FRONTAL SYSTEMS

As air masses move across planet earth they will eventually meet other air masses. The boundaries between such converging air masses are referred to as fronts. Fronts are labelled in reference to the relative temperature of the air that is approaching and replacing the existing air mass.

Because air masses have different characteristics depending on their (1) origin, (2) path over the earth’s surface, and (3) diverging or converging air, they usually give rise to distinct divisions between adjacent air masses, which are known as fronts. Fronts normally are found in low-pressure systems because a depression is usually made up of two different air masses.

There are four types of fronts;

• Warm fronts,
• Cold fronts,
• Stationary fronts, &
• Occluded fronts.

A Warm front occurs when a warm air mass overtakes an airmass of a colder temperature. A warm front will typically move slowly, approx. 10–25mph, and has a gradual slope that will slide over the top of a cooler air mass and move it out of the area. Such warm front will contain high humidity and have more stratiform, or layered, clouds along the frontal boundary.
Drizzle and decreasing visibility will be present as the temperature rises. Once the front passes rain showers are likely to occur, followed by gradual clearing.

A Cold front occurs when cold and dense air overtakes warmer air. Cold fronts move faster than warm air, averaging 25–30mph, and can reach speeds up to 60mph. Such fronts stay close to the ground and have a steeper angle than warm fronts.
As they move across the ground, they lift existing air at a very rapid rate. This rapid upward motion will force air to cool, release moisture and condense into cumulus clouds, with potential development for thunderstorms or even a line of them, called a ‘squall line’.
After the passing of a cold front, the weather will clear and improve with general wind flowing from the West-Northwest.

A Stationary front occurs when two air masses meet, but neither one moves the other out of the way. Stationary fronts can have a combination of weather from both warm and cold fronts. Such fronts can oscillate but will generally linger around for several days before one pushes the other out of the way.

Occluded fronts form when a fast moving cold front catches up to a slower moving warm front. The cold front will then lift up the warm front and eventually interact with the air in front of the warm front.
The stacking of fronts can give rise to a variety of weather conditions, from fog to thunderstorms.

The passage of a warm front has the following general characteristics and associated weather: