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1 Nov 2019

Fronts

This post post is all about fronts. It is a great tool to prepare for a lot of the questions on the Transport Canada Advanced RPAS exam.

What is a front?

A front is a zone of transition between two air masses. The zone, or front, can vary in size from very small to very large. On the Graphic Area Forecast (GFA) above you can see a warm front (red) that is over 500KM long.

Whether the front is warm or cold will be determined by the direction in which it is moving. If, for example, the front is moving colder air over the ground towards warmer air, it is a cold front. If the front is moving warm air over the ground towards the cooler air, it is a warm front. We will discuss this more below.

Types of fronts

There are several types of fronts worth discussing. The main differences between the types of fronts are how the air is moving and the properties of that air.

Warm front

A warm front is a front in which the warm air replaces cooler air at the surface.

Characteristics of a warm front

Although fronts can vary in how they show up and the conditions they bring with them, there are some characteristics that are typical of a warm front.

  • Very gradual slope
  • Slow moving
  • More gentle
  • Stratiform clouds
  • Continuous light to moderate rain
  • Preceded by cirrus clouds (1000 KM ahead of the front)
  • Preceded by altostratus or alto cumulus (500KM ahead of the front)
  • Possibility of surface layer clouds
  • Clear skies behind the front
  • Associated with a frontal temperature inversion
This GFA shows a warm front moving north. Warm fronts are represented by red lines with red half circles in the direction of movement.
Cold Fronts

A cold front is a front that replaces warm air at the surface with cold air.

Characteristics of a cold front:

  • A cold front is steeper than a warm front, about 1:100 slope
  • Cold fronts can move quickly
  • They are associated with more violent weather
  • Cold fronts move far and maintain their intensity
  • Preceded by:
    • Cirrus
    • Strong thunderstorms
    • Broad clouds immediately following the front
  • Can be associated with a squall line
This section of a GFA shows a cold front as a blue line with blue triangles pointing in the direction that the front is moving.

Stationary Fronts

A stationary front is a front that is relatively still. Often the winds on either side of the front are moving in parallel to the front. Often a stationary front is a clod front that has stalled.

Stationary fronts are associated with clouds and prolonged precipitation, though many types of weather could occur around those fronts.

Occluded Fronts

Because cold fronts are faster than warm fronts they can often overtake a warm front moving in a similar direction. The convergence of these fronts is called an occluded front.

The most common of occluded fronts is called a cold-front occlusion where the cold front forces itself under the warm front.

When the cold and warm fronts meet the curve poleward into the point where they occlude.

Frontal Turbulence

There is a separate article on frontal turbulence here