Ch-5, Part-7b:  Weather Fronts

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Frontal Lifting:  Convergence of Dissimilar Air Masses
 (Cold Front Example)
Warmer air always rises/goes "on top," not the colder air

 

FRONTS

What is a Front?

  • A front is a zone of contact between dissimilar air masses

Why should I care about fronts?

  • Because fronts are among the most commonly reported weather events during the cool season

  • Warm fronts, and cold fronts, in particular bring most of the precipitation we receive in the Pacific Northwest and in Idaho

  • Cold fronts can bring true storms for which you need to prepare

  • The astute, university-educated individual should be able to speak knowledgeably about the weather which will occur when a front is coming

What are some properties of fronts?

  • Remember, a front is the narrow zone where two dissimilar air masses are in contact

  • But, dissimilar air masses do not mix readily

  • Example: cold, dry Continental Polar vs moist, warm Marine Tropical air

  • The mixing which occurs is across a narrow zone of perhaps a fraction of a mile

  • "Weather" events occur along fronts

    • Can be "mild" weather

    • Can be violent, dangerous weather

  • Hence the name, "front", was devised in World War I

    • Scandanavian scientists saw similarity between weather fronts and the war "fronts"

    • Dissimilar air masses "battle" at weather fronts, especially at cold fronts

    • Ground troops battled at war fronts

Types of fronts and their names

  • Naming fronts

  • When two dissimilar air masses are in contact, one air mass is usually travelling faster, overtaking the other

  • Hence a front may be named after the faster moving of two air masses which are in contact

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  • Cold front ~

  • The colder of two air masses advances on a warmer air mass even though both are moving

  • The colder air is denser/heavier and wedges underneath the warmer air, forcing it up

  • Frontal slope

  • The front is not straight up and down as a weather map suggests

  • The front, the zone of contact is sloped, perhaps as much as 1:75

  • This means the front would rise 1 mile above the ground in 75 miles horizontal travel

  • This is much steeper than for a warm front

    •

  • Associated cold front weather ~

  • Warm air is quickly forced upwards

  • Hence, frontal lifting cools warm air to the saturation point

  • Clouds and precipitation form

  • Cold fronts can bring very blustery, stormy weather

  • May be thunder and lightening, even in winter

  • Can be intense and dangerous

  • Cold front's passage is short lived, say 15 to 30 minutes

  • The intense weather occurs along a narrow zone

  • The temperature drops distinctly with the passage of a cold front, say 10 degrees or more

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  • Warm front ~

  • The warmer of two air masses advances on a colder air mass even though both are moving

  • The colder air is heavier, so the faster moving, warm air advances on top of the cold air it is overtaking

  • Frontal slope

  • The slope of a warm front is almost horizontal

  • The slope might be 1:150, or 1:200 or so

  • This means that if the front was at the surface in Moscow, you would have to go half way to Seattle to have the front one mile above up!

  • So, the warm front appears to be level

    •

  • Associated weather ~

  • The warm front approaches slowly

  • May take many hours, or a day

  • First "tell-tale" cirrus clouds appear on the western horizon

  • During the course of a day a distinct sequence of clouds move closer, lower and lower

  • Cirrus

  • Cirrostratus

  • Altostratus

  • Stratus

  • Nimbostratus, precipitating

    •

  • Precipitation is gentle; not a blustery storm

  • Precipitation may last for many hours or a day or more

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  • Occluded front ~

  • A faster moving colder air mass overtakes an existing warm front

  • The existing warm front is lifted above the surfac

  • Abrupt uplift produces precipitation

    •

  • Stationary front

  • Neither of two air masses "wins the battle"

  • Two dissimilar air masses are in contact, but they are in place, stationary

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    • Fronts on a weather map ~

    • Fronts are depicted on printed maps, on television and on computer world wide web screens

    • If in color

      • Red stands for a warm front

      • Blue stands for a cold front

      • Purple stands for an occluded front, raised above the surface

    • Symbols

      • Triangles symbolize a cold front; they point in the direction of cold air mass movement

      • Half circles symbolize a warm front; they point in the direction of warm air mass movement

      • Alternating triangles and half circles, pointing in the same direction, symbolize an occluded front

      • Alternating triangles and half circles, pointing in opposite directions, symbolize a stationary front

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