- Air expands when heated and gets compressed when cooled. This results in variations in the atmospheric pressure.
- Conversely, when air is cooled, it becomes dense and sinks, creating an area of high pressure.
- The movement of air from high pressure to low pressure is what causes wind.
- Without this process, there would be large temperature differences between different latitudes, leading to more extreme weather conditions and potentially even making the planet uninhabitable.
- The wind is the movement of air horizontally from high-pressure areas to low-pressure areas, and it plays a crucial role in redistributing heat and moisture across the planet.
- When moist air rises, it cools as it gains altitude, and the water vapor in the air condenses into tiny droplets, forming clouds.
- Air has mass, and therefore, it also has weight. The weight of the air above a particular point on the Earth’s surface exerts a force in all directions, which is what we refer to as atmospheric pressure.
- The weight of the column of air that extends from the mean sea level to the top of the atmosphere is referred to as the atmospheric column, and the pressure exerted by this column on a unit area of the Earth’s surface is what we call atmospheric pressure.
- The choice of the unit often depends on the country or region in which the measurement is being made, as well as the discipline or application in which the measurement is being used.
Measurement of Air Pressure
- Atmospheric pressure is indeed measured using an instrument called a barometer, which measures the weight of the column of air above it at any given place and time.
- The standard unit used by meteorologists for measuring atmospheric pressure is the millibar (mb). One millibar is equal to the force of one gram on a square centimeter.
- A pressure of 1000 millibars is equivalent to a weight of 1.053 kilograms per square centimeter.
- In terms of the height of a column of mercury, a pressure of 1000 mb is equal to the weight of a column of mercury that is 75 centimeters high.
- Therefore, the normal pressure at sea level, which is approximately 1013.25 millibars, is equivalent to a column of mercury that is about 76 centimeters high.
- It’s worth noting that atmospheric pressure can vary widely depending on factors such as weather patterns, altitude, and geographic location.
Vertical Variation of Pressure
- This variation in pressure with height creates a pressure gradient, which is the driving force for the movement of air from high pressure to low pressure.
- The vertical pressure gradient force, as you mentioned, is much larger than the horizontal pressure gradient force, but it is generally balanced by the gravitational force, resulting in the absence of strong upward winds.
- The decrease in pressure with height is not constant and depends on several factors, including temperature, amount of water vapor, and gravity.
- On average, however, the atmospheric pressure decreases at a rate of about 34 millibars for every 300 meters of height.
- It’s also worth noting that temperature and density play a crucial role in determining atmospheric pressure.
- The variation in atmospheric pressure is a critical factor in determining the movement of air in the atmosphere, and its effects can be seen in weather patterns, air circulation, and other atmospheric phenomena.
- A rising pressure indicates fine, settled weather, while a falling pressure indicates unstable and cloudy weather.
Horizontal Distribution of Pressure
- Small differences in pressure are significant in determining wind direction and velocity, and isobars are a way to represent the horizontal distribution of pressure.
- Isobars are drawn on weather maps to connect areas of equal atmospheric pressure, allowing meteorologists to visualize areas of high and low pressure and track their movement over time.
- The seven pressure belts on Earth’s surface are the Equatorial Low-Pressure Belt, Subtropical High-Pressure Belts (two belts – one in each hemisphere), the Subpolar Low-Pressure Belts (two belts – one in each hemisphere), and the Polar High-Pressure Belts (two belts – one in each hemisphere).
- These belts are formed due to differences in solar heating and the rotation of the Earth, and they play a major role in determining global wind patterns and weather conditions.
The seven pressure belts are :
- Equatorial low-pressure belt.
- Sub-Tropical high-pressure belt – Northern hemisphere.
- Sub-Tropical high-pressure belt – Southern hemisphere.
- Sub-polar low-pressure belt – Northern hemisphere.
- Sub-polar low-pressure belt – Southern hemisphere.
- Polar high-pressure belt – Northern hemisphere.
Closed Isobars or Closed Pressure Centers
- In a low-pressure system, the pressure decreases outward from the center and is enclosed by isobars with progressively increasing pressure.
- On the other hand, in a high-pressure system, the pressure increases outward from the center and is enclosed by isobars with progressively decreasing pressure.
World Distribution of Sea Level Pressure
- It’s important to note that the standard atmospheric pressure at sea level is actually about 1013.25 millibars or 1 atmosphere (atm), which is equivalent to 1.01325 x 10^5 pascals (Pa) or 14.7 pounds per square inch (psi).
- This amount of pressure is exerted by the atmosphere at sea level on all animals, plants, rocks, etc.
- The equatorial low-pressure belt is also known as the Intertropical Convergence Zone (ITCZ) and is characterized by rising warm air, abundant rainfall, and thunderstorms.
- 30° N and 30° S are found in high-pressure areas known as subtropical highs.
- Further pole wards along 60° N and 60° S, the low-pressure belts are termed as the subpolar lows.
- The polar highs are areas of descending cold air and dry weather, characterized by high pressure and low temperatures.
Equatorial Low-Pressure Belt or ‘Doldrums’
- The equatorial low-pressure belt is also known as the Intertropical Convergence Zone (ITCZ) and is located between 10°N and 10°S latitudes.
- The width of the belt can vary between 5°N and 5°S and 20°N and 20°S. It is the zone of convergence of trade winds from two hemispheres from subtropical high-pressure belts, resulting in low pressure and rising air.
- The rising air leads to the formation of clouds and precipitation.
- The belt is also called the Doldrums because of the light and variable winds that make sailing difficult.
- The position of the belt varies with the apparent movement of the Sun.
- The intense heating over the equatorial region causes the air to rise, which creates an area of low pressure at the surface.
- This low-pressure area is known as the equatorial low-pressure belt or the inter-tropical convergence zone (ITCZ).
- The rising air cools as it ascends and forms clouds, leading to frequent precipitation in the region.
- This belt is characterized by extremely low pressure with calm conditions.
- The low-pressure belt along the equator is characterized by high humidity as a result of the large amounts of moisture carried by the winds over the oceans.
- As the warm, moist air rises in the equatorial low-pressure belt, it cools and water vapor condenses to form cumulonimbus clouds, which are often associated with thunderstorms. This process of rainfall is called convectional rainfall.
- The Coriolis force, which is caused by the rotation of the earth, deflects moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. At the equator, the Coriolis force is zero because the rotational speed of the earth is highest at the equator and decreases towards the poles.
Subtropical High-Pressure Belt or Horse Latitudes
- The subtropical highs extend from near the tropics to about 35°N and S.
- After saturation (complete loss of moisture) at the ITCZ, the air moving away from equatorial low-pressure belts in the upper troposphere becomes dry and cold.
- This dry and cold wind subsides at 30°N and S.
- So the high pressure along this belt is due to the subsidence of air coming from the equatorial region which descends after becoming heavy.
- The subsiding air from the equator is deflected towards the poles, creating a circulation cell known as the Hadley cell.
- The high-pressure belts at around 30°N and S are due to a combination of factors, including the subsidence of dry and cold air from the equatorial region and the Coriolis force, which deflects the subsiding air toward the poles.
- The subsiding air is warm and dry, therefore, most of the deserts are present along this belt, in both hemispheres.
- A calm condition (anticyclonic) with feeble winds is created in this high-pressure belt.
- The descending air currents feed the winds blowing towards adjoining low-pressure belts.
- This belt is frequently invaded by tropical and extratropical disturbances.
- The corresponding latitudes of sub-tropical high-pressure belts are called horse latitudes.
- In the early days, sailing vessels with a cargo of horses found it difficult to sail under the calm conditions of this high-pressure belt.
- They used to throw horses into the sea when fodder ran out. Hence the name horse latitudes.
In case you still have your doubts, contact us on 9811333901.
For UPSC Prelims Resources, Click here
For Daily Updates and Study Material:
Join our Telegram Channel – Edukemy for IAS
- 1. Learn through Videos – here
- 2. Be Exam Ready by Practicing Daily MCQs – here
- 3. Daily Newsletter – Get all your Current Affairs Covered – here
- 4. Mains Answer Writing Practice – here
Visit our YouTube Channel – here