Orographic rainfall, 'also called relief rainfall" is caused when air is forced to rise against a high mountain. The mountain barriers lying across the direction of air flow, force the moisture laden air rise along the mountain slope. This results in the cooling of the air, which leads to the formation of clouds and rain. This rainfall is called Orographic rainfall. The side of the mountain facing the wind is called the windward side and receives heavy rainfall. It is called the rainfed region. The other side of the mountain that does not face the wind is called the leeward side and receives less rainfall becomes rain shadow region.

(i) Atmosphere is a mixture of gases, water vapour and dust particles in different proportions.
(ii) Nitrogen (78%) and Oxygen (21%) are permanent gases of the atmosphere. They constitute 99% of the total composition and their percentages always remain the same without any change.
(iii) The remaining one percentage is occupied by Argon (0.93%), Carbon dioxide, (0.03%), Neon (0.0018%), Helium (0.0005%), Ozone (0.00006%) and Hydrogen (0.00005%). Krypton. Xenon and Methane are also present in trace. Water vapour (0 - 0.4%) is also found in the atmosphere, which plays an important role in predicting weather phenomenon.
(iv) The other solid particles present in the atmosphere includes dust particles, salt particles, pollen grains, smoke, soot, volcanic ashes etc.,
(v) Oxygen is most important for living organisms.
(vi) CO₂ absorbs heat and keeps the atmosphere warm by insulation and radiation.
(vii) Nitrogen acts as a diluent and is chemically inactive.
(viii) Ozone helps in protecting the earth from radiation.
(ix) The solid particles in the atmosphere acts as nuclei on which water vapour condense to form precipitation.

Precipitation
Falling down of condensed water vapour in different forms is called Precipitation. When the dew point is reached in the cloud water droplets become saturated and start to fall. Hence, they fall on the earth as precipitation.
The climatic conditions/ factors influencing the forms of precipitation mainly are:
• Temperature.
• Altitude.
• Cloud type.
• Atmospheric conditions.
• Precipitation process.
The main forms of precipitation include drizzle, rain, sleet, snow, hail, etc.
Drizzle: Falling of numerous uniform minute droplets of water with diameter of less than 0.5 mm is called a drizzle from low clouds. Sometimes drizzles are combined with fog and hence reduce visibility.
Rain: Rain is the most widespread and important form of precipitation in places having temperature above the freezing point. It occurs only when there is abundant moisture in the air. The diameter of a rain drop is more than 5 mm.
Sleet: Sleet refers to a precipitation, in the form of pellets made up of transparent and translucent ice. This precipitation is a mixture of snow and rain.
Snow: Snow is formed when condensation occurs below freezing point. It is the precipitation of opaque and semi opaque ice crystals. When these ice crystals collide and stick together, it becomes snowflakes.
Hails: Hails are chunks of ice (greater than 2 cm in diameter) falling from the sky, during a rainstorm or thunderstorm. Hailstones are a form of solid precipitation where small pieces of ice fall downwards. These are destructive and dreaded forms of solid precipitation because they destroy agricultural crops and human lives.

WIND
(i) The horizontal movement of air along the surface of the earth is called the 'wind'.
(ii) The vertical movement of air is called an 'Air current',
(iii) The winds always blow from a high pressure area to a low pressure area.
PLANETARY WINDS
(i) The winds which constantly blow in the same direction through the year are called the planetary winds.
(ii) These winds are Trade winds, Westerlies and Polar Easterlies.
TRADE WINDS
(i) Trade winds blow from the subtropical high pressure belt to the Equatorial low pressure belt in both the hemisphere.
(ii) They blow in a constant direction throughout the year.
(iii) These winds were very helpful to traders who depended on the winds while sailing in the seas.
WESTERLIES
(i) Westerlies are the permanent wind that blow from the tropical high pressure belt to the sub polar low pressure belt in both the hemispheres.
(ii) The velocity of Westerlies become vigorous.
(iii) It is called Roaring forties at 40^o, Furious fifties at 50^o and Screaming sixties at 60^o latitudes.
POLAR EASTERLIES:
(i) Polar easterlies are cold and dry polar winds that blow from the high pressure belt to the sub polar low pressure belt.

(i) Coastal zone is exposed to natural forces including cyclones and Tsunamis.
(ii) Cuddalore and Nagapattinam are situated on the Sea-Shore.
(iii) Cuddalore and Nagapattinam are vulnerable to storms.
(iv) During Summer Bay of Bengal subject to intense heat cause to raise the humid and unstable air masses that produce cyclone.

'Tropos' means 'turn' (or) change. Almost all weather phenomenon takes place in this layer. Hence Troposphere is called as weather making layer.

Absolute humidity: The amount of water vapour in the atmosphere is called Absolute Humidity.
Relative Humidity: The ratio between the amount of water vapour in the atmosphere and the amount of water vapour it can hold is Relative humidity.

(i) Air pressure decreases with altitude.
(ii) The air molecules became scattered and more widely spaced at higher altitudes.
(iii) The air pressure develops by 34 millibars per 300 metres increase in height.

(i) The location of the mountains influence the climate of a place.
(ii) The mountain chains act as natural barrier for the wind.
(iii) Sometimes they prevent the entry of cold winds into the country (or) the escape of monsoon winds, thus having a great influence over the climate.

Auroras are cosmic glowing lights produced by a stream of electrons discharged from the Sun's surface due to magnetic storms that are seen as unique multicolored fireworks hanging in the polar sky during midnight.

In 1772 CE Daniel Rutherford discovered Nitrogen in atmosphere.