The terms instability line and squall line are synonymous with violent winds, heavy rain, lightning, thunder, hail, and tornadoes. The terms are often used interchangeably and are incorrectly applied to any severe weather phenomena that moves through a region. However, there is a dif-ference between an instability line and a squall line.

An instability line is any nonfrontal line or band of convective activity. This is a general term and includes the developing, mature, and dissipating stages of the line of convective activity. However, when the mature stage consists of a line of active thunderstorms, it is properly termed a squall line. Therefore, in practice, the instability line often refers only to the less active phases.

A squall line is a nonfrontal line or band of active thunderstorms (with or without squalls). It is the mature, active stage of the instability line. From these definitions, instability and squall lines are air mass phenomenon because they are both nonfrontal occurrences. However, they are frequently associated with the fast-moving cold front.

NOTE: The term instability line is the more general term and includes the squall line as a special case.

A prefrontal squall line is a squall line located in the warm sector of a wave cyclone. They form about 50 to 300 miles in advance of fast-moving cold fronts and are usually oriented roughly parallel to the cold front. They move in about the same direction as the cold front; however, their speed is, at times, faster than the cold front. You can roughly compute the direction and speed by using the winds at the 500-mb level. Squall lines generally move in the direction of the 500-mb wind flow and at approximately 40% of the wind speed.

Figure 4-3-3.Cold front aloft.

FORMATION. There are several theories on the development of prefrontal squall lines. A generally accepted theory is that as thunderstorms develop along the fast-moving front, large quan-tities of cold air from aloft descend in downdrafts along the front and form a wedge of cold air ahead of the front. The wedge of cold air then serves as a lifting mechanism for the warm, moist, unstable air; and a line of thunderstorms develops several miles in advance of the front. Since the thunderstorms form within the air mass and not along the front, the squall line is considered as air mass weather (fig. 4-3-4). In the United States, squall lines form most often in spring and sum-mer. They are normally restricted to the region east of the Rocky Mountains with a high fre-quency of occurrence in the southern states.

WEATHER. Squall-line weather can be ex-tremely hazardous. Its weather is usually more severe than the weather associated with the cold front behind it; this is because the moisture and energy of the warm air mass tends to be released at the squall line prior to the arrival of the

Figure 4-3-4.Prefrontal squall line development.

Figure 4-3-5.Typical isobaric pattern associated with a prefrontal squall line.

trailing cold front. Showers and thunderstorms (sometimes tornadoes) occur along the squall line, and the wind shifts cyclonically with their passage (fig. 4-3-5). However, if the zone is narrow, the wind shift may not be noticeable on surface charts. There is generally a large drop in temperature because of the cooling of the air by precipitation. Pressure rises after the passage of the squall line, and, at times, a micro-high (small high) may form behind it. After passage of the squall line, the wind backs to southerly before the cold frontal passage. When the squall line dissipates, severe weather may develop along the fast-moving cold front.

Turbulence is severe in the squall-line thunderstorms because of violent updrafts and downdrafts. Above the freezing level, icing may occur. Hail is another possibility in the squall-line thunderstorm and can do extensive structural damage to an aircraft. Under the squall line, ceil-ing and visibility may be reduced because of heavy rain showers. Fog is a rare occurrence because of the strong wind and gusts, but it may be found in isolated cases. Tornadoes frequently occur with squall lines when the warm air mass is extremely unstable.

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