atmospheric pressure
- Atmospheric pressure is the pressure exerted by the weight of the air, which acts on everything around us. It is caused by the weight of the air present from sea level to the upper limit of the atmosphere.
- It is measured in millibars, and its average value at sea level is 1,013.2 millibars.
- Earth's gravity pulls the atmosphere downward, making the air denser and having higher air pressure near the surface.
- As altitude increases, air becomes rarefied, which reduces air pressure.
- Changes in air pressure are the main reason for the movement of air. Air always flows from high pressure areas to low pressure areas, which is called wind.
- Instruments like mercury barometer and aneroid barometer are used to measure air pressure.
- Mercury barometers use a column of mercury, while aneroid barometers do not require a liquid.
- These instruments help to measure air pressure accurately. Thus, atmospheric pressure affects wind speed and weather conditions.
Vertical variation in air pressure
- Air pressure changes with altitude. The vertical variation means that as you climb higher, the air pressure decreases faster.
- In the lower part of the atmosphere, air pressure decreases very rapidly with height. Typically, air pressure decreases by about 1 millibar for every 10 meters of height.
- However, air pressure does not always decrease at the same rate, as it depends on altitude and temperature.
- The vertical pressure variation, i.e. the change in pressure with height, is greater than the horizontal variation.
- However, the force of gravity balances out this difference, so vertical winds (up-down winds) are not very powerful.
Horizontal distribution of air pressure
- The study of horizontal distribution of air pressure is very important to understand the direction and speed of winds.
- This is shown with the help of isobar lines connecting places with equal air pressure.
- These lines, which connect places with equal air pressure above sea level, are used on weather maps to show the distribution of air pressure.
- After measuring the air pressure it is adjusted to sea level, so that the effect of different altitudes can be removed and accurate comparison can be made.
- In a low pressure system the air pressure is lowest at the centre, the isobar lines are round or oval, and this indicates a cyclone. In a high pressure system the air pressure is highest at the centre, this indicates an anticyclone.
World distribution of atmospheric pressure at sea level
- Air pressure varies in different parts of the world and its distribution varies seasonally, especially in January and July. Air pressure controls the speed and direction of winds on Earth.
- The equatorial low pressure area is near the equator. Due to excessive heat here, the air becomes lighter and rises, due to which the air pressure is the lowest in this area.
- The subtropical high pressure area is located near 30° north and south latitudes. This is called the subtropical high pressure belt, where the air pressure is high.
- The subpolar low pressure area is located at 60° north and south latitudes. The air pressure here is low and it is known as the subpolar low pressure belt.
- The polar high pressure area is located near the poles. The air pressure in this area is the highest and it is called the polar high pressure belt.
- These belts of air pressure are not permanent. Their location changes according to the seasons. They move towards the south in winters and towards the north in summers. This affects the structure of the atmosphere and the wind system.
Forces affecting the speed and direction of winds:
The difference in air pressure makes air flow from high pressure to low pressure, this is called the pressure gradient force. The Earth's rotation changes the direction of the wind, which is called the Coriolis force; the wind turns to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The height of the ground slows down the wind, this is called the friction force. The force of gravity pulls the air downward and keeps it stationary.
1. Pressure gradient force
- Pressure Gradient Force is generated by the difference in air pressure. This force causes the air to move from high pressure areas to low pressure areas.
- The rate of change in air pressure with respect to distance is called pressure gradient. The greater the pressure gradient force, the faster the wind will blow.
- The rate of change in air pressure with respect to distance is called pressure gradient. The greater the pressure gradient force, the faster the wind will blow.
2. Friction force
- Friction force is the force that slows down the speed of winds. This force is generated by the contact between air and the surface.
- The effect of friction is the greatest on the surface, and its effect can be felt up to a height of 1 to 3 km above the surface. In contrast, friction is very low on the sea surface, due to which the winds blow at a faster speed there.
3. Coriolis force
- The Coriolis force is a force generated by the Earth's rotation that changes the direction of the wind. This force was discovered by a French scientist in 1844, and it was named after him, the Coriolis force.
- Due to the rotation of the Earth, the wind turns to the right in the Northern Hemisphere, while in the Southern Hemisphere it turns to the left. The faster the wind blows, the greater is the effect of the Coriolis force. This force depends on latitudes. Its effect is the greatest at the poles, while it has no effect at the equator.
- Wind always flows from high pressure to low pressure. However, due to Coriolis force and pressure gradient force, it does not flow in a straight line but starts rotating around the low pressure. This is the reason why the direction of wind on the earth appears rotating rather than straight.
- There is no Coriolis force near the equator, so the air flows straight without deflection. This is why tropical cyclones do not form in this region. The Coriolis force plays an important role in affecting the Earth's wind system and weather.
4. Air pressure and winds
- The direction and speed of winds are mainly determined by the balance of pressure and various forces. Above the surface of the earth, at a height of about 2-3 km, the winds are free from friction and are only under the influence of pressure gradient force and Coriolis force.
- If the isobar lines are straight and there is no effect of friction, then the pressure gradient force and the Coriolis force come into a balance. In this case, the winds blow parallel to the isobar lines, which are called geostrophic winds.
- In low pressure areas, the winds move around the low pressure. This is called cyclonic circulation. In contrast, in high pressure areas, the winds move around the high pressure, which is called anticyclonic circulation.
- In low pressure areas, the air is pulled from the surface and rises up, whereas in high pressure areas, the air comes down from the height and spreads on the surface.
- Winds can rise due to many reasons. For example, air rises when it collides with mountains, due to the effect of convection currents or with the help of fronts. These processes affect the speed and circulation system of air in the atmosphere.
general circulation of the atmosphere
The general circulation of the atmosphere is the process in which air and winds flow due to differences in air pressure and temperature at different locations on Earth. This circulation also affects the oceans and the Earth's climate.
1. Main factors:
- Latitudinal variation in temperature
- Distribution of air pressure strips
- Displacement of air pressure belts due to solar rays
- Location of oceans and continents
- Earth's rotation
2. Work of atmospheric circulation
- ITCZ (Inter-Tropical Convergence Zone): Warm air near the equator rises, creating a low pressure area. This air cools down and falls at 30° north and south latitudes, forming a subtropical high pressure area.
- Hadley Cell: The flow of air from the equator to 30° latitude is called Hadley Cell. This air returns back to the equator in the form of easterly winds on the surface.
- Ferrel Cell: The flow of air from 30° to 60° latitude is called the Ferrel Cell, where westerly winds blow on the surface.
- Polar Cell: The flow of air from 60° to the poles is called the polar cell. The polar winds of this region are cold and dense, which flow towards the middle latitudes.
3. Effects on the oceans:
Wind blowing over the oceans creates slow and fast ocean currents. The oceans provide water vapor and energy to the atmosphere, which plays an important role in balancing the climate.
1. Seasonal winds
Seasonal winds change in different seasons. This change is mainly due to extreme changes in temperature, displacement of air pressure belts, and change in the direction of wind flow.
Effects of seasonal winds
- Southeast Asia offers the clearest example of monsoons, where wind directions change during the summer and winter seasons, profoundly affecting the region's climate and agriculture.
- Seasonal winds differ from the general circulation system, as they are based on local conditions and climatic anomalies.
2. Local winds
- Variations in the heating and cooling of the surface and the development of daily and annual cycles give rise to many local and regional wind flows, which give rise to distinctive climatic conditions.
3. Land and sea breeze
- Due to the temperature difference between the land and the sea, the direction of the winds changes, which are called land and sea breeze.
- During the day: The land heats up rapidly and forms low pressure, while the sea remains cool and forms a high pressure area. The wind blows from the sea towards the land and is called sea breeze.
- At night: The land cools down quickly and forms a high pressure area, while the sea remains warm and forms a low pressure area. The wind blows from the land towards the sea, it is called land breeze.
- This change happens regularly throughout the day and night.
4. Mountain and valley winds
In mountainous regions, changes in temperature change the direction of winds, which are called mountain and valley winds.
- During the day: The air on the slope of the mountain becomes light after getting heated by the sun and rises up. The air flows from the valley towards the mountain, which is called valley breeze.
- At night: The mountain slopes cool down and heavy air flows towards the valley, this is called mountain winds.
- Descending winds: Cold air flows from higher areas towards the valleys.
- Adiabatic winds: Moist air crosses mountains, becomes dry and warm and descends down the slope, thereby warming the atmosphere.
5. Air Masses
- When air stays in a particular area for a long time, it takes on the properties of that area (such as temperature and humidity). Such broad areas over which the air is affected are called homogeneous regions. These regions can be oceans, plains, or icy areas. This type of air is called an air mass.
- Air mass is a large portion of air which has similar properties like temperature and humidity.
The places where air masses are formed are called origin regions. These regions determine the properties of air masses. Their main types are as follows:
- Tropical and subtropical oceans - warm and humid air.
- Subtropical hot desert - hot and dry air.
- High latitude cold oceans – cool and moist air.
- Extremely cold, icy continents - cold and dry air.
- Permanently icy areas (Antarctic and Arctic) – extremely cold and dry air.
Types of air masses:
- Air masses are divided into five parts based on their origin:
- Tropical Oceanic (mT) - Warm and humid.
- Tropical Continental (CT) – Hot and dry.
- Polar Oceanic (mP) – cold and moist.
- Polar Continental (CP) – cold and dry.
- Continental Arctic (CA) - extremely cold and dry.
6. Fronts
When two different types of air masses meet, the contact area between them is called a front. The process of formation of fronts is called frontogenesis. This process occurs especially in mid-latitudes and is the main cause of weather changes.
There are four types of fronts:
1. Cold Front: When cold and heavy air pushes warm air upwards, it brings heavy rainfall and fall in temperature.
2. Warm Front: When warm air tries to rise over cold air, it causes light rain and cloud formation.
3. Stationary Front: When both the air masses become stationary and no air rises, there is not much change in the weather.
4. Occluded Front: When an air mass is completely lifted, it can cause clouds, rain and storms.
Characteristics of Front:
- Fronts cause weather changes by causing sudden changes in temperature, pressure and humidity. The meeting of air masses causes air to rise, leading to clouds and rain.
- These fronts form mainly in the middle latitudes and are most active there.
7. Extra-Tropical Cyclones
Extratropical cyclones form in middle and high latitudes away from the tropics, causing sharp and sudden weather changes. These cyclones develop near the polar front. When the pressure decreases, warm air moves north and cold air moves south, creating a cyclonic circulation.
Main components:
1. Warm Front: Warm air rises over cold air, causing clouds and rain
2. Cold Front: Moves at a fast speed and covers the warm front.
Occluded Front: When the cold front completely covers the warm front, the cyclone starts weakening.
- These cyclones bring rain, strong winds and drops in temperature, affecting the weather over large areas.
8. Tropical Cyclones
- Tropical cyclones are strong and destructive storms that form over tropical oceans, causing heavy damage in coastal areas.
- Cyclones are known by different names in different oceans and regions. In the Indian Ocean they are called cyclones, while in the Atlantic Ocean they are known as hurricanes. In the Pacific Ocean they are called typhoons, and in the region of Western Australia they are called Willy-Willies.
Conditions for the formation of a cyclone:
- Warm ocean: surface temperature 27°C or higher.
- Coriolis force : Helps the air to move around.
- Wind balance: less variation in vertical winds.
- Low pressure area: Places for air to circulate.
- Divergence in the upper atmosphere: Spread of air at upper levels.
The structure of a cyclone is divided into three main parts.
1. Eye: It is the centre of the cyclone, where calm and light winds blow, and the weather is relatively calm.
2. Eyewall: This is the most dangerous part of the cyclone, where the strongest winds (up to 250 km/h) blow and the heaviest rainfall occurs.
3. Rainbands: These are strips of clouds which help in bringing heavy rains and storms.
- The diameter of a cyclone is 600-1,200 km and it moves at a speed of 300-500 km per day. At the time of landfall, it weakens due to lack of ocean heat, but leaves impacts like heavy rain, strong winds and flooding.
- Cyclones leave a variety of destructive impacts. Strong winds damage buildings and trees, while heavy rainfall increases the risk of flooding. Storm surges cause severe damage to coastal areas, causing great loss of life and property. Timely warning and effective disaster management are extremely important to reduce the impact of cyclones.
9. Thunderstorms and Tornadoes
Lightning and tornadoes are local but extremely aggressive storms. They remain active for a short time and affect a limited area, but their intensity is very high.
1. Thunderstorm:
