A squall line is a development of a cold front. Cold fronts occur when a mass of cold air being drawn towards a low-pressure area becomes wedged below a mass of hot air, lifting the hot air. This most frequently occurs on the trailing edge of the area of low pressure.
Where the cold air meets the hot air invisible vortexes are set up of spiraling air, driven by the differences in temperature between the two masses of air. Hot air rising from the ground tips these vortexes into a vertical position and spirals of rising hot air are cooled as they pass through the colder air before spilling out of the top.
This leads to many areas of unstable air that form a line where the two masses of air meet. The moving air and sharp changes in temperature lead to storms, and the line of thunderstorms formed is known as a squall line. If one system forms it is known as super cell, but it is more common for a series of smaller storms to develop. There may be a super cell embedded within the squall line.
Squall lines cause shifting, gusting winds and heavy rain. Most of this activity takes place at the leading edge of the squall line, where most of the updrafts and down drafts are occurring. When temperatures are cold enough the storms will bring hail and snow.
The gusting winds at the edge of a squall line are a danger to aviation. They shift both direction and speed in an unpredictable manner as the differing air masses fight for direction and there are multiple up and down drafts inside the squall line as hot air rises and cold air falls.
Squall lines can be hundreds of miles long. They are commonly straight lines but will become curved from the force of the high-pressure system following them. This is called the bow wave pattern and can be a predictor of possible tornado activity. Squall lines often form in the early morning or late afternoon, which is the time of the greatest differences in air temperatures.
Squall lines can be seen approaching as a bank of cloud with a clear leading edge. Typically the cloud-bank rises high into the air and has a flat base. The approach can be felt as a drop in temperature and air pressure. Winds gradually increase and once the clouds arrive sudden bursts of heavy rain or hail occur.
When squall lines form over the sea they often fall apart very quickly on reaching land due to the lower temperature of the landmass relative to the sea.
Squall lines are found all over the world but are associated with the monsoon season in the Philippines and with tornado season in the United States.