IMD forecasts the formation of a low-pressure area over the southeast Bay of Bengal, thereby increasing the rainfall in the coastal districts of Tamil Nadu.

Details:

• The formation of a low-pressure area over the Southeast Bay of Bengal and the adjoining Andaman Sea is expected to intensify into a depression over central parts of the South Bay of Bengal.
• Prevailing upper air circulation would influence the formation of the fresh system.
• Weather models have indicated rain in the coastal parts of Tamil Nadu.

What is the pressure system?

• A pressure system is an area of the Earth’s atmosphere that has a particularly high or low pressure compared to the surrounding
• Types of pressure systems are:
  • Low-Pressure System
  • High-Pressure System

What is a low-pressure system?

• A low-pressure area is a region where the atmospheric pressure at sea level is below that of surrounding locations. Low-pressure systems form under areas of wind divergence that occur in upper levels of the troposphere.
• A low-pressure system is formed due to localized heating caused by greater sunshine over deserts and other landmasses.
  • Since the warm air in localized areas is less dense than the surroundings, the warm air rises, lowering the atmospheric pressure.
  • Large-scale thermal lows over continents help create pressure gradients that drive monsoon circulations.
  • Due to Earth’s spin and the Coriolis effect, winds of a low-pressure system swirl counterclockwise. This type of flow is known as cyclonic flow.
• Around the world, the low-pressure systems are most frequently located over the Tibetan Plateau and the lee of the Rocky Mountains. In Europe, the recurring low-pressure weather system is known as depressions.

High-Pressure System

• High-pressure systems are normally associated with light winds at the surface and subsidence at the lower portion of the troposphere.
• Subsidence dries out an air mass by adiabatic or compressional heating. Hence, high pressure usually results in clear skies.
• Since no clouds are present to obstruct the incoming shortwave solar radiation, the temperature rises during the day.
• At night, due to the absence of clouds, the outgoing longwave radiation is not absorbed and results in cooler low temperatures in all seasons
• A high-pressure system swirls in the opposite direction from a low-pressure system. This type of flow is known as anticyclonic flow.
• In high mountainous areas, rice takes more time to cook because low pressure reduces the boiling point of water.

Vertical Variation of Pressure

• In the lower atmosphere, the pressure decreases rapidly with height.
• In general, the atmospheric pressure decreases on average at the rate of about 34 millibars every 300 meters of height.
• The vertical pressure gradient force is much larger than that of the horizontal pressure gradient. But it is generally balanced by a nearly equal but opposite gravitational force. Hence, we do not experience strong upward winds.
• Due to gravity, the air at the surface is denser and hence has higher pressure. Since air pressure is proportional to density as well as temperature, it follows that a change in either temperature or density will cause a corresponding change in pressure.
• The pressure decreases with height. At any elevation, it varies from place to place and its variation is the primary cause of air motion, i.e., wind which moves from high-pressure areas to low-pressure areas.
• A rising pressure indicates fine, settled weather, while a falling pressure indicates unstable and cloudy weather.

Horizontal Distribution of Pressure

• The distribution of atmospheric pressure over the globe is known as the horizontal distribution of pressure.
• It is shown on maps with the help of
• The spacing of isobars expresses the rate and direction of change in air pressure.
• This change in air pressure is referred to as the pressure gradient.
  • The pressure gradient is the ratio between the pressure difference and the actual horizontal distance between two points.
• Close spacing of isobars expresses a steep pressure gradient while wide spacing indicates a gentle pressure gradient.
• The horizontal distribution of atmospheric pressure is not uniform in the world. It varies from time to time at a given place; it varies from place to place over short distances.
• The factors responsible for variation in the horizontal distribution of pressure are as follows:

1. Air temperature
2. The earth’s rotation
3. Presence of water vapor