What causes a low pressure
A low pressure area, or a ‘low’ in short, is a region where the atmospheric pressure is lower than that of a surrounding area. A low pressure system develops when warm and moist air rises from the Earth's surface and the air in the surrounding area rushes in to fill the empty space created by this rising air, thus forming a heavy inflow of wind. As this wind flows in from all directions, it forms a clockwise or anti-clockwise movement just like excess water in a hand-wash basin. “As the warm and humid air rises, it may become thick enough to produce rain or even snow at times’, says Brent McRoberts of Texas Atmospheric Sciences University.
The lowest air pressure in the world occurs in intense tropical cyclones where condensation of water vapor to form clouds and rain releases heat that warms the air column in the eye of the storm.
This image here shows what happens when a low pressure area is formed. The air rises around the low pressure and as it rises, it cools and then condenses into clouds and precipitation.
Low pressure brings bad weather
High and low atmospheric pressure are key to understanding the weather. A low pressure has a typical bad reputation of bringing bad weather. This is because areas of low pressure tend to be very cloudy and often cause rain or thunderstorms that are accompanied with strong winds. Likewise, areas of high pressure are usually associated with clear and sunny weather as they are the direct opposite of a low pressure system.
Interesting observations about Low pressure systems in India
- Every year, 2 to 3 low pressure systems form in the Arabian Sea whereas 4 to 5 low pressure systems form in the Bay of Bengal during the northeast monsoon.
- The Arabian Sea remains relatively dormant in terms of formations of any strong systems. Broadly, the intensity of the low pressure systems formed in the Bay of Bengal is much stronger than that in the Arabian Sea.
- Gonu is the only category-5 tropical cyclone on record, to have formed in the Arabian Sea.
- Low pressure systems which will further intensity into a cyclone, form only in the pre monsoon or post monsoon period. Cyclone Mahasen, earlier this year, occurred in the pre monsoon month of May.
- A low pressure system formed in the northeast monsoon (Oct – Dec) period is way stronger than the one formed during the southwest monsoon (June – Sept) period.
- Further, a low pressure system or a depression formed during the southwest monsoon period will rarely form into a cyclonic storm.
- Low pressure system forming into a cyclone in the month of October and November, affects Odisha, Andhra Pradesh and parts of Tamil Nadu(Cyclone Nilam in 2012 and Cyclone Phailin in 2013). The ones forming in December, generally bring bad weather over coastal Tamil Nadu, coastal Andhra Pradesh and Sri Lanka. (Cyclone Ward)
- Tamil Nadu and Andhra Pradesh receive 40 to 50% of rainfall in the months from October to December from these low pressure systems.
- A low pressure system, which eventually intensifies into a cyclone is always accompanied by strong to very strong winds, ranging from 70 to 200kmph or above. (Cyclone Phailin and Typhoon Haiyan had wind speeds of over 220 and 315kmph)