Class 7 Social Science Air Exam Notes Set 02

Class 7 Social Science Air Exam Notes Set B. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Ø INTRODUCTION

Class 7 Social Science – Air (Exam Notes, Set B)

Introduction

To understand the weather or climate of a place, we must learn about the basic elements, such as insolation, temperature, atmospheric pressure, humidity, winds and sky conditions. These are known as the elements of weather and climate.

A. Weather

1. Weather is defined as the condition of the atmosphere at a definite place at a fixed time.
2. Thus weather is the hour-to-hour or day-to-day condition of the atmosphere with respect to its basic elements.
3. The elements of weather change constantly.
4. The weather of a place can change at any time, and hence it cannot be generalised.

B. Climate

1. Climate is defined as the average weather conditions over a long period of time.
2. When we speak about the climatic conditions of a certain region, we mean the average atmospheric conditions of that region over a considerable length of time.
3. This involves keen observation and accurate recording and processing of the elements of climate, such as temperature, rainfall, air pressure and clouds.

C. Difference between weather and climate

1. Weather is the condition of the atmosphere in a particular area over a short period of time. It may change from day to day or even within the same day. For example, one day a place may be bright and sunny, whereas the next day it might have rain. Similarly, within the same region, on a particular day one town might have a clear sky whereas another town might have an overcast sky with strong winds. Thus weather is not constant.
2. We arrive at the climatic conditions of a place from the analysis of the daily weather recordings, such as rainfall, temperature, humidity and winds, for a period of several years. Climate thus refers to the average weather conditions of a large area over a long period of time, say 25 or 30 years. The climate of a place is of a permanent nature and does not change like the weather. For example, we say that India has a tropical monsoon climate and Antarctica has a polar climate. Climate also influences our ways of life: the clothes we wear, the houses we live in, the crops we grow and so on.
3. The elements of weather and climate are the same. They are temperature, pressure, winds, humidity, precipitation, sunshine and cloudiness. Among these, temperature is the most important factor determining climate.

Temperature

1. The degree of hotness or coldness of the air is known as temperature.
2. The temperature of the atmosphere changes not only between day and night but also from season to season. Summers are hotter than winters.
3. The sun is continuously radiating heat and light energy in all directions. This is known as solar radiation. The earth receives its heat from solar radiation.
4. The distribution of temperature is not uniform. The temperature of the atmosphere of a place depends upon a number of factors, such as insolation, the latitude of the place, the height of the place above sea level, the distance from the sea, the direction of the prevailing winds, natural vegetation, humidity, and the differential heating and cooling of land and water.

1. Insolation is one of the most important factors that influence the distribution of temperature on the earth. Insolation is defined as the incoming solar radiation that reaches the surface of the earth. The total amount of insolation received at the surface of the earth decreases from the equator towards the poles. The maximum insolation is received in the tropical zone.
2. Thus the temperature also decreases in the same manner, that is, it decreases from the equator towards the poles. The polar regions experience low temperatures, and these regions remain covered with snow almost throughout the year.
3. Any abnormal increase in temperature can be harmful for the survival of life on earth. It will be difficult to grow crops in extra-hot weather.
4. Cities are much warmer than villages. In cities, high-rise pucca concrete buildings trap the warm air, which increases the temperature.

Measurement of temperature

1. The temperature of the air can be measured with the help of a thermometer.
2. Two scales are generally used to measure temperature: the Centigrade (Celsius) scale and the Fahrenheit scale.
3. The standard unit of measuring temperature in our country is the degree Celsius. This scale was invented by Anders Celsius.
4. On the Celsius (Centigrade) scale, the freezing point of water is 0°C and the boiling point of water is 100°C, while on the Fahrenheit scale they are 32°F and 212°F respectively.
5. A thermometer with the Celsius scale has 100 divisions, while one with the Fahrenheit scale has 180 divisions.
6. To measure the highest and the lowest temperature in a day (24 hours), we use a specially developed thermometer known as Six's maximum and minimum thermometer. The readings from this thermometer have to be recorded only once in 24 hours to get the maximum and minimum temperatures.
7. The difference between the maximum temperature and the minimum temperature of a day is known as the daily range of temperature.
8. The difference between the mean temperature of the hottest month and the mean temperature of the coldest month is known as the annual range of temperature.

Atmospheric pressure

1. Air is a mixture of gases and, like other substances, it has weight.
2. The air exerts its weight as pressure on the earth's surface.
3. The weight of air on a unit area of the earth is called the atmospheric pressure or air pressure.
4. Air pressure is defined as the pressure exerted by the weight of air on the earth's surface.
5. The air also exerts pressure on our body, but we do not experience it. The air presses us from all directions, and our body exerts a counter-pressure to neutralise its effect.
6. The weight of the column of air is about one kg on one sq. cm of area.
7. The atmospheric pressure is highest at sea level, and it decreases with an increase in height above sea level and vice versa.
8. In those regions where the temperature is high, the air on the surface of the earth gets heated. It expands, becomes light and rises upward. This creates a low-pressure area, which is normally associated with cloudy skies and wet weather.
9. In those regions where the temperature is low, the air on the surface of the earth gets cooled. It becomes heavy and sinks downward. This creates a high-pressure area, which is normally associated with clear and sunny skies.

Measurement of atmospheric pressure

Atmospheric pressure can be measured with the help of an instrument called a barometer.

1. The mercury barometer has a long narrow tube filled with mercury. The normal atmospheric pressure at sea level is equal to the weight of a 76 cm high mercury column.
2. The aneroid barometer is also used for measuring atmospheric pressure. It does not contain mercury and is thus very handy.
3. For automatic and continuous measurements, barographs are used. The unit of measurement is the millibar (mb). The normal pressure at sea level is exactly 1013 mb.

Distribution of atmospheric pressure

All parts of the earth do not receive the same amount of insolation. Thus the distribution of atmospheric pressure is not uniform on the earth. There is no fixed pattern of distribution of atmospheric pressure on the earth, but there is a pattern of high-pressure belts and low-pressure belts over the earth.

Permanent pressure belts

There are seven distinct pressure belts. These are:

1. the Equatorial Low (1)
2. the Sub-tropical Highs (2)
3. the Sub-polar Lows (2)
4. the Polar Highs (2)

Except for the Equatorial Low, the other pressure belts have matching pairs in the Northern and Southern Hemispheres.

1. The Equatorial Low Pressure Belt

1. It extends from about 10°N to about 10°S latitude.
2. Due to intense heating, the air expands, becomes light and rises as convection currents, causing a low-pressure region known as the doldrums.
3. It enjoys absolutely calm conditions.

2. The Sub-tropical High Pressure Belts

1. These are near 30° north and south of the equator, where the rising equatorial air currents move downward, developing a high-pressure region.
2. This is also known as the horse latitudes. Winds blow from the sub-tropical region towards the equator as the trade winds.
3. Another set of winds blows towards the sub-polar lows. These are the westerlies.

3. The Sub-polar Low Pressure Belts

1. They are located between 60° and 70° in both hemispheres.
2. The winds coming from the sub-tropics and the polar regions converge in this belt and rise upward.
3. Due to the rotation of the earth and the centrifugal force, a low-pressure belt develops.
4. This region is marked by violent storms during the winter season.

4. The Polar High Pressure Belts

1. They are located at the North and South Poles, between 70° and 90° in both hemispheres.
2. The temperature is extremely low in these belts almost throughout the year.
3. The cold descending air develops high pressure in the polar regions.
4. These regions have permanent ice caps.

Winds

The air always moves from the high-pressure areas to the low-pressure areas. This horizontal movement of the air is called wind, while the vertical or nearly vertical movement of air is called an air current. A wind is always named after the direction from which it comes.

The direction and speed of the wind are controlled by the following factors.

1. The pressure gradient: The difference in atmospheric pressure between two places gives the pressure gradient. The greater the difference in pressure between two places, the steeper the pressure gradient and the higher the speed of the wind.
2. The Coriolis force: The axis of the earth is inclined, and the earth rotates around it. The rotation of the earth produces the Coriolis force, which deflects the wind. The winds are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The deflection is minimum at the equator and maximum at the poles.
3. The frictional force: Along the surface of the earth, the winds do not move freely because of the irregularities of the surface. The roughness of the surface causes friction, which reduces the speed of the wind and also changes its direction. The ocean surface is smooth, so the friction is less, and the wind blows smoothly and with higher speed over the oceans.

Types of winds

Winds can be broadly classified into permanent winds, seasonal winds and local winds.

The permanent winds

The winds that blow regularly and constantly in a particular direction throughout the year are the permanent or prevailing winds. The main permanent winds are the trade winds, the westerlies and the polar winds.

1. The trade winds blow from the sub-tropical high-pressure belts towards the equatorial low-pressure belt in both hemispheres. Their direction is from the north-east to the south-west in the Northern Hemisphere and from the south-east to the north-west in the Southern Hemisphere. These winds blow from cooler to warmer areas. Thus they are dry winds.
2. The westerlies blow from the sub-tropical high-pressure belts to the sub-polar low-pressure belts in both hemispheres. Their direction is from the south-west to the north-east in the Northern Hemisphere and from the north-west to the south-east in the Southern Hemisphere. These winds blow from warmer to cooler areas, and thus provide rainfall almost throughout the year. Because of their high velocity, they are also called the Roaring Forties.
3. The polar winds blow from the polar high-pressure belts to the sub-polar low-pressure belts in both hemispheres. These winds blow from the north-east in the Northern Hemisphere and from the south-east in the Southern Hemisphere. They are very cold winds, but when they blow over oceans, they become comparatively warm.

Permanent winds

The seasonal winds

These winds are caused by the unequal heating and cooling of land masses and water bodies. They change their direction with the change of seasons. The main seasonal winds are the monsoon, the land breezes and the sea breezes.

1. The monsoon winds blow from sea to land during the summer season and from land to sea during the winter season. The word monsoon is derived from the Arabic word mausim, meaning season. They are divided into the summer monsoon and the winter monsoon.
   During the summer season, the land is hot and has a low-pressure area, while the adjoining sea is comparatively cool and has a high-pressure area. Thus the winds blow from sea to land and cause heavy rainfall in the months from June to September.
   During the winter season, the conditions are just the reverse. High pressure develops over the landmass, while there is low pressure over the Indian Ocean. Thus the winds blow from land to sea and bring cool and dry weather in the months from November to February.
2. The land and sea breezes are experienced in narrow coastal regions. These winds are caused by the unequal rate of heating of the land and the adjoining sea.
   In the daytime, the land is heated faster than the adjoining sea. This creates a low-pressure zone on the land and a comparatively high-pressure zone on the sea. Thus the winds blow from sea to land and are called sea breezes.
   At night the conditions are reversed. The land is cool and the sea is still warm. Thus the winds blow from land to sea and are called land breezes.

The local winds

These winds are restricted to a small area and blow only during a particular period of the day or year. They can be warm or cold depending on the area from which they blow. The well-known local winds are the Loo, Foehn, Chinook, Mistral, Harmattan and Sirocco.

1. In the plains of northern India, very hot and dry winds blow in the afternoons during the months of May and June. These are called the Loo.
2. The strong, dry and hot wind that develops on the leeward side of the Alps is called the Foehn.
3. The strong dry wind that blows over north-west Africa from the north-east is called the Harmattan.
4. The warm, dry south-westerly wind that blows down the eastern slopes of the Rockies in the USA and Canada is called the Chinook.

Moisture in the atmosphere

Water is found on the surface of the earth in oceans, seas, gulfs, bays, lakes, ponds, rivers and even in the soil. The heat of the sun converts the water into water vapour through the process of evaporation. The most important source of moisture in the atmosphere varies not only from place to place but also from time to time.

1. The process by which water enters the atmosphere as water vapour on heating is known as evaporation.
2. The process by which water vapour is converted into water droplets on cooling is known as condensation.
3. The process by which water droplets fall on the ground in liquid, solid or frozen form is known as precipitation.

Humidity

The amount of water vapour or moisture present in the atmosphere at a particular time and place is known as humidity. Humidity is expressed as absolute humidity and relative humidity.

The actual amount of moisture present in the atmosphere is known as absolute humidity. The ratio between the actual amount of moisture present in the air at a given temperature and the maximum capacity of the air to hold moisture at that temperature is known as relative humidity. Relative humidity is always expressed as a percentage.

Humidity in the atmosphere is measured with the help of a hygrometer, which is also called the wet and dry bulb thermometer.

Condensation

Condensation is the reverse process of evaporation. The process of converting water vapour into water or ice is called condensation. It takes place due to loss of heat. When the water vapour in the atmosphere condenses, it may take the form of dew, frost, fog, mist, clouds and so on. These should not be confused with the forms of precipitation, such as rain, drizzle, snow, sleet and hail.

Clouds are the most important form of condensation. They are formed when condensation of moisture occurs far above the ground. This condensation occurs around tiny solid particles like dust or smoke particles. The tiny droplets do not fall to the ground and remain suspended in the air. The clouds then rise upward. More and more droplets are added, and the clouds slowly grow in size. When the air is fully saturated, these droplets may fall down as raindrops. All clouds do not give precipitation.

Depending on their shape and formation, clouds are of three major types.

1. Cirrus clouds are fleecy, like wool, and are generally at a high altitude.
2. Cumulus clouds have a cauliflower-like shape.
3. Stratus clouds have a layered structure.

Precipitation

Small droplets of water, ice crystals and so on fall to the ground by the process of precipitation. The small drops of water join together to form large drops, which become too heavy to remain suspended in the air. Thus precipitation takes place. The main forms of precipitation are drizzle, rain, snow, sleet and hail.

1. Drizzle: Drizzle consists of raindrops of smaller size and less intensity.
2. Snow: Snow is formed when water droplets rise higher and freeze on account of a fall in temperature.
3. Sleet: Sleet is a mixture of rain and ice.
4. Hail: Vertical air currents may push water droplets higher. They form solid ice balls and fall as hail.
5. Rain is the most important and most common form of precipitation. Rainfall occurs only when the cloud droplets change to raindrops. The average diameter of a raindrop is about 5 mm, and one raindrop has about 5 million cloud droplets.

Depending on the way in which the condensation and cooling of the warm, moist air takes place, rainfall can be of three types: convectional rainfall, orographic rainfall and cyclonic rainfall.

1. Convectional rainfall: The heated air with moisture rises in the form of convection currents, leading to the development of clouds at a height of about 10 km. The ascending hot and humid air current causes condensation of the clouds, resulting in heavy rainfall for a short duration. Such rainfall is also accompanied by thunder and lightning. It occurs mostly in equatorial regions.
2. Orographic rainfall: It is also known as relief rainfall, because it occurs from the cooling of warm, moist air that ascends above a mountain barrier lying in the direction of the prevailing winds. The sudden ascent cools the air, which causes condensation and precipitation. Most of the precipitation in the world is orographic in nature.
3. Cyclonic rainfall: This type of rainfall is associated with tropical and temperate cyclones. When warm and cold air masses meet, the warmer (lighter) air generally climbs above the colder (heavier) air. The boundary between the warm and cold air masses is called a front. The rising air is cooled and causes heavy precipitation. Tropical cyclones are also known as typhoons, hurricanes and tornadoes.

Rainfall is very useful, as it brings fresh water to the earth's surface. This is needed for the survival of human beings, animals and plants. More rainfall can cause floods, and less rainfall can cause drought conditions.

factor of climate.

air that increases the temperature in the cities.

 Ø MEASUREMENT OF TEMPERATURE

 Ø MOISTURE IN THE ATMOSPHERE

 Ø HUMIDITY