FRONTS

WHAT IS A FRONT?

A given air mass may have a rather sharply defined boundary between itself and a neighboring air mass. This discontinuity is termed as a FRONT. Simply we can say that the line of contact along the separation plane between air masses and the surface is termed as a Front. We can find an example of a front in the contact between Polar and Tropical air masses below the axis of the jet in upper air waves.

CHARACTERISTICS OF FRONTS

Fronts may be near vertical, as in the case of air masses having little motion relative to one another; of they may be inclined at an angle not far from the horizontal, in cases where an air mass is sliding over another. A front may be most stationary with respect to the earth surface, but nevertheless, the adjacent air masses may be in relatively rapid motion with respect to each other along the front.

Polar Fronts

The depressions are formed due to the meeting of a warm sector consisting of light warm equatorial air with heavy and colder polar air. The cold heavy air will try to force its way under the warm air and will raise it. This holds that there are two currents of air having different temperatures and velocities. They are flowing side by side and the surface of discontinuity separating the two, is known as the Polar Front.
Such surfaces of discontinuity may come into being at any place, provided that ideal conditions exist. But the following are the three surfaces of discontinuity, permanently available on the surface of the earth:
1. Tropopause or the ceiling of the convective zone of the atmosphere.
2. At the sub-tropical high pressure belt, the line of contact between the colder trade winds, blowing towards the equator, and the warmer westerlies, blowing away from it.
3. The boundary which separates the cold polar air, blowing out from the poles, and the warmer westerlies, blowing towards them.
On the surfaces or polar fronts, due to the rotation of the earth, atmospheric disturbances develop and are pushed forward by cold air pushing the warm one and the latter giving rise to squalls and rains when they get colder after expansion. Due to the distribution of land and sea, and their unequal heating, this surface of discontinuity is always wavy or irregular.

Cold Front

A frontal contact zone in which cold air is invading warm air zone is termed as a “cold front”. The cold air mass, being heavier, remains in contact with ground and forces the warmer air mass to rise over it. The slope of the cold front is being actually of the order of slope of 1 in 40 to 1 in 80.
Cold fronts are often associated with strong atmospheric disturbances, the warm air thus listed often breaks out in violent thunderstorms. These may also occur along a line well in advance of the cold front, a “squall line”. Thunderstorms can be seen on the radar screen. (see Strahler 190)
Weather associated with Cold Front
The north-westerly winds to the west of a cyclone carry relatively cold air southward and eastward. As the cold front advances into the area previously occupied by the warm air, there is a tendency for the warm air to be displaced upward by the cold dense air. Thus along the cold front, in the region of increased ascent, a relatively narrow zone of clouds and precipitation often develops. Occasionally cold fronts are accompanied by no more than a few clouds and no precipitation since the air behind a cold front is normally colder than the surface over which it is moving. Cumulus clouds develop during day time hours, these clouds are insufficient to produce showers.

Warm Front

A warm front is that in which warm air is moving into a region of colder air. Here, again, the cold air mass remains in contact with the ground, and the warm air mass is forced to rise as if ascending a long ramp. Warm fronts have lower slopes than cold fronts, being of the order of 1 in 80 to as low as 1 in 200.
Warm fronts are attended by stable atmospheric conditions and lack the turbulent air motions of the cold front. Of course, if the warm air is unstable it will develop convection cells and there will be heavy showers and thunderstorms.
Weather associated with Warm Front
As warm, moist air ideally (mT) moves polewards to the east of a low center, it may rise slightly because of the general conversions of boundary layer air within the cyclone, but the main ascent occurs when the air reaches the warm front and begins to glide up over the colder surface air. Thus to the north and east of a typical Northern Hemisphere cyclone, a broad area of clouds and precipitation develops. Far in advance of the front, sometimes as much as 1500 km (900 miles), cirrus clouds are found. The location, amount and type of cloudiness and precipitation ahead of a warm front vary depending on the moisture content and stability of the ascending warm air and also on the slope of the warm frontal surface.

Occluded fronts

Cold fronts normally move along the ground at a faster rate than warm fronts. Hence, when both types are in the neighborhood, as they are in the cyclonic storms, the cold front may over take the warm front. An “occluded front” then results. The colder air of the fast moving cold front remains next to the ground, forcing both the warm air and the less cold air to rise over it. The warm air mass is lifted completely free of the ground.