Understanding Fronts
Fronts may be defined as sloping zones of pronounced transition in the thermal and wind fields. They
are characterized by a combination of relatively large horizontal temperature gradients, static stability and
both horizontal and vertical wind shear. In addition, the strength of a front may be defined by the magnitude of
the horizontal temperature gradient.
Frontal and air mass analysis had it's roots in the 19th century when it was recognized that cyclones (lows) form
along the boundary of air masses which possess different characteristics of temperature and humidity. The first
operational exploitation of the notion of air masses and fronts was initiated by the Norwegian School of
meteorologist following the First World War, which we often refer to as "the early concept".
The essential idea was that the extratropical cyclone forms on a frontal surface, wherein a major part of
the temperature contrast between neighbouring air masses is concentrated into a narrow transition layer. The
theory was developed so that many of the day-to-day changes in weather could be described with the formation
and movement of "fronts" between air masses.
Observations of the temperature, wind direction, humidity, and other physical phenomena during unsettled periods
in the weather showed that discontinuities often persist in the transition zone between air masses. This zone of
discontinuity, which is located where air masses meet, is commonly referred to as the baroclinic zone, mixing
zone or frontal zone.
Fronts of North America
From the viewpoint of the air mass model, the dome of warm tropical air encloses three other domes; the
Maritime Polar air mass, the Maritime Arctic air mass and then the Continental Arctic air mass. The Continental Arctic
air makes up the smallest dome and the outline of it's edge forms the Arctic Front. The next larger dome
is that of Maritime Arctic Air that encases the arctic air mass. The southern boundary of this dome is the
Maritime Front. The boundary between the Maritime Polar air mass and the Maritime Tropical air mass is the Polar
Front.
Types of Fronts
Warm/Cold Fronts
A cold front is the leading edge of an advancing cold air mass; a warm front is the retreating edge of a cold air mass.
Quasi-stationary Front
If the cold air is neither advancing nor retreating, the front is called Quasi-stationary.
Frontal Wave
A frontal wave is the term used to describe a horizontal wave-like deformation of a front usually associated with
cyclonic circulation (low). The point where the "cold" portion of the front meets the "warm" portion of the front is referred
to as the wave crest.
Trowal
By the classical theory, the word trowal is short for a "trough of warm air aloft". According to the early concept,
when the cold air mass behind a cold front advances faster than the air mass ahead of the warm front retreats,
the warm sector area between the warm and cold fronts becomes increasingly smaller. Ultimately, the region of warm air at
the surface is forced aloft, thus forming a trowal. This process occurs when the crest of the wave detaches itself
from the low center. The phenomena is also known as an occlusion or occluded front.
Cold Front |
Warm Front |
Trowal |
Frontal Clouds and Weather
Frontal clouds and weather depend in a great extent to the moisture profile of the warmer air. This is the air ahead of a cold front, or behind a warm front. The type and intensity of weather that will occur along a given front will be dependent upon the amount of water vapour in the warm air, the stability of the warm air, the strength of the vertical motion (lift) that the front generates as it moves, and the strength of the baroclinic zone (change in temperature). Stable air with enough lift and moisture will produce stratiform or layer cloud and continuous or intermittent precipitation. If the lift is great enough and the stability less, the type of could generated will be convective with the associated showery precipitation.
Warm Frontal Weather
Along a warm front with humid and stable air in the warm sector (behind the frontal surface) expect a transition from cirrus through cirro-stratus and alto-stratus to nimbo-stratus cloud. Along the front expect continuous precipitation with frontal fog behind the surface front.
Humid and unstable air along a warm front will bring the same transition in cloud as above with the addition of embedded cumulus-type cloud ahead of the front. This can be anything from towering cumulus through cumulo-nimbus cloud with associated showery precipitation with the heavier showers in the zone just ahead of the surface front.
A slow-moving cold front in humid, stable air will produce stratiform cloud along the frontal surface with strato-cumulus and alto-cumulus with the possibility of precipitation in a band along both sides of the surface front.
Cold Frontal Weather
An active cold front with humid, unstable air will produce these bands of strato-cumulus cloud with lines of cumulo-nimbus and alto-cumulus-castellanus producing showery precipitation along the front.
Trowal Weather
TROWALS or occlusions will produce mixtures of all these types of cloud and weather in various degrees depending upon the speed of the fronts, the moisture and temperature profiles of the warmest air and the amount of lift and stability.
Upper Warm Frontal Weather
Hazardous Frontal Weather
Anytime that you have a situation where a warm front lifts above freezing air above a layer of below freezing air the possibility of freezing precipitation is present. This is precipitation that starts liquid and falls through a cold air mass becoming super-cooled. When the super-cooled droplets touch any object they tend to freeze on contact coating the object with ice in various amounts. Should the object be an aircraft the windshield, flying surfaces and propellors may become iced causing a hazard to flight.
Along vigorous cold fronts the amount of convective cloud, and the type of precipitation could be hazardous. The typical lines of thunderstorms seen in the summer would be weather generally associated with cold fronts and these lines may develop hail or in extreme cases tornadoes.
Understanding Atmospheric Pressure
Table of Contents