Weather Fronts - OK-First

Weather Fronts

Principle

Fronts are zones of transition between two different air masses. The zone may be 20 miles across or it may be 100 miles across, but from one side of a front to the other, one clearly would sense that the properties of an air mass had changed significantly (e.g., contrasts in temperature and dew point, wind direction, cloud cover, and on-going weather). The frontal zone represents the leading edge of a wedge of cold/cool air. If the wedge is moving into an area of warmer air, the front is called a cold front. If the wedge is retreating and warmer air is moving into an area previously occupied by cool air, the front is termed a warm front.

Figure 1 - Types of Fronts

Definition

• Fronts are boundaries between air masses of different temperatures.

  Fronts are actually zones of transition, but sometimes the transition zone, called a frontal zone, can be quite sharp.

• The type of front depends on both the direction in which the air mass is moving and the characteristics of the air mass.

  There are four types of fronts that will be described below: cold front, warm front, stationary front, and occluded front.

• To locate a front on a surface map, look for the following:
  • sharp temperature changes over relatively short distances,
  • changes in the moisture content of the air (dew point),
  • shifts in wind direction,
  • low pressure troughs and pressure changes, and
  • clouds and precipitation patterns.

  Not all of these patterns may be obvious or even occur, but these are some signs.

Figure 2 - Side View of a Typical Cold Front

Cold Fronts

• Cold front- a front in which cold air is replacing warm air at the surface.

• Some of the characteristics of cold fronts include the following:
  • The slope of a typical cold front is 1:100 (vertical to horizontal).
  • Cold fronts tend to move faster than all other types of fronts.
  • Cold fronts tend to be associated with the most violent weather among all types of fronts.
  • Cold fronts tend to move the farthest while maintaining their intensity.
  • Cold fronts tend to be associated with cirrus well ahead of the front, strong thunderstorms along and ahead of the front, and a broad area of clouds immediately behind the front (although fast moving fronts may be mostly clear behind the front).
  • Cold fronts can be associated with squall lines (a line of strong thunderstorms parallel to and ahead of the front).
• In winter, cold fronts move into Oklahoma mainly from the Canadian prairies but sometimes from the Arctic Circle or the eastern Pacific.

• Cold fronts almost always are easier to locate on a weather map than are warm fronts, primarily because of the strength of the high pressure system to the north and west of the cold front compared to that north of a warm front.

• Cold fronts usually bring cooler weather, clearing skies, and a sharp change in wind direction.

Figure 3 - Side View of a Typical Warm Front

Warm Fronts

• Warm front- a front in which warm air replaces cooler air at the surface.

• Some of the characteristics of warm fronts include the following:
  • The slope of a typical warm front is 1:200 (more gentle than cold fronts).
  • Warm fronts tend to move slowly.
  • Warm fronts are typically less violent than cold fronts.
  • Although they can trigger thunderstorms, warm fronts are more likely to be associated with large regions of gentle ascent (stratiform clouds and light to moderate continuous rain).
  • Warm fronts are usually preceded by cirrus first (1000 km ahead), then altostratus or altocumulus (500 km ahead), then stratus and possibly fog.
  • Behind the warm front, skies are relatively clear (but change gradually).
  • Warm fronts are associated with a frontal inversion (warm air overrunning cooler air).
• If a warm front exists on a weather map, it will be northeast of the cold front and often, to the east of a surface low pressure area.

• Clouds and precipitation are quite prevalent to the north of the warm front.

  This results from the fact that low-level southerly winds in the "warm sector" of the cyclone rise up and over the cooler, more dense air at the surface located north of the warm front. The lifting leads to saturation, cloud formation, and, ultimately, to some form of precipitation.

• In Oklahoma, warm fronts are rare in the winter and non-existent in the summer.

Stationary Fronts

• Stationary front- a front that does not move or barely moves.

• Stationary fronts behave like warm fronts, but are more quiescent.

• Many times the winds on both sides of a stationary front are parallel to the front.

• Typically stationary fronts form when polar air masses are modified significantly so as to lose their character (e.g., cold fronts which stall).

Figure 4 - Development of an Occluded Front

Occluded Fronts

• Because cold fronts move faster than warm fronts, they can catch up to and overtake their related warm front. When they do, an occluded front is formed.
• Occluded fronts are indicative of mature storm systems (i.e., those about to dissipate).
• The most common type of occlusion in North America is called a cold-front occlusion and it occurs when the cold front forces itself under the warm front.

  The weather ahead of the cold occlusion is similar to that of a warm front while that along and behind the cold occlusion is similar to that of a cold front.

End

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