On a weather chart, the lines joining values of equal pressure are called isobars. Sometimes the isobars form a wave-like pattern, showing “troughs” of low pressure and “ridges” of high pressure. Meteorologists follow the movement of the trough from one region to another and also monitor the pressure to see if it lowers further as time goes by.

If an isobar on the weather chart assumes the shape of a circle or an ellipse, with pressure within it less than what is outside it, a “low pressure area” is said to have “formed”. This is just the beginning of a sequence of developments.

If the pressure falls further, two concentric isobars can be drawn, and the low is said to have “become well-marked”. The isobars are normally drawn at an interval of 2 hPa. Low pressure areas usually do not have a clearly defined centre. Winds around a low pressure area are light, of the order of 30 km per hour or less.

A well-marked low may “organize” into a “depression”. The depression could “concentrate” into a “deep depression”. Depressions and deep depressions could be over land as well as sea. In the monsoon season, the northern parts of the Bay of Bengal, called the Head Bay, are the area of formation of what are called “monsoon depressions”. These low pressure systems move inland and normally follow a west-north-west track all the way up to Rajasthan. Some systems may, however, take a more southerly track and go towards Gujarat, while others may recurve and end up on the Himalayan foothills. During their 4 to 5 days of journey, monsoon depressions give copious rains, particularly in their southwest sector. They may linger on over catchment areas, and cause heavy flooding of peninsular rivers. On an average, 4 to 6 monsoon depressions form during the monsoon season, but this number may vary greatly from one year to another. The number of depressions and their paths have an important bearing on the monsoon rainfall distribution over India.

The centre of a depression can be identified from the isobar patterns and satellite images and given in terms of its latitude and longitude over the ocean. Over land, however, it suffices to talk in terms of sub-divisions, saying for example, that a monsoon depression over Chhatisgarh is likely to move westwards into east Madhya Pradesh. Depressions over land are essentially heavy rain systems and do not cause damage due to winds, which are of the order of 30-50 km per hour in a depression and 50-60 km per hour in a deep depression.

Over the ocean, there is a good chance for a deep depression to “intensify” into a “tropical cyclone”. This does not happen over land, as only a warm ocean (sea surface temperature 27 deg C or more) is capable of providing the storm with the required energy to sustain it. As pressure falls, winds build up in strength to 60-90 km per hour and blow anticlockwise around the cyclone centre. If atmospheric and oceanic conditions remain favourable, a marginal tropical cyclone can grow into a furious and deadly storm. Meteorologists maintain a constant vigil on all tropical cyclones because of their destructive potential and use certain terms to describe their current and predicted states. A tropical cyclone is said to “intensify further” into a “severe cyclonic storm” with winds of 90-120 km per hour. The next stage to which it can “intensify still further” is called a “very severe cyclonic storm” in which the winds blow at a speed of about 120-220 km per hour. The term “supercyclone” is used when the winds exceed 220 km per hour. Supercyclones such as the one which crossed the Orissa coast on 29 October 1999, are rare phenomena.

Centres of cyclonic storms can be defined accurately from satellite and radar imagery, especially when an eye is seen. The centre is given in terms of the latitude and longitude. However, it is also the practice to give its distance from a well-known place on the coast, usually the place towards which the storm seems to be heading, so that people on the coast can perceive how far away it is from them. What is called a “very severe cyclonic storm” over the Arabian Sea and the Bay of Bengal, is called a “hurricane” over the north Atlantic Ocean and east and central Pacific Oceans, and a “typhoon” over the northwest Pacific Ocean. But for the names, all these systems have similar meteorological characteristics.

When tropical cyclones cross the coast, the process of weakening can be very rapid, but they do go through all the above stages in reverse order and finally dissipate.