A squall line is a long line of thunderstorms. They are formed by a common lifting mechanism. So what are those lifting mechanisms? They include fronts, gravity waves, outflow boundaries, and isentropic lifting.
Squall lines normally form in conditions of instability, moisture, and lift. Usually the winds near the surface are very different than the winds that are higher up. This creates a large vertical wind shear. The clouds have the appearance of being tipped over.
A classic squall line develops ahead of a cold front or dry line is the boundary that divides tropical dry air from tropical moist air. A dry line is sited by the wind shift that occurs and the dew point gradient. You can see clear examples of dry lines across Oklahoma and Texas in the spring of the year.
~moves east during the day and west during the night
~low density gradient
~moves steadily in one direction
~strong density gradient
Once the squall line begins the thing that keeps the squall line a vibrant force is the outflow. The outflow boundary is caused by the way the downdraft reacts to the surface. The speed and the angle of the downdraft determine how the air and fluid will meet the earth’s surface. One side of the downdraft will flow against the prevailing low level flow. The other side will be flowing with the flow. Crash.
Meteorologist can track the edge of the outflow boundary with Dopplor radar. A gathering of aerosols, dust, and bugs at the leading edge will promote a higher clean air signature. Another clue or sign is that clouds and new thunderstorms develop on the leading edge of the outflow boundary. You can detect two storms and often the clouds will tip or bow in direction that is the fastest moving.
Squall lines can develop rather quickly and they can last for days. As long as there is lift, moisture and instability they can keep moving. Why do they matter to us? They let us know that tornadoes are a very distinct possibility.
Because of where I was raised I rarely look to the sky and wonder if a tornado is going to drop down. However, my husband watches clouds all time. I have only seen one funnel cloud and for that I am grateful. Mother Nature doesn’t have to show me Tornado Alley. Learning about it is enough.