Solar Radiation, Heat Balance And Temperature

On 21^st June the sun is directly overhead in the northern hemisphere which is 23.5°N. This phenomenon is known as Summer Solstice.

Tiruvanantapuram is located near to the equatorial line and therefore the days are longer compared to other cities.

The long wave radiation is absorbed by the atmospheric gases particularly Carbon dioxide and other green house gases. So they don’t allow the terrestrial heat to escape from the earth. This increases the temperature.

Explanation

(i)The incoming solar radiation to the earth’s atmosphere is known as Insolation

(ii) The percentage of light reflected by an object is known as albedo.

(iii) The lies joining the places of equal temperature is known as Isotherm

(iv) Annual range is the difference between the mean temperature of the warmest and the coldest months.

The main reason that the earth experiences highest temperature in the subtropics in the northern hemisphere rather than at the equator is that subtropical areas have longer day hours in the summer than the equator.

(i) The air and the ocean currents are responsible in the variations in weather and climate. As the earth receives uneven isolation from the sun due to its tilt axis, the areas get unevenly heated. More over the air and water currents carry the heat from one area to other and therefore the climate changes.

(ii) The factors that control temperature distribution on the surface of the Earth are as follows:-

• The latitude of the place- The temperature depends on the isolation received which differs from place to place

• The altitude of the place- The temperature decreases with increase in altitude known as normal lapse rate i.e. 1°C per 167 m.

• Distance of the sea- This depend on the land and sea breeze which regulates the temperature of a place.

• Air-mass and Ocean currents- The air and ocean currents regulates the temperature of a place.

(iii) India is located around the northern hemisphere. Summer sets in India from March end. So in May the summer is at its peak therefore the day temperature is maximum. On 21^st June, the sun rays fall directly on the northern hemisphere i.e. 23.5°N. After 21^st June, the monsoon sets in India which brings a cooling effect to the country.

(iv) Annual range of temperature is the difference between the mean temperature of the warmest and the coldest months. In Siberia the mean January temperature both at 80°N and 50°N is -20°C and in July it is more than 10°C. Therefore the annual range of temperature is high.

(i) The amount of insolation a place receives depends upon the inclination of the rays. This depends on the latitude of the place. The higher the latitude, the less is the angle they make with surface of the earth resulting in slant sun rays. The area covered by vertical rays is always less than the slant rays. If more area is covered, the energy gets distributed and the net energy received per unit area decreases. Moreover, the slant areas are required to pass through greater depth of the atmosphere resulting in more absorption, scattering and diffusion.

The axis of the earth is tilted at an angle of 23.5°. So the sun’s rays are different in both the hemispheres at the same time. The sun’s rays fall vertically on the tropic of cancer from 1^st March to 21^st July and on Tropic of Capricorn from 23^rd September to 22^nd December. As we go towards the poles, the temperature keep decreasing as the earth is tilted. Therefore the latitude and tilt in the axis of rotation of the Earth affect the amount of radiation received at the earth’s surface.

(ii) The sun’s energy reaches the earth’s surface by the process of radiation. The earth maintains the temperature by absorbing the rays and reflecting the terrestrial radiation. Of the total energy radiated by the sun, some amount of energy is reflected, radiated and absorbed. Out of the remaining part, 35 units are reflected back to the space before reaching the earth’s surface. Of these 27 units are reflected back from the top of the clouds and 2 units from the snow and ice –covered areas of the earth. The remaining 65 units are absorbed, 14 units within the atmosphere and 51 units by the earth’s surface. The earth radiates back 51 units in the form of terrestrial radiation. Of these 17 units are radiated to space directly and the remaining 34 units are absorbed by the atmosphere. 48 units absorbed by the atmosphere are also radiated back into space. Thus, the total radiation returning from the earth and the atmosphere respectively is 65 units which balance the total of 65 units received from the sun. This is known as the heat balance of the earth’s surface.

(iii) In general the effect of latitude on latitude is well pronounced on the map as the isotherms are generally parallel to the latitude. The deviation from this general trend is more pronounced in January than in July especially in the northern hemisphere. In the northern hemisphere the land surface area is much larger than in southern hemisphere. Hence the effects of land mass and ocean currents are well pronounced. In the January the isotherms deviate to the north over the ocean and to the south over the continent. This can be seen in the Atlantic Ocean. The presence of warm ocean currents, gulf streams, and North Atlantic drift make the Northern Atlantic Ocean warmer and the isotherms bend towards the north. Over the land the temperature decreases sharply and isotherms bend south in Europe. The effect of ocean is well pronounced in the southern hemisphere. Hence the isotherms are more or less parallel to the latitudes and the variation in temperature is more gradual than in the northern hemisphere. The isotherm of 20°C, 10°C and 0°C runs parallel to 35°S, 45°S and 60°S latitudes respectively.

i) Altitude of Kolkata is 9.14m Latitude of Kolkata is 88.3639° E and Longitude of Kolkata is 22.5726° N

ii) Mean of Daily Maximum is the average highest temperature recorded in a month.

Mean of Daily minimum is the average lowest temperature recorded in a month.

iii) Daily mean monthly temperature of Kolkata

v) Annual Range of temperature = Mean Maximum temperature in the Month of May – Mean Maximum temperature in January

= 36°C-26°C

= 10°C

vi) Months in which daily range of temperature is highest- April, May, June

Months in which daily range of temperature is lowest- December, January

vii) Kolkata is located near to the tropic of Cancer where it experiences the scorching heats of sun from March to 21^st June. And the Maximum temperature rises in May. After that the monsoon sets in and the weather becomes cool. So July is cooler. Towards the end of October as the monsoons draw back the place becomes cool as now the Tropic of Capricorn experiences the scorching heat of the Sun and so from October the temperature dips down and in January it becomes lowest. January is the coldest month in Kolkata.