Detailed explanations in West Bengal Board Class 6 Geography Book Solutions Chapter 7 Weather and Climate offer valuable context and analysis.
WBBSE Class 6 Geography Chapter 7 Question Answer – Weather and Climate
Objective Type Questions [1 mark each]
Question 1.
What is the normal lapse rate of temperature?
Answer:
1°C for 165 metre.
Question 2.
Which rays are hotter-vertical or slanting ?
Answer:
Vertical
Question 3.
Name the condition when the temperature increases with height.
Answer:
Inversion of temperature
Question 4.
When is daily maximum temperature recorded?
Answer:
In the afternoon.
Question 5.
In which areas is daily range of temperature low?
Answer:
- coastal areas.
- clouded areas.
Question 6.
In which areas is daily range of temperature high?
Answer:
- In interiors of continents
- In deserts
Question 7.
What are isobars?
Answer:
Lines of equal pressure.
Question 8.
What change takes place in the process of evaporation?
Answer:
Water is changed to water vapour.
Question 9.
What is condensation?
Answer:
Cooling of saturated air.
Question 10.
At what temperature, does condensation take place ?
Answer:
At dew point.
Question 11.
What is the percentage of humidity at dew point?
Answer:
100 per cent.
Question 12.
State the different forms of condensation.
Answer:
Frost, snow, dew, fog, mist, and clouds.
Question 13.
In which unit, is relative humidity measured?
Answer:
In percentage.
Question 14.
What is the relation between relative humidity and rainfall?
Answer:
When relative humidity is 100 per cent, rainfall takes place.
Question 15.
Name the different forms of precipitation.
Answer:
Rain, snow, hailstone.
Question 16.
What is the general cause of rainfall?
Answer:
Cooling of saturated air.
Question 17.
Which instrument is used to measure mean daily temperature?
Answer:
Maximum and Minimum Thermometer.
Question 18.
Who invented Maximum and Minimum Thermometer?
Answer:
J.Six.
Question 19.
Which instrument is used to measure atmospheric pressure ?
Answer:
Barometer.
Question 20.
Name the instrument used to measure relative humidity.
Answer:
Wet and Dry bulb thermometer.
Define the following briefly : 2 marks each
Question 1.
Insolation.
Answer:
Insolation means the incoming solar radiation on the earth’s surface.
Question 2.
Conduction.
Answer:
It is the process of transfer of heat through contact.
Question 3.
Inversion of Temperature. ar
Answer:
It is the condition when the temperature increases with height.
Question 4.
Isotherms.
Answer:
Isotherms are the lines joining the places of equal temperature reduced to sea level.
Question 5.
Atmospheric Pressure.
Answer:
It is the weight of a column of air extending from the ground to the uppermost layer of the atmosphere.
Question 6.
Millibar.
Answer:
One millibar is equal to force of one gram per sQuestion cm or a force euqal to 1000 dyne per sq.cm.
Question 7.
Pressure gradient.
Answer:
It is the rate of which the horizontal pressure changes.
Question 8.
Humidity.
Answer:
Humidity means the amount of water vapour in air.
Question 9.
Absolute humidity.
Answer:
Absolute humidity is the weight of actual water vapour present per unit volume of air.
Question 10.
Relative humidity.
Answer:
Relative humidity is the ratio between the actual humidity of air and the maximum vapour capacity at a given temperature.
Comparison type questions: 3 marks each
Question 1.
Weather and Climate:
Answer:
Weather | Climate |
1. Weather stands for actual atmospheric conditions for a short period. | 1. Climate is composite pictures of atmospheric conditions for a longer period (30 years). |
2. The weather changes from day-today. | 2. The climate is everlasting and does not change often. |
3. Weather is actually present at a place. It is what you get. | 3. Climate is what you expect. |
4. Weather refers to a particular place. | 4. Climate refers to a particular area. |
5. Weather is uncertain and we can only forecast it for a short time ahead. | 5. Climate has a high degree of accuracy and it is similar year after year. |
Question 2.
Condensation and Evaporation
Answer:
Condensation | Evaporation |
1. Condensation is the process by which water vapour is changed to water. | 1. Evaporation is the process by which water is changed to water vapour. |
2. Cooling of air leads to condensation. | 2. High temperatures favour evaporation. |
3. This change takes place from gaseous to liquid condition. | 3. This change takes place from liquid to gaseous condition. |
Question 3.
Relative Humidity and Absolute Humidity
Answer:
Relative Humidity | Absolute Humidity |
1. It is a ratio between the actual amount of water vapour present in an air at the temperature and the maximum amount of water vapour that the air can hold at the temperature. | 1. The total amount of water vapour present in an air at a particular temperature is absolute humidity. |
2. It is a ratio between the absolute humidity and vapour capacity of the air. | 2. It is defined as weight of water vapour per unit volume of air. |
3. It is maximum in equatorial region, but is lowest in hot deserts. | 3. It is maximum over oceans and lowest in high pressure areas. |
Question 4.
Dew and Dew Point
Answer:
Dew | Dew Point |
1. The droplets of water deposited on rocks, grass, leaves and the ground are termed as dew. | 1. The temperature at which an air becomes saturated is called dew point. |
2. Air, near the ground is condensed to form drops. | 2. Condensation of air begins at this temperature. |
3. Frozen drops of dew are known as frost. | 3. The relative humidity at dew point is 100 % |
Question 5.
Fog and Mist
Answer:
Fog | Mist |
1. Fog is condensed water vapour hanging in the air. | 1. Thin fog is called mist. |
2. The visibility is less than 200 metre. | 2. Visibility is more than 200 metre. |
3. Fog is formed due to cooling of air on dust particles. | 3. Mist is formed due to cooling of wet air by radiation. |
Question 6.
Precipitation and Rainfall
Answer:
Precipitation | Rainfall |
1. It is the collective name given to different forms of release of moisture afer condensation. | 1. It is a type of precipitation when moisture falls on the earth in the form of droplets of water. |
2. Precipitation has two forms : (a) liquid (b) solid. | 2. When cloud particles grow heavy, they fall as rain drops (in liquid form). |
3. Rainfall, snow, hail are the common forms of precipitation. | 3. Three types of rainfall are convectional relief and frontal. |
Answer the following questions briefly : 3 marks each
Question 1.
Define Temperature of a place.
Answer:
The Temperature means degree of hotness of a body. The temperature of a place means the temperature of the air at that place. It is measured in shade of the air upto the height of at least one metre.
Question 2.
What is Daily Mean Temperature?
Answer:
Daily Mean Temperature : The average of the maximum temperature and minimum temperature recorded during a day (24 hours) is known as daily mean temperature.
Daily Mean Temperature :
= \(\frac{\text { Max.Temp. }+ \text { Min.Temp. }}{2}\)
For example, if a place has the maximum temperature of 39° C and minimum temperature of 27° C for a particular day, the daily mean temperature
= \(\frac{39^{\circ}+27^{\circ}}{2}=\frac{66^{\circ}}{2}=33^{\circ} \mathrm{C} \text {. }\)
Question 3.
What do you mean by Monthly Mean Temperature?
Answer:
Monthly mean temperature
: The average of the daily mean temperature of all days of a month is known as monthly mean temperature.
Monthly Mean Temperature of July
= \(\frac{\text { Daily Mean Temp. of } 1+2+3 \ldots 31 \text { days }}{31}\)
Question 4.
Name the factors which affect the range of temperature of a place.
Answer:
(a) Latitude
(b) Distance from the sea
(c) Ocean currents
(d) Prevailing winds
(e) Soils
(f) Cloudiness.
Question 5.
What is inversion of Temperature?
Answer:
Inversion of Temperature : Under normal conditions the temperature decreases with increasing height at the rate of 1° C per 165 metre. This is known as ‘Normal Lapse Rate’. But sometimes, the conditions are reverse so that the temperature increases with height locally and temporarily. This condition in which the colder air nearer the earth and the warmer air above is called inversion of temperature.
Question 6.
What is Absolute Humidity?
Answer:
Absolute Humidity :
Definition: Absolute humidity refers to the total amount of water vapour present in a given volume of air. It is the weight of actual amount of water vapour present in a unit volume of air. According to D.S. Lal, The weight of water vapour per volume of air is referred to as the absolute humidity. Unit : The unit used to express absolute humidity is gram per cubic centimetre. For example, if the absolute humidity is 10 gm / cu cm. it means that in a sample of one cubic centimetre of air. the amount of water vapour present is 10 gm.
Question 7.
What is Relative Humidity?
Answer:
Relative Humidity : Definition: The ratio of the amount of water vapour present in the air mass at a given temperature of a place relative to the maximum quantity that could be present is called Relative Humidity. So Relative humidity is a ratio between the actual amount of water vapour present in the air and the maximum amount of water the air can hold at that temperature. Unit : Relative humidity is a ratio, but it is always expressed as a percentage.
Formula : Relative Humidity
Question 8.
What is Specific Humidity?
Answer:
Specific Humidity is defined as the mass of water vapour in gram contained in a kilogram of air and it represents the actual quantity of moisture in a definite air. According to H.J. Critchfield, Specific humidity is the ratio of the mass of water vapour actually in the air to a unit mass of air including the water vapour. Specific humidity is expressed as a gram/kilogram.
Question 9.
What do you mean by precipitation?
Answer:
Precipitation : Condensation of water vapour in the air in the form of water droplets and ice and their falling on the ground is collectively called Precipitation. It is a collective name given to various forms of moisture falling on the earth from condensation of water vapour in the atmosphere. In metereology, precipitation is defined as fall of water through the atmosphere to the earth’s surface in the forms of rainfall including drizzle, snowfall, sleet or hail.
Question 10.
Define rainfall ?
Answer:
Rainfall : Definition : When moisture-laden air rises up, condensation takes place and clouds are formed. If the clouds are further cooled, more condensation occurs and drops of water that are formed big in size. These big drops are too heavy to remain in the air and fall down towards the ground in liquid form. The falling of these drops of water from the clouds is known as rain. Rain drops are 0.5 mm to 6.35 mm in diameter.
Identify the correct answers :
Question 1.
About 35% of insolation returned to space is called
(a) Effective solar radiation
(b) Earth’s Albedo
(c) Heat balance
(d) Condensation
Answer:
(b) Earth’s Albedo
Question 2.
The lapse rate of temperature is
(a) 4.6° C
(b) 5.6° C
(c) 6.4° C
(d) 7.5° C
Answer:
(c) 6.4° C
Question 3.
The annual range of temperature is minimum in
(a) Equatorial region
(b) Tropical region
(c) Polar region
(d) Desert region
Answer:
(a) Equatorial region
Question 4.
Standard sea level air pressure is called
(a) 1013.2 mb
(b) 1014.2 mb
(c) 1015.0 mb
(d) 1016.0 mb
Answer:
(a) 1013.2 mb
Question 5.
An imaginary line on map joining places with same air pressure is called
(a) Isotherm
(b) Isohyte
(c) Isobar
(d) Isoplate
Answer:
(b) Isohyte
Question 6.
Closely located isobars denote
(a) High pressure
(b) Low pressure
(c) Medium pressure
(d) No pressure
Answer:
(b) Low pressure
Question 7.
The actual mass of water vapour in the air is called
(a) Relative humidity
(b) Absolute humidity
(c) Specific humidity
(d) Cloud
Answer:
(a) Relative humidity
Question 8.
The instrument used to measure humidity is
(a) Barometer
(b) Hygrometer
(c) Anemometer
(d) Thermometer
Answer:
(b) Hygrometer
Question 9.
The ratio of weight (in gram) of water vapour to the total weight (in kilogram) of moist air is expressed as
(a) Specific humidity
(b) Relative humidity
(c) Absolute humidity
(d) None of these
Answer:
(a) Specific humidity
Question 10.
The actual mass of water vapour per unit volume of air is known as
(a) Relative humidity
(b) Absolute humidity
(c) Specific humidity
(d) None of these
Answer:
(a) Relative humidity
Question 11.
Wind speed is measured by instrument
(a) Sextant
(b) Rain gauge
(c) Hygrometer
(d) Anemometer
Answer:
(d) Anemometer
Question 12.
Energy is released when water vapour
(a) precipitates
(b) condenses
(c) evaporates
(d) rains
Answer:
(b) condenses
Question 13.
The temperature varies primarily according to latitude mainly because
(a) the earth is a large planet
(b) the earth rotates on its axis
(c) the earth is spherical in shape
(d) the earth is far away from the sun
Answer:
(c) the earth is spherical in shape
Question 14.
Normal lapse rate of the atmosphere is 1° for every
(a) 132
(b) 165
(c) 185
(d) 225
Answer:
(b) 165
Fill in the blanks with correct words : 1 mark each
1. _____ is the weight of air on 1 sQuestion cm. surface at sea level.
Answer:
1.3 kg
2. _____ millibars is the normal pressure of air.
Answer:
1013.2
3. _____ are lines of equal pressure.
Answer:
Isobars
4. Speed of sunrays is _____ km per second.
Answer:
3 lakh
5. The incoming solar energy intercepted by the earth is defined as _____
Answer:
Insolation
6. The _____ gas absorbs some of the ultra-violet radiation of the sun.
Answer:
Ozone
7. Land gets heated through the process of _____.
Answer:
Conduction
8. Water surfaces get heated by the process of _____.
Answer:
Convection
9. The amount of rainfall depends on the _____ content of the air.
Answer:
moisture
10. Snow storms are called _____.
Answer:
blizzard
11. The _____ scale is used to show the strength of the wind.
Answer:
Beaufort.
If the sentence is true, write ‘T’ and if false write ‘F’ against the following : 1 mark each
1. The weather keeps changing from day to day and can even change within a few hours.
Answer:
True
2. The climate of a region is the average weather condition over a span of 30-40 years.
Answer:
True
3. 51% of the heat of sun that reaches the earth’s surface is almost entirely reflected back at night. This is known as Terrestrial Radiation.
Answer:
True
4. The difference between the highest and lowest temperature in a day at a place
is known as the annual range of temperature.
Answer:
False
5.Water starts boiling at 32° F or 0° C.
Answer:
False
6. Cloudy nights are usually hot and sultry.
Answer:
True
7. Temperature increases with increasing altitude.
Answer:
False
8.Temperature increases gradually from the Equator to the poles.
Answer:
False
9. The higher we go the heavier the air becomes.
Answer:
False
10. When the sky has 25 % of cloud cover we say it is almost cloudy. cover we say it is almost cloudy.
Answer:
False
Weather and Climate Class 6 WBBSE Notes
Weather : Weather is the sum total of atmospheric conditions at any place, at any specific time (a short period). The weather changes from day-to-day and hour-to-hour. The Indian Meteorological Department publishes the daily Indian weather maps.
Climate : Climate is the description of atmospheric conditions of an area over a long period (say 35 years). Thus climate is the average weathering of a place over a long period of time.
Elements of weather and climate: The elements of weather and climate are-
- Temperature
- latitude
- Altitude
- Distribution of land and water
- Pressure distribution
- Prevailing winds
- Ocean currents
- Mountain barriers.
Introduction :
Every now and then we talk about weather. When we discuss about the weather of a place, we talk about how hot or cold the day is, whether it is rainy, cloudy or sunny and the like. In this chapter we shall learn about weather and climate. Weather is the name given to the atmospheric conditions formed by changing atmospheric elements, like air temperature, air pressure, air humidity, rainfall/snowfall, winds, sunshine, cloudiness etc. of a particular place or a particular day or a part of a day. Weather is essentially a day-to-day or even an hour-to-hour phenomenon. Climate is a description of many different kinds of weather that are generally found at a given place over a long period of time.
Definition of weather : Weather refers to the sum of the atmospheric conditions at any place or in an area at a particular time. Weather is the description of the atmospheric conditions of a particular place at a particular time or a short period of time. Weather is essentially a day-to-day or even an hour-to-hour phenomenon.
Definition of climate: The average weather conditions and variations in these conditions over a long period of time such as a season or a year is termed as climate. The climatic data is based on calculated averages of data recorded over a period of 35 years.
Elements of Weather and Climate –
Difference between Weather and Climate –
Weather | Climate |
1. It describes the atmospheric conditions for a short period of time. | 1. It describes the atmospheric conditions over a long period of time. |
2. Weather is essentially a day-today or even an hour-to-hour phenomenon. | 2. Climate is generally a variation over a long period of time such as a season or a year. |
3. The description covers a smaller area. | 3. The description covers a larger area. |
4. Weather data is obtained through direct observation or by the use of instruments. | 4. Climate data is based on calculated averages of data recorded over a period of 30-35 years. |
5. It refers to a particular place. | 5. It refers to a particular region. |
Major Elements of Weather and Climate
What do you mean by the term of ‘Elements of Weather and Climate’?
The atmospheric conditions, which make up the sum total of climate, are called elements of weather and climate. These elements vary from place to place and from time to time. All these elements comprise the following :
- Air Temperature
- Air Pressure and Wind
- Cloudiness and Rainfall.
Air Temperature : Temperature is the amount of heat that a place receives and it is known by measuring the temperature of air of a place. The air and minute dust particles of air become hot by the rays of the sun. The warmth of the air is known as Air Temperature. The temperature of a particular place denotes the degree of hotness of air at that place. Thermometer is the instrument that measures air temperature. The sun is 150 million km away from the earth.
The sun is 1,00,000 times bigger than the earth. The surface temperature is estimated at 6000° C. The sun’s core has an estimated temperature of 14,000,000° C. The sun radiates energy in the form of electro-magnetic waves of short wavelength.
These waves travel in all directions from the sun. The earth, owing to its small size and great distance from the sun, intercepts only a small fraction of the energy \(\frac{1}{2000,000,000}\) radiated by the sun. The radiant energy received by the earth is called the incoming solar radiation or insolation. The incoming solar energy intercepted by the earth is defined is insolation.
According to H.C. Critchfield, “Radiant energy from the sun that strikes the earth is called isolation.” If we consider 100 % of insolation entering the earth’s atmosphere, nearly 35 % of the total insolation scattered and reflected by clouds (26%), dust particles molecules (7 %) and surface of the earth (2 %) (26 % + 7 % + 2 % = 35 %).
This loss of insolation is called Albedo (35%). The 35 % insolation is therefore, not used for heating the atmosphere. Remaining (100-35) % = 65 % of the incoming solar radiation takes part in warming up the atmosphere directly and indirectly and is called an Effective Solar Radiation. The atmosphere directly absorbs only 14 % of the incoming insolation. The rest (65-14) % or 51 % of incoming solar radiation is transmitted to the surface of the earth. This incoming solar radiation (insolation) first heats the solid and liquid surfaces and then the earth radiates back to the air in contact with it and atmosphere is gradually heated. These are called indirectly Effective Solar Radiation.
Processes of Heating of the Atmosphere : The air is heated by the following main processes :
Radiation : It is a process by which heat waves are sent out by the earth’s surface. At first, the surface of the earth is heated by the sun. The heat of the earth’s surface is then radiated to the lower atmosphere. The air in the lower layer of the atmosphere gets heated.
Conduction: A particle transmits heat to another particle in contact with it. This process is called conduction. The lower layers of air are heated by conduction of heat from the earth’s surface. The upper layers are heated by lower layers through conduction.
Convection : The air particles being heated by their contact with the earth’s surface become lighter and rise up. The comparatively cool air particles in the upper layers are heavier and come down slowly. In this way air currents are formed and these are known as convection currents.
Difference in Temperature between day and night :
Maximum Temperature: The highest temperature of the day is called Daily Maximum Temperature. The sun is the highest at noon but the highest temperature does not occur at 12.00 noon because the atmosphere receives heat from the earth’s surface slowly. The maximum temperature is attained by 2.00 p.m. after the noon. If we note the highest temperature of a month, it is called Monthly Maximum Temperature.
Minimum Temperature: The lowest temperature of a day is called the Daily Minimum Temperature. The Daily Minimum Temperature at a place does not occur at 12:00 midnight but at about 4:00 a.m. in the morning because radiation of heat continues up to the sunrise.
Daily or Diurnal Range of Temperature: The difference between the maximum and minimum temperatures recorded during a day is called Diurnal Range of Temperature. Daily range of temperature is minimum in equatorial region and maximum in tropical desert region.
Annual Range of Temperature: The difference between the mean temperature of the hottest month and that of the coolest month in a year is called the Annual Range of Temperature.
When the diurnal ‘range of temperature’ is high, days are usually very hot and nights are cold. High annual range of temperature makes the summers scorching hot and the winters very cold.
Measurement of Temperature:
Temperature is measured in degree celsius (centigrade scale) i.e. °C or Fahrenheit scale i.e. °F. Temperature of melting point of ice is taken as 0° C or 32° in Fahrenheit scale and the temperature at which water boils is taken as 100° C and there are 100 divisions. Following formula is used for inter conversion of temperature between Centigrade scale and Fahrenheit scale. °F = (°C × \(\frac{9}{5}\)) + 32.
Conversions : Celsius (°C)
= Fahrenheit -32° × \(\frac{5}{9}\) and Fahrenheit (°F) = (Celsius .× \(\frac{9}{5}\))+32°.
The temperature of air is always measured in the shade because in the open direct radiation from the sun will affect the reading. Low and moderate temperates usually up to 500° C is called thermometry. Temperatures above this range are called Pyrometry. Water freezes to ice at 32° Farhenheit (F) or 0° Centigrades (C). Water starts boiling at 212° F or 100° C.
Instrument for measuring temperature: The degree of heat of any body is measured by its temperature. A clinical thermometer is used to measure the temperature of our body. We can also use a thermometer for measuring atmospheric temperature. We can measure the highest and lowest temperatures in a day by a ‘Six’s Thermometer’.
Six’s Maximum and Minimum Thermometer : This thermometer is designed so as to record both the maximum (highest) and minimum (lowest) temperature experienced during a day. The instrument consists of a U-shaped glass tube with a long cylindrical bulb bent downwards at one end and a spherical bulb at the other end.
Factors affecting air temperature :
(a) Latitude : Due to the spherical shape of the earth the angle of incidence of sunrays is not the same everywhere on the surface of the earth. Direct rays of the sun produce more heat than the slanting rays.
The region near the Equator gets more direct rays of sun and are therefore, hot throughout the year. On the other hand, outside the tropics, the rays are spread over a larger area and pass through a longer distance. Therefore temperature gradually decreases from the Equator to the Poles.
(b) Altitude : The density of air decreases as you go higher and there are less impurities away from the surface of the Earth, the temperature decreases with the increase in height above sea level. Up to a certain height the temperature decreases at the rate of 1° C for 165 m height above the sea level. Or we may say, temperature decreases at the rate of 6.5° C per kilometer altitude. It is known as the normal lapse rate of temperature.
(c) Distance from the sea: Due to the phenomena of land and sea breezes the temperatures of the coastal margins is comparatively cooler than that of a place situated far away from the sea.
Inversion of Temperature :
Under normal condition, temperature decreases at the rate of 6.5°C per 1000 metre. But sometimes, the conditions show the reverse of the normal situation. So that temperature increase with height, locality and temporarily. This condition in which the cooler air is nearer on the earth’s surface and the warmer air above on it is called inversion of temperature. It generally occurs in mountain valleys, especially in winter on calm clear nights.
(d) Length of Day and Night: Summer days are longer than winter days. Longer periods of daylight heat the earth more. So summer temperature is higher than winter temperature.
(e) Cloud : Clouds check solar radiation. Absence of clouds make a day very hot and a night very cold.
Temperature Belts of the World –
We can divide the globe into 5 temperature belts on the basis of variation in temperature from the equator towards the poles:
- The Torrid belt
- The North Temperate belt
- The South Temperate belt
- The North Frigid belt
- The South Frigid belt.
(i) The Torrid belt : This belt extends between the Tropic of Cancer (23 \(\frac{1}{2}\)° N) and the Tropic Capricorn (23 \(\frac{1}{2}\)° S). This is the hottest part of the earth with mean temperature above 20° C. The sun shines over this belt more or less vertically throughout the year.
(ii) & (iii) The North and South Temperate belts : Temperate belt extends between 23 \(\frac{1}{2}\)° and 66 \(\frac{1}{2}\)° latitudes in both the hemispheres. This is the belt of moderate temperature (between 10° to 20° C ).
The summers are not very hot, and the winters are not very cold. The temperate zones of both the hemispheres can be divided into two parts: (a) warm temperate-zone between 23 \(\frac{1}{2}\)° and 50° latitudes and (b) cool temperate zone between 50° and 66 \(\frac{1}{2}\)° latitudes.
(iv) & (v) North and South Frigid belts : Frigid zones extends between 66 \frac{1{2 and the Pole in both the hemispheres. Temperature is constantly low throughout the year because of slanting rays of the sun. These are the coldest parts of the earth with mean temperature below 10° C. The land remains frozen almost throughout the year.
Isotherms : Definition : Isotherms are imaginary lines drawn on a map joining places of the same average temperature for a specified period, supposing them to be at sea level (reduced to sea level).
Characteristics : (i) Isotherms bend to east-west, generally following the latitudes, (ii) The July isotherms bend towards equator over the oceans and polewards on land, (iii) The January isotherms bend just reverse of it, (iv) Isotherms take sudden bends where land-water contrasts are maximum, (v) The spacing of isotherms indicates latitudinal thermal gradient i.e. steepness or slow gradual nature of temperature change.
Humidity : The amount of water vapour present in the air varies from place to place and from time to time at the same place. The amount of water vapour present in the atmosphere is referred to as humidity. Humidity of the atmosphere is high when it has large quantities of water vapour. The water content of the atmosphere is indicated by Absolute Humidity. Relative humidity is a more useful measure of humidity of the atmosphere than absolute humidity.
The water vapour holding capacity of the air is related to its temperature. Hot air can hold more water vapour than cold air. Relative humidity is the ratio between the quantity of water vapour present in the atmosphere at a given temperature to the maximum quantity of water vapour that the atmosphere can hold at that temperature. Relative humidity is expressed in percentage. Relative humidity is high in the equatorial regions and is the minimum in the tropical deserts.
When the relative humidity of air is 100 %, the air is said to be saturated. The temperature at which air gets saturated is called the dew point. Humidity of air is measured by hygrometer.
Saturated air : Air is said to be saturated when it contains water vapour in its full capacity. The temperature at which the air is saturated and below which condensation (the transformation of water vapour into water or ice) begins is called the dew point.
Determination of humidity : Humidity is measured by an instrument named Hygrometer which comprises wet and dry bulb thermometers [Fig-7.8]. Atmospheric humidity is expressed in a number of ways – absolute humidity, relative humidity and specific humidity.
The actual amount of water vapour present in the air at a particular temperature is called the absolute humidity. It is expressed in gram per cubic centimetre of air. Relative humidity is defined as the ratio of the amount of water vapour in the air to the amount the air can hold at that temperature. Relative humidity is generally expressed in percentage. For example, if the humidity capacity and absolute humidity of air having temperature of 20° C are 8 grams and 4 grams per cubic centimeter respectively, the relative humidity will be as follows :
Specific humidity is defined as the mass of water vapour in grams contained in a kilogram of air and it represents the actual quantity of moisture in a definite air.
Role of humidity in the atmosphere :
1. The atmospheric humidity is of vital climatic importance because different forms of precipitation, storms and turbulence etc. depend on humidity.
2. Human comfort depends considerably on the humidity of the atmosphere especially in hot countries. High relative humidity means discomfort and low humidity causes skin to become rough and cracked.
3. Relative humidity determines the rate and amount of evaporation. So it is an important climatic factor.
4. Water vapour absorbs terrestrial radiation and thus helps in heating the earth’s surface and the lower atmosphere.
5. Through the condensation of water vapour, huge amount of energy is released into the atmosphere in the form of latent heat of condensation, driving force for most of the storms.
Air Pressure and Wind : The atmosphere surrounding the earth exerts pressure on the earth’s surface owing to its weight. We do not feel the effect of this pressure because we are accustomed to it from our birth.
Definition of air pressure : Atmospheric pressure is the pressure at any point on the surface of the earth due to the weight of the column of air above that point. Thus air pressure, considering total weight of various layers of air in atmosphere, is maximum at sea level or on the surface of the earth.
Importance of air pressure : Air pressure is important for the following reasons :
- It creates winds
- It influences weather
- Changes in air pressure give important clues in forecasting weather.
Units used to express atmospheric pressure:
Atmospheric pressure is not the same at all places at sea level. Pressure varies from place to place according to changes in temperature. At sea level, when the temperature increases, the atmospheric pressure decreases. The mercury barometer is the standard instrument for measuring atmospheric pressure. Pressure is read in centimetres or inches of mercury, a true measure of the height of the mercury column. Standard sea level pressure is 76 cm or 29.92 inches on this scale. Atmospheric pressure is measured with an instrument called ‘Barometer’.
Another pressure unit is used by meteorologists in drawing weather charts. At the earth’s surface the average air pressure is 1.03 kg. per sq. cm. This pressure is expressed in millimetres (mm) of mercury or millibar’si’ (mb). One bar is divided into 1000 milibars. Millibars are now known as hectopascals. One atmospheric pressure (76 cm of mercury) is equal to 1013 m b. The term ‘high pressuri denotes the state of the atmosphere when the barometer reads about 76 cm(1013 mb) or more. The term ‘low pressure’ denotes the state of the atmosphere when the barometer reads about 74 cm(986 mb) or less. At sea level, the barometer rarely falls below 71 cm(946 mb) or rises above 79 cm(1050 mb).
Isobars: The pressure of air is shown on weather maps by means of lines known as isobars, meaning ‘equal weight’. An isobar is an imaginary line drawn on a map (or a weather chart) joining all places with equal atmospheric pressure.
Factor affecting atmospheric pressure
The atmospheric pressure varies from place to place, region to region and from time to time due to the following factors :
i. Temperature : Air pressure has an inverse relation with temperature. The higher the temperature, the lower is the pressure. As air is heated, it expands and rises upwards. It becomes less dense and exerts less pressure. Conversely, cold air contracts; it becomes heavier and descends. This descending cold air exerts high pressure. Higher temperature is, therefore, associated with low pressure, an upward current of air and inflowing winds. On the other hand lower temperature is associated with high pressure, a downward current of air and outflowing winds.
ii. Altitude : Altitude variation causes variations in air pressure. The lower layers of air are denser than the upper layers. Moreover, the lower layers are compressed under the weight of the upper layers of air and that is why the lower layer exerts high pressure. As we go higher up, the atmospheric pressure decreases. Air pressure decreases at the rate of 10 mm per 110 metres rise in altitude.
iii. Water vapour : Air containing water vapour is lighter than dry air. The more the water vapour the lighter the air is. The humid air contains comparatively less nitrogen and oxygen but the dry air contains more of those. This makes the humid air lighter than the dry air. For this reason land winds are heavier than sea winds which are laden with water vapour.
iv. Rotation of the earth : The rotation of the earth and the centrifugal force resulting in swing of air in different areas, exerts some influence on air pressure. The sub-polar low pressure belts between 60° and 65° north and south are due to the rotation of the earth.
v. Distribution of land and water : Pressure conditions on land and water on the same latitude are not the same. During summer, the land gets heated more than the water, thus a low pressure region is created on the land compared to water. The opposite condition occurs during winter.
Mechanism and Types of Winds –
Definition : A current of air moving with any speed in horizontal direction is called wind while it is moving upwards and downwards, it is called air current. So air that moves parallel to the surface of the earth is known as wind and vertical movement of air is called air current. Wind is intimately related to pressure.
Wind Direction : A wind is named according to their direction from which it blows. For example, an easterly wind is one which comes from the east, westerly wind comes from the west and a north wind comes from the north.
Wind Speed : The difference of pressure between any two places is called pressure gradient or barometric slope. There is close relationship between pressure gradient and air circulation. The wind blows from areas of high pressure to areas of low pressure. It may be mentioned that the direction of pressure gradient is always from high pressure to low pressure. So air movement follows the barometric slope. When the barometer slope is steep, wind blows rapidly; when it is weak, wind becomes weak.
Measurement of Wind Direction : Wind direction is easily determined by an instrument named windvane or weather cock.
Measurement of Wind Speed : To measure wind speed, an instrument called the anemometer is used.
Unity of Wind Speed : The unit used to measure the wind speed or velocity is Knot (metre/mile per hour ; 1 Knot = 1,877 km / hours).
Beaufort Scale : Before adoptation of the instrument anemometer in early 1950s, Beaufort Scale was widely used for the measurement of the velocity of wind. This scale of wind strength was devised by Sir Francis Beaufort of the British Navy in 1805 to help sailors at sea. It was originally based on the effect of wind speed on a sailing ship. The scale is still in use today. There are 13 levels of wind strength ( 0 to 12) on this scale, ranging from dead calm to hurricane.
Clouds and Rainfall –
Definition of clouds : Clouds are formed when condensation occurs at altitudes far above the ground. Clouds consist of million of tiny water droplets and ice particles. Clouds are formed owing to the ascent of warm moist air. The cloud droplets are so tiny that they remain in suspension and do not drop on the ground. Clouds move about in response to winds and air currents and take on different shapes and sizes.
The measurable proportion of cloud in the sky is called ‘cloud cover’ or ‘cloudiness’.
When the air near the earth rises into the sky, it cools and some of its water vapour condenses into tiny droplets of water. This happens when the temperature of air is below the dew-point.
Condensation : Condensation is the process by which water vapour is converted into water. Condensation can only occur when the air becomes saturated. Air becomes saturated in two ways-
(i) By cooling of air : When the temperature of air decreases, its capacity to hold water decreases. The air becomes saturated with the same quantity of water vapour by a fall in temperature.
(ii) By the addition of more water vapour : When more evaporation occurs, the air becomes saturated. In nature, saturation of air in this manner is not a very common occurrence.
Precipitation: The process through which water from the atmosphere falls down on the earth is called precipitation.
Rain : Precipitation that reaches the ground in the form liquid is called rain. Rain drops are 0.5 mm to 6.35 mm in diameter.
Temperature and rainfall are the two most important elements of climate. Climate = temperature + rainfall. Precipitation which takes
place where the atmospheric temperature is below 0° C is called snowfall. The lightest form of precipitation is drizzle, which occurs in fine drops falling close together. Snow is precipitation composed of ice crystals. Sleet is a mixture of rain and snow. The rain gauge is the instrument to measure rainfall.
Elements of weather Measuring Instruments | Forms of Precipitation |
1. Air temperature-Thermometer | 1. Rain : Precipitation that reaches the ground in liquid form. |
2. Atmospheric pressure-Barometer | 2. Drizzle : Less intense rain. |
3. Humidity of air-Hygrometer | 3. Snow : Powdery mass of ice crystals. |
4. Rainfall-Rain gauge | 4. Sleet: Mixture of rain and ice. |
5. Wind speed-Anemometer | 5. Hail : Pellets of ice, associated with thunderstorms. |