Chapter - 10
Atmospheric Circulation and Weather Systems
️ Atmospheric pressure :-
Atmospheric pressure is the weight of the air column of one unit area from the sea level to the end of the atmosphere.
Atmospheric weight or pressure is measured in millibars and hectopascals.
The average air pressure on the ocean surface is 1013.25 millibars.
The air pressure on the map is shown by isobars or isobars.
️ Decreased air pressure :-
Air pressure is higher in the lower part of the atmosphere and decreases rapidly with increase in altitude, this decrease rate is 1 millibar per 10 meters of altitude.
️ Even pressure lines Isobar :-
Lines drawn joining places of equal air pressure above sea level are called isobars. It is drawn at equal intervals.
What does the near or far distance of equal pressure lines indicate?
If even pressure lines are close to each other, then the pressure gradient is high and if the pressure gradient is far away, then the pressure gradient is less.
️ Pressure gradient :-
The difference in pressure from one place to another is called pressure gradient.
️ Local Winds :-
Due to the variation of temperature and other weather-related factors, the movement of winds in a particular place is called local wind.
️ Tornado :-
In the mid latitudes, local storms take a terrible form with thunderstorms. There is very little air pressure in its center and the air comes aggressively from top to bottom like the trunk of an elephant, this phenomenon is called tornado.
️ Air Sign :-
When the air stays over a wide area for a sufficiently long time, it acquires the properties (related to temperature and humidity) of that area. This air with temperature and specific properties is called Vayu Rashi. They are hundreds of kilometers wide and have many layers.
️ Coriolis Force :-
The wind does not always move from high pressure to low pressure across isobars. They also get deflected due to the rotation of the earth. This deflection of the winds is called the Coriolis force or effect.
How does the Coriolis Force effect the direction of winds?
Due to the influence of this force, the winds turn to their right in the Northern Hemisphere and to their left in the Southern Hemisphere.
The effect of the Coriolis force is zero at the equator and maximum at the poles.
This deflection was proved by a scientist named Ferrell, so it is called Ferrel's Law.
️ Types of Winds :-
There are three types of winds :-
- PlanetaryWinds
- Seasonal Winds
- Local Winds
️ PlanetaryWinds :-
The winds blowing in the same direction throughout the year over a wide area of the earth are called global winds. These winds move regularly from one high pressure zone to another low pressure zone.
These are mainly of three types :-
- Passage or trade winds.
- West winds.
- The celestial winds
️ Transit or trade winds :-
The winds that move from the sub-tropical high air pressure zone to the equatorial low air pressure zone are called convective winds.
According to the Coriolis force, they deflect from their path and move in the North-East direction in the Northern Hemisphere and in the South-East direction in the Southern Hemisphere.
Trade winds are called trade winds in English. In German, trade means a definite path.
Reaching the equator, it gets saturated with water vapor and rains heavily throughout the year near the equator.
️ Westerly winds :-
Flows from the high air pressure zone to the subpolar low pressure zone.
In both the hemispheres, their expansion is between 30 ° degree to 60 ° latitude.
Their direction is from South-West in the Northern Hemisphere and from North-West in the Southern Hemisphere.
Like the trade winds, these winds are calm and not regular in direction. Cyclones and anticyclones often keep coming in this zone.
️ Polar Winds :-
These winds blow from the polar high pressure zone to the subpolar low pressure zone.
Their expansion is between 60 ° latitudes and poles in both the hemispheres.
These winds are very cold and dry due to coming from snowy areas.
️ Seasonal Winds :-
These are the winds which change their direction according to the season or season. They are called occasional winds. Monsoon winds are a good example of this.
️ Local Winds :-
These winds are produced by the difference between the heating and cooling of the surface. They affect a limited area locally. Land Sameer and Samudra Sameer, Lu, Phone, Chinook, Mistral etc. are such local winds.
️ Monsoon winds :-
The word monsoon is derived from the Arabic word 'Mausim'. Which means season. Therefore, monsoon winds are those winds whose direction is completely reversed according to the season.
These winds blow from sea to land in six months of summer and from land to sea in six months of winter.
These winds are divided into two categories, summer monsoon and winter monsoon. These winds blow in the Indian subcontinent.
❇️ Location - Sameer & Sea - Sameer: -
Sthal Sameer: -
These winds blow from the land towards the sea during the night. Because during the night the land cools down quickly and the sea cools down late, due to which an area of low air pressure develops over the sea.
Sea Breeze: -
These winds blow from sea to land during the day. Because when the sun shines during the day, the land gets hot sooner than the sea. Due to which an area of low air pressure develops at the site.
These winds are humid.
️ Valley Sameer :-
During the day, in calm, clear weather, vegetationless, sun-facing, slopes heat up rapidly and the air coming in contact with them also heats up and rises. To take its place, the wind blows from the valley upwards.
Their speed is very fast at two o'clock in the day.
Sometimes clouds are formed due to these winds, and it starts raining on the mountain slopes.
️ Mountain Sameer :-
During the night, the air of the mountain slopes becomes cold and heavy due to terrestrial radiation and starts descending in the valley.
Due to this the temperature of the valley becomes very low till some time before sunrise. Due to which there is an inversion of temperature.
Their speed is very fast just before sunrise. These breezes are dry.
️ Cyclone :-
When low air pressure is established in an area and there is high air pressure around it, then the winds are attracted towards the low pressure and due to the rotational motion of the earth, the winds are in the Northern Hemisphere in the opposite direction of the clockwise and the other. They move in a hemisphere according to clockwise.
️ Anticyclone :-
There is high air pressure in the center of this system. Therefore, the winds from the center move around towards the low air pressure. In this, the winds in the Northern Hemisphere correspond to the hands of the clock and Hemisphere moves in the opposite direction.
Vatagra: -
When two different types of air signs meet, then the boundary area between them is called Vatagra.
These are of four types –
- cold front
- warm air
- constant front
- Occupied airtight
️ Horizontal distribution of air pressure :-
The latitudinal distribution of atmospheric pressure is called horizontal distribution of atmospheric pressure. The difference in temperature at different latitudes and the effect of Earth's rotation create seven zones of air pressure on the Earth.
Which are as follows :-
Equatorial low air pressure zone: -
The extension of this zone is between 5 ° north and 5 south latitudes.
In this zone, the sun's rays fall straight throughout the year. Therefore, the air here is always heated and rises up.
Winds do not blow in this zone. Only vertical convective air currents rise upwards. Therefore, this zone remains a wind-less peaceful region. That's why it is called calm zone or doldrum.
Subtropical high air pressure zone: -
This zone is spread between 30 ° to 35 ° latitudes in both the northern and southern hemispheres.
The air in this zone is almost calm and dry. The sky is clear cloudless. All the hot deserts of the world are located in this zone in the western parts of the continents because the direction of the winds is from the land to the sea (Off Shore). Hence these winds are dry.
Subpolar low air pressure zone: -
The extension of this zone is between 60 ° to 65 ° degree latitudes in both the northern and southern hemispheres.
Depression (cyclone) comes in this zone, especially in winter.
Polar High Air Pressure Tropic: -
Their expansion is in the areas near the North and South Poles (90 ° North and South Poles).
The temperature here remains very low permanently. Therefore, the surface is always covered with snow.
