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Three chief types of winds are Trade winds, Westerlies, and Polar winds.

In the northern hemisphere the winds deflect towards right and in Southern hemisphere deflect towards left due to the effect of the rotation of the earth, e.g. the trade winds take the Southwest direction in place of southern direction in northern tropical belt and in southern tropical belt these winds take northwest direction in place of northern direction.

As the air become thinner with increasing height, so, the air pressure also decreased. The rate of decrease is 100 millibars per 1000 metres of height gained. So mountain hikers take oxygen cylinder with them to compensate the air inhaled.

Doldrums is a low pressure belt along the equator from 5°N to 5°S. In this belt due to high amount of evaporation, the atmosphere is very calm and quiet due to no circulation of winds and continuous rising air mass creating a hollow like condition in this belt. Inspite of air pushed towards the equator, it is immediately deflected to right and left, creating a vacuum like condition resulting in a belt of calm named as ‘Doldrums’

When the temperature increases and the air start to rise upwards. creating a low pressure area, but when the temperature decreases, the air start to sink downwards creating a high pressure area on land surface. So, the temperature and pressure are inversely related to one another.

Due to the earth’s changing aspect with respect to the sun, the pressure belts shift northwards by 5° in summer and southwards in winter resulting in particular seasonal changes, e,g. The cold waves blow in India in winter due to the sub¬tropical belt shifting along 30°N touching the Himalayas. On the other hand in summer the trade winds become powerful in India and push the monsoon winds from east to west in Northern Plain of India.

Due to the rotation of the earth from west to east, the winds are deflected to the right e.g. the trade winds blowing from north, to south take north easterly direction.

Permanent winds : 

1. These blow throughout the year. 

2. These are named Trade winds, Westerlies and Polar winds. 

Periodic winds : 

1. These blow in particular area and seasons. 

2. These are named monsoon winds, Chinook, Foehn, Mistrel winds etc.

Vertical Variation : 

1. The Temperature decreases as the height increases 

2. The rate is 1°C for every 166 metres of height. 

Horizontal Variation : 

1. The temperature decreases as the latitude increases. 

2. Highest temperatures are found along Equatorial and tropical regions, while least temperature is recorded in polar areas.

In tropical region the air is greatly humid due to excess of evaporation creating a low pressure conditions, while the cold regions with less humidity are high pressure areas. So, the humid air is lighter than dry air.

Mediterranean lands come along the belt of Westerlies along 30°S in place of 40°N in winter. So, the Mediterranean countries along 30°N get ample rainfall. Similarly in the Southern Hemisphere the Westerlies blow along 30°S, in winter, so, the southern Mediterranean countries get winter rainfall.

Isobars : 

1. These are the lines joining the places of same air pressure. 

2. The closeness of these lines indicate the intensity of pressure. 

Isotherms : 

1. These are the lines joining the place of same air temperature. 

2. The closeness of these lines indicate the intensity of temperature.

The three chief types of winds are Trade winds, Westerlies and polar winds. 

Trade winds : blow along the tropical belt from sub-tropical High to equatorial low pressure belts. The name trade is given due to the past time, when trade ships were sailed according to the direction of these winds. 

Westerlies : are the winds blowing from western direction. These are south-westerly in northern and north-westerly in the southern hemispheres. 

The Polar : winds are icy cool winds blowing from poles towards Circum polar low pressure belts, which are also known as Blizzards.

Atmospheric Pressure refers to the force per unit area exerted against a surface by the weight of the air above that surface. Pressure is expressed in millibars (mb) and measured with a mercury barometer. The average atmospheric pressure at sea level is 1013.25 mb or 760 mm (the) height of the column of mercury in a barometer at sea level). Atmospheric Pressure decreases with height. 

Factors affecting Atmospheric Pressure are as below : 

1. Altitude : The atmospheric pressure decreases with height or altitude. The decrease in pressure is about 1 cm of mercury for every 110 m of ascent. The atmospheric pressure is highest at sea level. This is because at higher altitudes the air is thinner or less dense than the air at the sea level. The maximum air density is at the earth’s surface ; air density decreases with height because the pull of the earth’s gravity is less. The fewer number of gas molecules at higher altitudes means fewer molecular collisions and a decrease in air pressure. 

2. Temperature : Atmospheric pressure decreases with increase in temperature. This is because when the temperature rises, air expands. The molecules of air move far apart (become less dense) and hence exert less pressure. On the contrary, with decrease in temperature the air gets compressed and the space between molecules decreases (becomes more dense) and exerts more pressure on the region. 

3.  Water Vapour : Water vapour concentration affects atmospheric pressure because the molecular weight of water (18 g/mol) is less than the average molecular weight of air (about 29 g/mol). When water evaporates and enters the atmosphere as a gas, the water vapour molecules take the place of other gas molecules in the air. So, a volume of wet (or humid) air weighs less than an equal volume of dry air. Therefore, humid air is less dense and exerts less pressure than dry air. 

4.  Rotation of the Earth : Due to the rotation of the earth, bulk of the air at the Poles is thrown away towards the Equator. Since the Equatorial region receives great amount of heat throughout the year, the air becomes warm and light and therefore, it rises and creates low pressure. At the Poles, the cold heavy air sinks down and creates high pressure. In fact, temperature and rotation of the earth together contribute to the formation of world pressure belts.

Cyclones : 

1. These are the circular winds with low pressure in the centre. 

2. The winds blow anticlockwise from outside to the centre in Northern Hemisphere. 

3. These bring rainfall, clouds thunder lightening. 

Anticyclones : 

1. These are circular winds with high pressure in the centre. 

2. The winds blow clockwise from the centre towards all sides in Northern Hemisphere. 

3. These bring clear weather and cold winds.

Summer Monsoon : 

1. These blow in summer season from July to September. 

2. These bring heavy rainfall with thunder lightening

Winter Monsoon : 

1. These blow in winter mainly in December and January. 

2. These bring dry cold weather and cold waves.