Introduction

Weather v. Climate

1. • Weather changes frequently but Climate is the average atmospheric conditions of an area over a considerable time.

1. • The climate of temperate latitudes is far more variable than the tropics.

Points to remember about the Weather effects

1. • Death rates in the tropics are higher than in deserts as germs find it difficult to transmit easily in dry conditions compared to humid conditions.

1. • Rainfall, snow, sleet, and hail are types of precipitation and are measured by a Rain gauge.

1. • Places with the same mean annual rainfall are joined by the lines called “Isohyet”.

1. • Mercury is used in the Barometer as it is the heaviest known liquid. The mercury column at sea level is 30 inches approx. or 1013 milli-bar / 760 mm of Hg [Mercury].

1. • Lines called isobars to join regions of high pressure.

1. • The Temperate latitudes see higher variations in pressure which leads to cyclones and anti-cyclones. As one goes higher in altitude the pressure decreases. The drop is 1 inch for every 900 feet ascent in height.

1. • Airplanes use a modified barometer called an Altimeter.

1. • Temperature decrease is a 1 Fahrenheit drop for 300 feet ascent in altitude.

1. • Humidity is the measure of dampness in the atmosphere. It has two types:

1. • Absolute and Relative humidity. Absolute humidity is the amount of water vapor in grams per cubic meter.

1. • The relative humidity is the amount of water vapor present in the air at a particular temperature compared to the total amount of water vapor, air can hold at that temperature.

Relative humidity = 80%; This means Air holds 4/5th of the water vapor it can hold.

Important instruments related to Weather

1. • Wind director – Wind vane

1. • Wind speed – Anemometer

1. • Relative Humidity – Hygrometer

1. • Places with equal sunshine duration are joined by “Isohels”.

1. • “Isonephs” are lines that join places with an equal degree of cloudiness.

Formation of thunderstorms:

Heavy rainfall with thunder and lightning. They usually last for a short amount of time. Cumulonimbus clouds are formed. This leads to atmospheric instability and convectional rainfall.

1.  Air motion is upwards.
2.  The accumulation of water increases and water droplets descend as the surrounding is cooled. This leads to more downward drafts and spreads throughout the entire cumulonimbus.

 Types of thunderstorms:

1.  Thermal thunderstorms – localized, intense heating of the ground.
2. Orographic thunderstorms – obstruction due to mountains leads to it as forcible vertical movement is needed.
3. Cold front thunderstorms – hot and cold air collide to get this.

Tornadoes:

Occurs mostly in temperate regions where cold and warm air meets. Southeast of USA is called “Tornado Alley”.

Polar stratospheric clouds:

They are formed in winter and contain nitric acid, sulfuric acid, and water. Nitric acid reacts with CFC to create chlorine. This creates a chlorine concentration in winter. This chlorine reacts with oxygen molecules of ozone and destroys the ozone layer.

Ozone depletion is more at the southern pole and is called ozone hole – “Thinning of the ozone layer in stratosphere below threshold” [not an actual hole].

El – Nino and La – Nina

There are oscillations in the pressure gradient and air circulation every 2-3 years in the South Pacific Ocean. This is called El Nino Southern Oscillation.

The term “El Niño” refers to “the boy” / “Child christ”, so named because of the pool of warm water in the annual weak warm ocean current that ran southwards along the coast of Peru and Ecuador around Christmas. “La Niña”,  the ‘opposite’ of El Niño, translates to “the girl”.

During a normal year the following events are seen:

•  The south equatorial current takes warm water to Australia and creates a low-pressure region. This heats the air and it rises upwards. The resultant atmospheric instability leads to rainfall in Australia.
•  The air that rises up cools and diverges. It comes to the South American coast. Due to this high pressure is created and has a desiccating effect on the Atacama Desert.
•  This is High pressure and low-pressure formation is called Walker Cells.
• The south equatorial current takes warm water to the west from the east. This causes up-welling at Peru’s coast and rich fisheries in Peru.

During El Nino year:

1. The south equatorial current weakens and water piling is reduced to Australia.
2. The abnormal cooling of the Australian region and warming of the Peruvian coast is seen. The low-pressure condition over Peru and high pressure over Australia cause drought. Reversing of walker cells is observed.
3. This causes rainfall in the Atacama Desert. The up-welling on the Peru coast is affected and the fishery business goes down.
4. El Niño causes droughts in India and Indonesia as well.

During La Nina year:

1. Walker cells are intensified. This leads to higher rainfall in Australia and higher up-welling in Peru due to which fishes are more.
2. India too receives good rainfall.

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