WEATHER DISTURBANCES

INTRODUCTION

The atmosphere exerts stress – severe at times – upon man and other life forms through weather disturbances involving extremes of wind speeds, cold and precipitation. Under the general category of storms, these phenomena constitute environmental hazards, not only directly, but indirectly through their attendant phenomena – storm waves and storm surges (gushes) of the seas and river floods, mud flows and land slides of the lands. Weather disturbances of lesser magnitude are among the beneficial environmental phenomena, for they bring precipitation to the land surfaces, and thus recharge the vital supplies of fresh water, upon which man and all other terrestrial life forms depend.
An understanding of weather disturbances of all intensities enables man to predict their times and places of occurrence, and thus to give warnings and allow protective measures to be taken.

TYPES

1. Extra-tropical or Temperate or Wave Cyclones
The dominant type of weather disturbance of middle and high latitudes is the Wave Cyclone, a vortex that repeatedly forms, intensifies, and dissolves along the frontal zone between cold and warm air masses.
These cyclones are most dominant over the North Atlantic Ocean and specially during the winter season. They are much larger in diameter but possess weaker pressure gradient. The shape and arrangement of isobars is v-shaped and they are accompanied by anti-cyclones. The speed is also different and there is a marked temperature and rainfall difference between their front and back. They are, however, not born of convection and earth’s rotation.

2. Tropical and Equatorial Weather Disturbances
Weather systems of the tropical and equatorial zones show some basic differences from those of mid latitudes. The coriolis effect is weak close to the equator, and there is a lack of strong contrast between air masses. Consequently, clearly defined fronts and large intense wave cyclones are missing. On the other hand, there is an intense atmospheric activity in the form of convection cells because of the high moisture content of the maritime air masses in these latitudes. The main weather disturbances of these latitudes are as under:

a. Easterly Wave
It is one of the simplest forms of weather disturbances. It is a slowly moving trough of low pressure within the belt of tropical easterlies (trade winds). These waves occur in latitudes 5o to 30o north and south over oceans, but not over the equator itself. The wave is simply a series of indentations in the isobars to form a shallow pressure trough. The wave travels westwards at a rate of 325 to 500 km/day. Air near the surface converges on the eastern, or rare side of the wave axis. Moist air is lifted and produces scattered showers and thunder storms. The rainy period may last for a day or two.

b. Weak Equatorial Low
It is another related disturbance, which forms near the center of the equatorial trough. Moist equatorial air masses converge on the center of the low causing rainfall from many individual convectional storms.

c. Polar Outbreaks
A distinctive feature of tropical – equatorial weather is the occasional penetration of powerful tongues of cold polar air from the mid latitudes into very low latitudes. These tongues are known as Polar Outbreaks. They bring usually cool, clear weather, with strong steady winds moving behind a cold front with squalls (storm). The polar outbreak is best developed in Americas. The outbreak that moves southwards from the United States over the Caribbean Sea and Central America are called ”Northers” or “Nortes”; those that move north from Patagonia Region into Tropical South America are called

d. Tropical Cyclones
These are of the most powerful and destructive types of cyclonic storms. They are also known as the hurricane or typhoon. The storm develops over oceans in latitudes 8o to 15o north and south, but not close to the equator, where the coriolis effect is extremely weak.
The tropical cyclone is almost a circular storm center of extremely low pressure into which winds are spiraling at high speed accompanied by very heavy rainfall. Storm diameter may be 150 to 500 km. Wind speeds range from 120 to 200 km/hour, sometimes much higher. Barometric pressure in the storm center commonly falls to 950 mb or lower.