Fronts are the boundary layer between air masses (where weather happens)
Fronts are named according to the temperature of the advancing air, relative to the air it is replacing
The word 'front' came about during WWI during the times of trench warfare where the two opposing fronts came out and battled, similar to how the two air masses battle in a weather front
Air Mass:
Uniform temperature moisture
Classified by temperature on surface relative
4 discontinuities between Air masses:
Temperature
Dew Point
Wind
Pressure
Weather Fronts:
Aviation Weather Handbook, Front Symbology
The severity of the clouds and precipitation occurring along a front are dependent on:
The amount of moisture available (shown by the dew point)
The degree of stability of the lifted air
The slope of the front
The speed of the frontal movement
The contrast in the amounts of temperature and moisture between the two air masses
There are four types of fronts: [Figure 1]
Cold fronts, depicted in blue with triangles protruding in the direction of movement
Warm fronts, depicted in red with half circles protruding in the direction of movement
Stationary fronts, depicted in blue and red with opposing red half circles and blue triangles
Occluded fronts, depicted in pink with half circles and blue triangles protruding in the direction of movement
Aviation Weather Handbook, Front Symbology
Cold Front:
Aviation Weather Handbook, Cold Front
Cold Front Passage
A cold front is a body of cold, dense, and stable air overtaking and replacing warmer air
It is so dense, it stays close to the ground and acts like a snowplow, sliding under the warmer air and forcing the less dense air aloft [Figure 2]
Cold fronts have a steep slope, and the warm air is forced upward abruptly
This often leads to a narrow band of showers and thunderstorms along, or just ahead of, the front if the warm rising air is unstable
The rapidly ascending air causes the temperature to decrease suddenly, forcing the creation of clouds
The type of clouds that form depends on the stability of the warmer air mass
A cold front in the Northern Hemisphere is normally oriented in a northeast to southwest manner and can be several hundred miles long, encompassing a large area of land
Cold fronts typically move at a faster rate than warm fronts (~25-30 mph)
Extreme cold fronts have been recorded moving at speeds of up to 60 mph
Cold fronts tend to move faster in the summer than in winter
Movement is usually eastern while the front is usually a NE-SW line
Precipitation always falls in front of a cold front
Fast Moving Cold Fronts:
Fast-moving cold fronts are pushed by intense pressure systems far behind the actual front
As surface friction slows a front, the leading edge tends to bulge out and steepen the front's slope
This results in a very narrow band of weather, concentrated along the leading edge of the front
If the warm air being overtaken by the cold front is relatively stable, overcast skies and rain may occur for some distance behind the front
If the warm air is unstable, scattered thunderstorms and rain showers may form. A continuous line of thunderstorms, or squall line, may form along or ahead of the front
Squall lines present a serious hazard to pilots as squall-type thunderstorms are intense and move quickly
Behind a fast-moving cold front, the skies usually clear rapidly, and the front leaves behind gusty, turbulent winds and colder temperatures
Squal Lines:
Squal lines are the result of two airmasses colliding
Intensity is relative to the speed and difference between the airmasses, with highly diverse airmasses potentially colliding at ~50 knot speeds
Weather products that mention potential for squall lines include:
Convective outlooks and mesoscale products from the Storm Prediction Center website (spc.noaa.gov) will certainly mention their probability, as will convective sigmets on the Aviation Weather Center website (aviationweather.gov)
Weather imagary/radar displays squall lines as thin red or magenta lines
Slow Moving Cold Fronts:
Less than 15 knots
More like a warm front in properties and weather
Aviation Weather Handbook, Cold Front
Cold Front Passage
Warm Fronts:
Aviation Weather Handbook, Warm Front
Warm Front Passage
A warm front is a body of warm air overtaking and replacing cooler air
Warm fronts typically move at a slower rate than cold fronts (~10-25 mph)
The slope of the advancing front slides over the top of the cooler air and gradually pushes it out of the area
Warm fronts typically have a gentle slope, so the warm air rising along the frontal surface is gradual [Figure 4]
This favors the development of widespread layered or stratiform cloudiness and precipitation along, and ahead of, the front if the warm rising air is stable
In the summer months, cumulonimbus clouds (thunderstorms) are likely to develop
Aviation Weather Handbook, Warm Front
Warm Front Passage
Stationary Front:
When the forces of two air masses are relatively equal, the boundary or front that separates them remains stationary and influences the local weather for days
Less than 5 knots of movement
Winds blow 180° off from one another on each side
Weather in a stationary front is a mix of cold and warm front characteristics
Stationary frontal slope can vary, but clouds and precipitation would still form in the warm rising air along the front
Wind blows parallel to fronts, but opposite directions to one another
Occluded Front:
Occurs when a fast-moving cold front catches up to a slow-moving warm front
The temperature within each front is the primary determinant as to the type of front/weather to be expected
Cause persistent weather lasting over 24 hours
Classified cold or warm based on what is in contact with the ground
Cold Front Occlusion:
A cold occlusion results when the coldest air is behind the cold front
The cold air replaces the cooler air at the surface and forces the warm air aloft
High potential for serious weather
Warm Front Occlusion:
Warm occlusion results when the coldest air is ahead of the warm front
The cold air forces the cooler air of the advancing front aloft
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