Pressure & wind system

Pressure and wind system-

Air pressure-

• The weight of a column of air at a given place and time is called air pressure.

• Barometer is the instrument which measures air or atmospheric pressure.

• The average air pressure at sea level is equivalent to the pressure produced by a column of water about 10 meters (or about 76 cm of mercury column).

Units of Atmospheric Pressure Units of Atmospheric Pressure

Pascal (Pa)-

• It is a SI  unit for air pressure. 1 Pa = force of 1 newton acting on a surface of one square meter.

• 1 hectopascal (hPa) = 1 millibar (mb) [hecto = one hundred =100]

Bar-

• A more popular unit for air pressure. 1 bar = 1000 hPa = 1000 mb. 

Millibar(kilopascals)

• It is also he unit of measurement of atmospheric pressure.

• One millibar is equal to the force of nearly one gram per square centimetre. 

Factors Affecting Air Pressure-

Temperature-

• With increase in temperature there is decrease in air pressure because increase in temperature increases the volume of the air and thus reduce its density and pressure.

• Hence, air pressure is low in equatorial regions and it is higher in polar regions.

 Height from Sea Level or Altitude-

• As there is heavy gases in lower atmosphere but these gases decreases with altitude therefore pressure exerted by air also decreases with altitude. 

The Earth’s Rotation-

• The earth’s rotation generates centrifugal force.

• This results in the deflection of air from its original place, causing decrease of pressure.

• The earth’s rotation also causes convergence and divergence of moving air.

• Areas of convergence experience low pressure while those of divergence have high pressure. 

• Also equator have low pressure and poles have high pressure due to more centrifugal force at equator and less at polar region.

Gravitation pull-

• Equator have low gravity as comparison to poles due to shape of the earth therefore equator have less gravitational pull which cause less air pressure as comparison to poles. 

 Moisture in Air-

• Air with higher quantity of water vapour has lower pressure and that with lower quantity of water vapour has higher pressure because water vapours are light in weight therefore they rise up which decreases air pressure.

• That’s why we have high pressure in winters due to dry air as comparison to summers (due to moist air).

Distribution of Pressure-

Vertical Distribution-

• The lower layers of the atmosphere have higher density, hence, exert more pressure.

• Conversely, the higher layers  are less compressed and, hence, they have low density and low pressure.

• Therefore in general there is decrease in pressure as we move towards higher altitudes.

Due to Low pressure in Hilly areas-

• Rice takes more time to cook because low pressure reduces the boiling point of water.

• Breathing problem such as faintness and nose bleedings are also faced by many trekkers from outside in such areas because of low pressure conditions in which the air is thin and it has low amount of oxygen content. 

Why we do not experience strong upward winds ?

• 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 and that’s why we do not experience strong upward winds. 

Horizontal Distribution of Pressure-

• Horizontal distribution of pressure is studied by drawing isobars at constant levels. 

Isobar-

•  A line on a weather map that joins places that have the same air pressure at a particular time. 

• The spacing of isobars expresses the rate and direction of change in air pressure. This charge in air pressure is referred to pressure gradient.

Pressure gradient -

• It is the ratio between pressure difference and the actual horizontal distance between two points.

• Close spacing of isobars expresses steep pressure gradient while wide spacing indicates gentle pressure gradient.

Pressure Belts of Earth-

• There is a pattern of alternate high and low-pressure belts over the earth.

• This is due to the spherical shape of the earth due to which different parts of the earth are heated unequally. 

Classification/types of Pressure Belt-

On the basis of mode of genesis pressure belts are divided as:

1.Thermally induced pressure belts: -

(i) Equatorial low pressure belt. 

(ii) Polar high pressure belt. 

2. Dynamically induced pressure belts-

(i) Sub-tropical high pressure belt. 

(ii) Sub-polar low pressure belt. 

Equatorial Low-Pressure Belts-

• This belt extends from equator to 100N and 100S latitudes.

• Due to the vertical rays of the sun here, there is intense heating which causes low pressure in this area.

• Due to excessive heating horizontal movement of air is absent here and only conventional currents are there.

• Therefore this belt is called doldrums which means the zone of calm.

• This zone is also Inter Tropical Convergence Zone (ITCZ) because there is convergence of winds flowing from sub tropical high pressure belts. 

The Sub-tropical High Pressure Belts-

• The sub-tropical high pressure belts extend from the tropics to about 350 latitudes in both the Hemispheres.

• In the northern hemisphere it is called as the North sub-tropical high pressure belt and in the southern hemisphere it is known as the South sub-tropical high pressure belt.

• This belt is not thermally induced. 

Why there is high Pressure-

• The rising air of the equatorial region is deflected towards poles due to the earth’s rotation. After becoming cold and heavy, it descends in these regions and creates heavy pressure.

• It is also called as the Horse latitude because of prevalence of frequent calms. 

Sub-polar low pressure belt-

• These belts located between 60° and 70° in each hemisphere are also known as Circum-Polar LowPressure Belts.

• Winds coming from the sub-tropical and the polar high belts converge here to produce cyclonic storms or low pressure conditions therefore this belt is dynamically induced.

• This region is marked by violent storms in winters.

Polar High-Pressure Areas-

• This is Thermally induced pressure belt (due to low temperature in this region )and it is located between 70° to 90° North and South.

• Winds from these belts blow towards sub-polar low pressure belts. 

Seasonal distribution of Pressure-

Conditions in January-

• In January, The sun moves southwards due to which the equatorial low pressure belt shifts a little south of the mean equatorial position.

• Areas of lowest pressure occurs in South America, Southern Africa and Australia. This is because the land tends to get hotter rapidly than water.

• Oceans of southern hemisphere have high sub tropical pressure cells.

• The belt of high pressure gets disrupted by the presence of continental land masses, which experience significantly higher temperatures compared to the surrounding oceanic regions.

• They are well developed in eastern part of the ocean where cold ocean currents dominate. 

• In the absence of landmass, the sub-polar low pressure belt in the Southern Hemisphere forms a continuous belt of low pressure that encircles the earth without being divided into cells.

• In Northern hemisphere high pressure cells are not properly developed in semi-tropical areas but appear as a long high pressure ridge.

• In eastern central Asia there is a fully developed and broad based high pressure cells which has all the characteristics favourable for intense heating.

• In the Northern Hemisphere, two low-pressure cells known as Iceland low and Aleutian low form over the North Atlantic and North Pacific oceans, respectively.

Conditions in July-

• There is shifting of equatorial low pressure trough towards north due to shifting of The sun towards north.

• The whole of the pressure system shifts to the north following the shift of the sun.

• The sub polar high pressure system extends in the form of a long trough in southern hemisphere but the low pressure cells in northern hemisphere are very feeble.

• Centres of low pressure have developed in Asian continent and north-west North America. This disrupts the sub-tropical high pressure system.

• During the winter months, landmasses develop high-pressure systems while the Aleutian and Icelandic lows weaken over the oceans, resulting in the formation of extensive low-pressure cells.

• In Asia also a low pressure develops.

• The subtropical heights of the northern hemisphere are more developed over the oceans - Pacific and Atlantic.

• The sub-tropical high-pressure belt in the Southern Hemisphere is a continuous feature.

Wind-

• Wind refers to the movement of air caused by variations in air pressure within the Earth's atmosphere.

Factors affecting winds-

1.Pressure gradient

• Winds moves from high pressure to low pressure due to pressure difference.

• The pressure gradient force is a physical force that arises due to variations in atmospheric pressure between two regions.

• Greater the pressure difference between two point ,greater will be gradient force(and steeper will be pressure gradient) which will cause winds to move faster. 

2.Coriolis force (Earth’s Rotation)-

• Coriolis force is a force that appears to act on objects in motion within a rotating frame of reference and is not a real force, but rather an apparent force that arises due to the rotation of the reference frame.

The magnitude of Coriolis force depends on-

• The rotation of the Earth.

• The speed of the moving object.

• Its latitudinal location.

The stronger the speed, the stronger the Coriolis force.

The higher the latitude, the stronger the Coriolis force that’s why The Coriolis force is zero at the equator. 

3.Frictional Force -

• It is a force of opposition which slows air in motion.

• It is more near the earth surface and decreases as we move upward.

• Because friction reduces wind speed it also reduces Coriolis deflection. 

4.Centrifugal action of wind-

• Curved isobars occur when there is an imbalance between other forces, resulting in this atmospheric phenomenon. 

• The imbalance referred to is essential to induce the required change in direction for the curved movement of fluids.

• This force helps to change the direction of the wind.

Types of winds-

In general winds are classified as given below-

1. Planetary winds or permanent winds. 

2. Periodic winds. 

3. local winds. 

1. Planetary winds or permanent winds-

• These are permanent winds which flows from high pressure to low pressure.

• They blow over vast area of continents and oceans.

They are of following types-

A.The Easterlies-

• The winds blows from sub-tropical high pressure areas towards equatorial low pressure areas.

• They are also called as Trade winds.

• Because of the Coriolis effect the northern trade winds move away from the subtropical high in north-east direction.

• In the Southern Hemisphere, the trade winds move away from the subtropical high and toward the equatorial low from the southeast direction.

Why they are called Easterlies ?

• These winds generate from Eastern direction,therefore they are called Easterlies. 

Inter Tropical Convergence Zone (ITCZ)-

• The Intertropical Convergence Zone (ITCZ) refers to the belt that encircles the Earth near the equator where the trade winds of the Northern and Southern Hemispheres converge.

• There is seasonal shift in this ITCZ. 

Hadley cells-

• The Hadley Cells are a type of low-latitude atmospheric circulation that involves the upward movement of air at the equator and the downward movement of air around 30° latitude. This pattern of circulation plays an important role in the Earth's climate system and the distribution of heat and moisture around the globe.

• Ocean currents regulate weather patterns in the low latitudes and create the trade winds in the Tropics.

B.The Westerlies-

• These winds moves poleward from the sub-tropical high pressure in both Hemisphere

• In the northern hemisphere they are  detected to the right and thus blow from the south west.

• In the southern hemisphere they are deflected to the left and blow from the north-west.

• They are found between 30-60 degree latitude.

• These winds start descending down above the sup-tropical high pressure belt and polar high pressure belt to form cells.

• These cells are called Ferrel cell and Polar cell respectively.

The Ferrel cell

• The Ferrel cell refers to the general movement of air in the mid-latitudes, which takes place at latitudes ranging from 30 degrees to 60 degrees North and South.

• The westerlies are stronger in cold region and they are also stronger in southern hemisphere as comparison to northern hemisphere because of presence of less landmass in southern hemisphere. 

Furious fifties and Screaming sixties-

They are known by different names in Southern Hemisphere as-

• Roaring forties- Along 40 S latitudes.

• Furious fifties - Along 50 S latitudes.

• Screaming sixties -Along 60 S latitudes.

C.Polar Easterlies

• Polar easterlies blow from polar regions towards sub-polar low pressure regions.

• Their direction in the northern hemisphere is from north-east to southwest and from south-east to north-west in the southern hemisphere.

Polar cell - 

• The air rises, diverges, and moves towards the poles.

• Once over the poles, the air sinks, forming the polar highs.

• At the surface air diverges outward from the polar highs and forms the polar cell. 

Important One Liner on Winds and Pressure Belts of Earth. 

What is air pressure?

Air pressure is the weight of a column of air at a given place and time.

Which instrument is used to measure air pressure?

A barometer is used to measure air or atmospheric pressure.

What is the average air pressure at sea level?

The average air pressure at sea level is equivalent to the pressure produced by a column of water about 10 meters (or about 76 cm of mercury column).

What is Pascal?

Pascal is the SI unit for air pressure. 1 Pa = force of 1 newton acting on a surface of one square meter.

What is the more popular unit for air pressure?

The more popular unit for air pressure is bar. 1 bar = 1000 hPa = 1000 mb.

How does temperature affect air pressure?

With an increase in temperature, there is a decrease in air pressure because an increase in temperature increases the volume of the air, reducing its density and pressure.

How does altitude affect air pressure?

As altitude increases, the pressure exerted by air decreases as there are fewer heavy gases in the atmosphere.

How does the Earth's rotation affect air pressure?

The Earth's rotation generates centrifugal force, which results in the deflection of air from its original place, causing a decrease of pressure.

How does moisture in the air affect air pressure?

Air with higher quantities of water vapor has lower pressure, and air with lower quantities of water vapor has higher pressure because water vapor is lighter in weight and rises up, which decreases air pressure.

What are isobars?

Isobars are lines on a weather map that join places that have the same air pressure at a particular time.

What is the spacing of isobars used for?

The spacing of isobars expresses the rate and direction of change in air pressure, which is referred to as pressure gradient.

Why do we not experience strong upward winds?

The vertical pressure gradient force is much larger than that of the horizontal pressure gradient. However, it is generally balanced by a nearly equal but opposite gravitational force, which is why we do not experience strong upward winds.

What are the pressure belts of Earth?

There is a pattern of alternate high and low-pressure belts over the Earth, which is due to the spherical shape of the Earth and the unequal heating of different parts of the Earth.

What are the two types of pressure belts?

On the basis of the mode of genesis, pressure belts are divided into thermally induced and dynamically induced pressure belts.

What causes the Coriolis effect?

The Coriolis effect is caused by the Earth's rotation and results in the deflection of moving air and water.

What is a monsoon? 

A monsoon is a seasonal wind pattern that brings heavy rain to certain regions during specific times of the year.

How does pressure gradient force affect wind speed? 

The pressure gradient force is responsible for the acceleration of wind speed from areas of high pressure to areas of low pressure.

How do high and low pressure systems affect weather patterns?

High pressure systems are generally associated with clear and dry weather, while low pressure systems are often associated with clouds, precipitation, and storms.

What is the jet stream?

The jet stream is a high-altitude wind that flows from west to east and can influence weather patterns in the mid-latitudes.