Polar front theory was developed by
Jacob Bjerknes, derived from a dense network of observation sites in
Scandinavia during World War I. This theory proposed that the main inflow into a
cyclone was concentrated along two lines of convergence, one ahead of the low and another trailing behind the low. The trailing
convergence zone was referred to as the squall line or cold front. Areas of clouds and rainfall appeared to be focused along this convergence zone. The concept of frontal zones led to the concept of air masses. The nature of the three-dimensional structure of the cyclone was conceptualized after the development of the upper air network during the 1940s.
Life cycle s. Arrows indicate wind flow relative to the storm. Area C is most prone to supporting tornado development. Organized areas of thunderstorm activity reinforce pre-existing frontal zones, and they can outrun cold fronts. This outrunning occurs within the
westerlies in a pattern where the upper-level jet splits into two streams. The resultant
mesoscale convective system (MCS) forms at the point of the upper level split in the wind pattern in the area of best low-level inflow. The convection then moves east and toward the
equator into the warm sector, parallel to low-level thickness lines. When the convection is strong linear or curved, the MCS is called a squall line, with the feature placed at the leading edge of the significant wind shift and pressure rise. If squall lines form over arid regions, a dust storm known as a
haboob may result from the high winds in their wake picking up dust from the desert floor. Well behind mature squall lines, a wake low can develop on the back edge of the rain shield, which can lead to a heat burst due to the warming up of the descending air mass which is no longer being rain-cooled. Smaller
cumulus or
stratocumulus clouds, along with
cirrus, can be found ahead of the squall line. As
supercells and
multi-cell thunderstorms dissipate due to a weak shear force or poor lifting mechanisms, (e.g. considerable
terrain or lack of daytime heating) the
gust front associated with them may outrun the squall line itself and the synoptic scale area of low pressure may then infill, leading to a weakening of the cold front; essentially, the thunderstorm has exhausted its updrafts, becoming purely a downdraft dominated system. The areas of dissipating squall line thunderstorms may be regions of low
CAPE, low
humidity, insufficient wind shear, or poor synoptic dynamics (e.g. an upper-level low filling) leading to
frontolysis.
Characteristics Updrafts The leading area of a squall line is composed primarily of multiple updrafts, or singular regions of an
updraft, rising from ground level to the highest extensions of the
troposphere, condensing water and building a dark, ominous cloud to one with a noticeable overshooting top and anvil (thanks to
synoptic scale winds). Because of the chaotic nature of updrafts and
downdrafts, pressure perturbations are important.
Precipitation-cooled air from downdrafts usually spreads outwardly just above the surface and lifts air into the updrafts unless gushing too far out and cutting off this
inflow. Visually this process may take the form of a
shelf cloud, often with a turbulent appearance.
Pressure perturbations Pressure perturbations around thunderstorms are noteworthy. With
buoyancy rapid within the lower and mid-levels of a mature thunderstorm, updraft and downdraft create distinct mesocenters of pressure. As thunderstorms organized in squall lines, the northern end of the squall line is commonly referred to as the cyclonic end, with the southern side rotating anticyclonically (in Northern hemisphere). Because of the
Coriolis force, the northern end may evolve further, creating a "comma shaped" wake low, or may continue in a squall-like pattern. The updraft ahead of the line create a
mesolow too while the downdraft just behind the line will produce a mesohigh.
Wind shear Wind shear is an important aspect of a squall line. In low to medium shear environments, mature thunderstorms will contribute modest amounts of downdrafts, enough to help create a leading edge lifting mechanism – the gust front. In high shear environments created by opposing low level jet winds and synoptic winds, updrafts and consequential downdrafts can be much more intense (common in supercell
mesocyclones). The cold air
outflow leaves the trailing area of the squall line to the mid-level jet, which aids in downdraft processes.
Severe weather indicators Severe squall lines typically bow out due to the formation of a stronger mesoscale high-pressure system (a
mesohigh) within the convective area due to strong descending motion behind the squall line, and could come in the form of a
downburst. The pressure difference between the mesoscale high and the lower pressures ahead of the squall line cause high winds, which are strongest where the line is most bowed out. Another indication of the presence of severe weather along a squall line is its morphing into a line echo wave pattern (LEWP). A LEWP is a special configuration in a line of convective storms that indicates the presence of a low-pressure area and the possibility of damaging winds, large hail, and tornadoes. At each kink along the LEWP is a mesoscale low-pressure area, which could contain a tornado. In response to very strong outflow southwest of the mesoscale low, a portion of the line bulges outward forming a bow echo. Behind this bulge lies the mesoscale high-pressure area.
Depiction on maps Squall lines are depicted on
National Weather Service (NWS)
surface analyses as an alternating pattern of two red dots and a dash labelled "SQLN" or "SQUALL LINE". ==Variations==