Atmospheric Pressure and Winds

Atmospheric pressure refers to the force per unit area exerted against a surface by the weight of the air above that surface.

The atmospheric pressure is measured with the help of Mercury Barometer or aneroid barometer.

The atmosphere can be considered as a closed container bounded by land and sea from below and force of gravity from above that does not allow air to escape to outer space. The gas molecules are in a constant state of collision and move freely. These molecules exert pressure which causes atmospheric pressure.

The factors which affect the atmospheric pressure of a place are:

1. Altitude
2. Temperature
3. Earth rotation

The world's pressure belts are:

1. Equatorial low pressure belt (5°N to 5°S)
2. Sub-tropical high pressure belt (25° to 35° N and S)
3. Sub-polar low pressure belt (55° to 60° N and S)
4. Polar high pressure belt (75° to 90° N and S)

Any object or fluid moving in the northern hemisphere tends to be deflected to the right of its path of motion. In the southern hemisphere, a similar deflection is toward the left of the path of motion. This is known as Ferrel's law. It states the direction of action of the Coriolis Force.

A zone of low pressure lying parallel to equator where North-East and South-East trade winds converge and meet each other, producing calm and light surface winds and a strong upward movement of air is called Doldrums.

It is called so because the word Doldrums is associated with a gloomy, listless mood, perhaps reflecting the sultry air and variable breeze found there.

Trade Winds are the surface winds which move from the Horse Latitudes to the Doldrums. In the Northern Hemisphere, they are the North-East trades and in the Southern Hemisphere, the South-East trades.

The wind always blow from high pressure area towards low pressure area. The trade winds are caused by movement of winds from Sub-Tropical high pressure belts towards equatorial low pressure belt.

The Westerly Winds which blow with great frequency and regularity in regions lying on the poleward sides of the subtropical high pressure areas or Horse latitudes are known as the Anti-Trade Winds or Westerlies.

The Roaring Forties and Furious Fifties are westerlies blowing in Southern Hemisphere. They are called so because they blow from 40° and 50° S with great strength. Their strength is because of obstruction-less path.

The Planetary Winds are:

1. Trade Winds
2. Westerlies
3. Polar Winds / Polar Easterlies

The decrease of pressure between two points along a line perpendicular to the isobars divided by the distance between the points is called the Pressure Gradient. It can be defined as the rate of change of pressure per unit horizontal distance.

The causes of shifting of pressure belts are inclination of Earth's axis and change in relative position of Earth with Sun -

1. At the time of Summer solstice the Sun is vertical over the Tropic of Cancer and therefore, all the pressure belts except the northern polar high pressure belt shift northward.
2. After Autumn Equinox, there is southward migration of the Sun which becomes vertically over the Tropic of Capricorn at the time of Winter Solstice and therefore the pressure belt shifts southward except in the southern polar high pressure belt.
3. The Sun is overhead over the Equator at the time of Spring Equinox and Autumn Equinox and hence all the pressure belts occupy their normal positions.

The shifting of pressure belts has the following consequences:

1. It influences the areas of Mediterranean climate and the position of Westerlies which move northward in the summer season (June) and southward in the winter (December) season.
2. The summer and winter monsoons and the complete reversal of wind direction are also the results of shift of pressure belts.
3. Both the trade and anti-trade winds are thus closely influenced by the shift of belts.

If the earth did not rotate on its axis, winds would follow the direction of the pressure gradient. Instead, Earth's rotation produces a force which tends to turn the flow of air. This force is called the Coriolis Force. It results in the deflection of air towards the right in the Northern Hemisphere and toward the left in the Southern Hemisphere.

Local winds are types of winds which blow for a short duration and affect the climate of the region where they blow. These winds originate in a specific region and blow over a limited area. Local winds are generally known by a common name throughout the world. Some examples of local winds are Loo, Mistral, Fohn, Chinook, Norwester, etc.

The Land Breeze is a diurnal wind blowing from the land out to sea. It is caused by the differential cooling of land and sea. The cooling of the air over the land by radiation during the night causes the air to descend and flow seawards. Therefore during the night there is low pressure over the heated sea and high pressure over land. Hence, winds blow from land to sea during night.

The Sea Breeze develops along sea coasts or large inland water bodies in summer when the land heats much faster than the water on a clear day and a low pressure develops over the land. Under these conditions, in the day, the air blows from the sea to the land as there is high pressure on the sea and low pressure over the land.

The Sea Breeze develops along sea coasts or large inland water bodies in summer when the land heats much faster than the water on a clear day and a low pressure develops over the land. Under these conditions, in the day, the air blows from the sea to the land as there is high pressure on the sea and low pressure over the land. Below diagram explains the formation of sea breeze:

Monsoon Winds are periodic winds like the sea and land breezes on a large scale. The monsoon is a seasonal wind which blows in a fixed direction during a particular season. It reverses its direction as the season changes. They are divided into two Wind Systems the Summer Monsoon and the Winter Monsoon.

Theses winds develop due to differential heating of land and sea over a large area.

Land and sea breezes are local periodic winds on a diurnal basis. They develop due to differential heating of land and sea. These winds have a moderating effect on the temperature of the place. The temperature is neither too hot nor too cold. These winds also provide relief from the stagnant humid heat.

Cyclones are strong winds which can blow at any place and time. These are the centres of low pressure which may last for a few days or few hours. Cyclone winds blow with great speed towards the centre which is the eye of the storm. The movement of wind is anticlockwise in northern hemisphere and clockwise in southern hemisphere.

Cyclones are caused due to formation of low pressure area which becomes the centre of cyclone. The air from all directions rushes towards the low pressure area with great force in clockwise or anticlockwise direction.

Due to extreme heat, there is development of a strong low pressure area which initiates cyclones. They are generally associated with heavy rains, cloudy skies and stormy weather. After cyclones, the weather becomes pleasant.

Anticyclone is a high pressure area. In the Northern Hemisphere, winds blow in a clockwise spiral away from the centre while in Southern Hemisphere, winds blow in an anti-clockwise spiral.

Anticyclones are caused due to descending of warm air which is completely devoid of moisture.

Tropical Cyclone — Tropical Cyclones start within 8° and 15° - 20° North and South of the Equator where surface-sea temperature reaches 27°C. The air above the warm sea is heated and rises up. This causes the low pressure. The weather system produces heat which increases the wind speed. These cause the tropical cyclones to sustain itself. These cyclones are associated with high temperature and high humidity.

Temperate Cyclone — They are common near the sub-polar low pressure belts. The warm moist Westerlies from the South converse with the polar Easterlies from the north. The cold air mass is known as cold front and the warm air mass is known as warm front. When these fronts meet, it causes rainfall. Front is the zone of contact between the two. They are more frequent in winter·than in summer.

Variable winds are those winds which blow in a small area and are related to the pressure systems. They are known as variable winds as they do not blow in a definite direction and their speed and velocity varies with the pressure system. Two main kinds of variable winds are cyclones and anti-cyclones.

Two special types of tropical cyclones are:

1. Typhoons in South China Sea.
2. Hurricanes in Caribbean Sea.

Typhoons are experienced in South China Sea.

Isobars are imaginary lines drawn on a map to join places having same or equal mean atmospheric pressure reduced to a sea level.

The factors affecting air pressure are:

1. Altitude.
2. Temperature.
3. Earth rotation.

Weather maps consist of isobars to show atmospheric pressure because different pressure systems cause different types of weathers. The spacing of isobars indicate the pressure gradient. The closer the isobars, the steeper the gradient of pressure and the weather is stormy. When isobars are far apart from each other, there is a little difference in the atmospheric pressure. Therefore, weather maps are important in weather forecasting.

The main causes for development of local winds are:

1. Unequal heating of land and sea resulting into land and sea breezes.
2. Heating and cooling of the mountain slopes.
3. Local winds originating because of the deformation of air currents, crossing of mountain ranges and physical barriers.
4. Convectional Local Winds caused by steep pressure gradients and steep variations in local temperatures.

Jet streams are fast flowing narrow air current found in the upper atmosphere or in troposphere of some planets including earth. The main jet streams are located near the altitude of the tropopause.

The horizontal movement of air relative to earth's surface, produced essentially by air pressure differences from place to place and also influenced by the Coriolis force and surface friction is called Wind.

Wind originate due to uneven heating of the Earth by the Sun and due to earth's rotation.

The important pressure belts of the Earth are:

1. Equatorial low pressure belt (5°N to 5°S)
2. Sub-tropical high pressure belts (25° to 35° N and S)
3. Sub polar low pressure belts (55° to 60° N and S)
4. Polar high pressure belts (75° to 90° N and S)

The winds that blow constantly throughout the year in a particular area, due to general pressure belts, are called permanent winds. For example: Trade winds, anti-trade winds and polar easterlies.

Permanent winds derive their name due to fact that they blow throughout the year.

Trade Winds are called the North-east Trades in the Northern Hemisphere.

Trade Winds are called South-east trades in the Southern Hemisphere.

Periodic winds are the seasonal winds that reverse their direction with season. These winds blow at regular intervals or in regular cycles.

These winds develop as a result of variation in local temperature, pressure and humidity. Their origin is attributed to formation of air currents, crossing mountain ranges, valleys and physical barriers.

Two local winds are:

1. Chinook
2. Loo

Two wind systems of monsoons are:

1. The Summer Monsoon
2. The Winter Monsoon

These winds can be best noticed in South West Asia and Northern Australia.

The horizontal movements of air relative to earth's surface, produced essentially by air pressure differences from place to place, also influenced by the Coriolis force and surface friction are called Winds.

The force exerted on the surface by the air above it as gravity pulls it towards earth is called atmospheric pressure.

A dynamically or thermally caused low pressure area of converging and ascending air flows is known as Cyclone.

El Nino is a warm ocean current that occasionally appears off the coast of Peru in South America as a temporary replacement of the Cold Peru Current.

Isobars are imaginary lines drawn on a map to join places having same or equal mean atmospheric pressure reduced to a sea level.

The Coriolis Effect is the effect produced by a coriolis force, namely, the tendency of all particles of matter in motion on the earth's surface to be deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

As we go higher upwards, the atmospheric pressure decreases at an average rate of 34 millibars per 300 metres of latitude. It is because the air in the lower layer of the atmosphere is denser than at higher level. The air is denser in lower layer because of gravitational pull of earth and also because the lower layers are compressed under upper layers of atmosphere.

The Tropical Belt of Calms is also known as the Horse Latitude because in olden days because of the absence of surface winds ships had to unload the cargo to make them lighter. At times the cargo used to be horses.

The rotation of the earth produces another force other than the pressure force which is called the Coriolis force. It causes deflection of winds. Thus, the winds are directed to the right of their flow in the Northern Hemisphere due to Coriolis effect.

The Roaring Forties and Furious Fifties are found in the Southern Hemisphere as they do not face any obstruction to lower their strength and speed.

The causes of shifting of pressure belts are inclination of Earth's axis and change in relative position of Earth with Sun -

1. At the time of Summer solstice the Sun is vertical over the Tropic of Cancer and therefore, all the pressure belts except the northern polar high pressure belt shift northward.
2. After Autumn Equinox, there is southward migration of the Sun which becomes vertically over the Tropic of Capricorn at the time of Winter Solstice and therefore the pressure belt shifts southward except in the southern polar high pressure belt.
3. The Sun is overhead over the Equator at the time of Spring Equinox and Autumn Equinox and hence all the pressure belts occupy their normal positions.

Cyclone Winds blow at a great speed, usually at a rate of 100 - 150 km/h, and form a fierce storm. Therefore, they cause heavy damage to life and property.

During winters the land masses are cooler than the water bodies. The air over the land masses is cooler and condensed, creating high pressure over the land masses while the air over water bodies becomes heated and rises up creating low pressure over the water bodies.

Cyclones require a sea surface temperature of about 26°C to develop and maintain their strength. They also need a high relative humidity. These conditions are only found in the tropical oceans during summer. Therefore, cyclones mostly occur in summer in the tropical regions.

The word Chinook means snow eater. Chinooks are warm, dry, South-Westerly adiabatic winds which blows down the slopes of the Rockies in parts of U.S.A. They are known for their ability to rapidly melt snow and ice. This can lead to a rapid disappearance of snow cover, even in the middle of winter. Hence they are called snow eater.

The equatorial region has a low-pressure belt because the Sun's rays strike the equator almost directly, leading to intense heating of the air near the surface. As pressure is inversely proportional to temperature, hence due to high temperatures in equatorial region, low pressure belt develops there.

Below is the diagram of the planetary wind system showing the direction of winds:

Below diagram shows formation of land breeze:

Below diagram shows formation of sea breeze:

Below is the labelled diagram of a cyclone in Northern Hemisphere:

Below is the labelled diagram of cyclone in Southern Hemisphere:

Below is the labelled diagram of an anticyclone in Northern Hemisphere:

Below diagram shows the Major Pressure Belts of the World:

Increasing temperature & altitude

Isobars

Coriolis force

Sub polar low pressure belt

North east trade winds

Land breeze & sea breeze are the phenomena of interior parts of the country.

Australia

Cyclone is associated with cloudy skies & very heavy rainfall but anti-cyclone is associated with clear skies & dry conditions.

Trade wind

Peru

South west

Jet stream

Tropical cyclones form between 8° and 20° north and south of the Equator, where sea surface temperatures are around 27°C or higher. The warm ocean heats the air above, causing it to rise and create a zone of intense low pressure. As a result, powerful winds spiral inward and upward, leading to the formation of a cyclone. These storms bring extremely strong winds, heavy rainfall, and storm surges, which can cause massive destruction to life and property, making them highly devastating natural disasters.

1. These winds are known as the Roaring Forties and Furious Fifties.

2. These westerly winds are particularly strong in the Southern Hemisphere between 40° and 50°S because there is very little land to interrupt their flow. The continuous stretch of open ocean allows the winds to gain speed and blow with great force, helping ships sail faster from west to east.

Both A and R are true and R is the correct explanation of A.

Explanation — At higher altitudes, atmospheric pressure is lower, which means the boiling point of water decreases. As a result, food takes longer to cook. A pressure cooker helps increase the pressure inside, thereby raising the boiling point of water and making cooking faster and more efficient.

A is true but R is false.

Explanation — North east monsoon blows from land to sea. Since these winds travel over land and not moisture-laden oceans or seas, they carry less moisture, resulting in less rainfall.