, the entire lightning sequence takes place in less than two-tenths of a second. Sheet lightning consists of bright flashes across the sky indicating cloud-to-cloud lightning. Heat lightning is simply light reflected by clouds from distant thunderstorms that occur beyond the horizon. Lightning heats the air along the narrow conducting path to temperatures that may exceed 25,000ûC (45,000 ûF).
Light travels with a speed of 300,000 km/sec (186,000 mi./sec), which is a million times faster than speed of sound. One can determine the distance between the observer and a thunderstorm by counting the time in seconds between the lightning flash and the thunder and dividing by 5. For example, if the thunder is heard 10 sec. after lightning is seen, the lightning strike occurred about 2 miles from the observer's location.
Lightning is deadly! on the average it kills more people each year than hurricanes and tornadoes combined. It also damages tens of millions of dollars value of electrical equipment each year. A lightning detection network (LDN) provides real-time information on the location and severity of cloud-to-ground lightning strokes.
Downburst: downward directed strong and potentially destructive winds that diverge horizontally as they strike the Earth surface. It is associated with a thunderstorm and with or without rainfall. Downbursts blow down trees, flatten crops, and wreck buildings. Based on size, a downburst is classified as either a macroburst or a microburst.
Macroburst is a downburst that affects a path of longer than 4 km (2.5 mi) with surface winds topping 210 km/hr (130 mi/hr).
Microburst is a downburst that affects a path of 4 km (2.5 mi) or shorter with surface winds topping 270 km/hr (167 mi/hr). It is accompanied by rapid changes in wind speed and direction, wind shear.
Microburst is particularly dangerous for airplanes during takeoff and landing. Federal Aviation Administration (FAA) now requires airlines to install an approved mircoburt detection system on their aircraft by the end of 1995. A microburst is shorter-lived, but more destructive than a macroburst.
Flash flood: a sudden rise in river or stream levels causing flooding. It is generally associated with a stationary or slow-moving intense thunderstorm. This stage of a thunderstorm results from weak winds aloft (less than 35 km/hr above 3 km) and/or the presence of persistent flow of humid air up a mountain slope.
Flash flooding is a special hazard in mountainous terrain, where excess water runs off to creeks, streams, rivers, or sewers, or collects in other low-lying areas. Because of their design and composition, urban areas are prone to flash floods during intense rainfall.
Hail: the precipitation particles in the form of conical or lump shape ice crystals with or without a thin surface of liquid water. It develops in intense thunderstorm cells characterized by strong updrafts, great vertical development and an abundant supply of supercooled water droplets.
Hailstones range from pea size to the size golfball or even larger. The largest hailstone ever recorded in the United States has a weight of 758 g (1.67 lb.) and circumference of 44.5 cm (5.5 in.) - about the size of a softball.
A hailstone forms when an ice pellet grows by accretion (addition) of freezing water droplets. In general, stronger the updraft, the larger will the ice crystal grow producing larger hailstones. A hailstone results from an alternating layers of clear (graze) and opaque (rime) surfaces. As many as 25 layers have been counted in a single large hailstone.
Hailsreak is the accumulation of hail in a long, narrow path along the ground. A typical hailstreak may be 2 km (1.2 mi) wide and 10 km (6.2 mi) long, and a single thunderstorm may produce a severe hailstreak. It forms following hail formation, weakening updraft and hailstones reaching the ground. It takes about 15 to 20 minutes from the formation of hail to the depositing hail at surface.
Figure 13.23, page #328 (Moran and Morgan)
Hail can break windows and dent cars, batter roofs of homes, but the most costly damage is to crops. It usually falls during the growing season (spring) wiping out the agricultural products. Hail damage is around hundred millions of dollars annually in the United Sates.
Hail frequency is not necessarily related to thunderstorm frequency. For example, Central Florida, experiences the highest frequency of thunderstorms in the US, is almost hail free region. In North America, hail is most frequent on the High Plains just east of the Rocky Mountains, where it can be expected to fall from 10% of all thunderstorms.
Figure 13.24, page #329 (Moran and Morgan)