Most people are familiar with the notion that thunderstorms occur due to extreme convection, rapidly rising columns of air which produce cumulonimbus, anvil shaped, clouds which can reach a height of 10,000 meters. Often, the heating of the land by the summer sun is enough to trigger such events if the air is moist and unstable.
Squall lines are associated with thundery activity but occur typically when a mass of cold air ('a cold front') moves into a mass of warm, moist air. The temperature difference of the air masses causes violent convection along a line of considerable distance and a series of thunderstorms can develop along this. Giant cumulonimbus clouds rise high into the atmosphere as the warm air lifts above the encroaching cold air. Often a line of low pressure develops as this warm air is lifted up, and cold air rushes down from high altitude, enhanced by falling water droplets or ice particles. At ground level this manifests itself as violent gusts of wind which can reach 65 miles an hour, causing significant structural damage and uprooting trees.
A 'typical' thunderstorm has a fairly short lifecycle. In its early stage, lasting about 10 to 15 minutes, warm updrafts of air rise at 16-32 feet per second, producing massive clouds. In stage two, lasting for the next 15 to 30 minutes, the now towering cumulonimbus clouds develop cold downdrafts which are accompanied by heavy rain, or hail. It is at this stage that thunder and lightning occur. In stage three, which may last for an hour, the cold downdrafts eventually cut off the supply of warm, rising air. Rain decreases then stops and the cloud thins.
Line squall weather systems can remain active for much longer periods of time as the cold front moves across the land. This is because the down flow of cold air from extreme height lifts warm, moist, air into the atmosphere as the front advances, slowing or preventing the decay of the storms which would normally occur in a more typical thundery outbreak. In this sense line squalls resemble 'multicell' storms, but extend along a far greater distance.
The severity of the weather associated with line squalls will vary according to the temperature difference between the two contending air masses. The greater the temperature gradient, the more likely it is that the resulting winds at ground level will be extreme and accompanied by torrential rain or heavy hailstorms.