Insolation

The incoming solar radiation is known as insolation.

The importance of insolation is as follows:

1. It provides energy in the form of heat and light.
2. It maintains the temperature of Earth.
3. It help plants make their food.

The three processes by which air gets heated are:

1. Convection — Convection occurs when heat is transferred through a gas or liquid. It is transmission of heat from one part of a liquid or gas to other part by upward movement of its particles.

2. Conduction — Conduction is a process in which heat is transferred directly through matter from a point of high temperature. Conduction transfers heat between adjacent molecules till the temperature is equal.

3. Radiation — It is the mode of transfer of heat which does not require any medium. It is a process by which a body emits radiant energy. It leads to loss of heat and leads of cooling.

Various factors influencing the temperature of a place are:

1. Latitude — Places near equator are hotter than places near poles.
2. Altitude — Temperature decreases with increasing altitude at a rate of 1°C per 166 metre.
3. Distance from sea — Places near sea have equable climate i.e. low diurnal and annual range of temperature than the places situated away from the sea.
4. Ocean currents — Ocean currents along the coast make the coastal areas warmer and cold currents make it cooler than usual.
5. Clouds and rainfall — Absence of clouds results in high temperature while presence of clouds moderates the temperature.
6. Slope of land — South facing slopes in Northern hemisphere are warmer.
7. Vegetation — Vegetation checks radiation from upper layer of soil. They have moderating effect on temperature.
8. Nature of soil — The more the water soil can retain, the less rapidly it heats or cools. Dark coloured soil absorbs more heat and gives rise to differences in temperature.

Only 51% of the insolation reaches the surface of the Earth as out of the remaining 49%, 35% of the insolation is directly reflected back to space by the atmosphere and the rest of the 14% is absorbed by the ozone layer.

Places close to the Equator have higher temperature and are warmer than places away from the equator. This is because, at the low latitude the Sun's rays are direct and have to travel a lesser extent through the atmosphere. Hence, the heat of these rays is more intense. But at higher latitudes, the Sun's rays are slanting and have to pass through a greater extent of atmosphere, due to which they lose their heat and are not intense.

The land absorbs more solar radiation and reflects a little while water reflects more solar radiation and absorbs a little. The differential heating of land and water is caused because of different rates at which the land and water are heated up. Land gets heated faster than the water. Therefore, during day land gets hotter than water. This results in wind blowing from sea to land during day. During night the land cools down faster and becomes cooler than the water bodies. This results in wind blowing from land towards the sea.

The temperature decreases at the rate of about 1°C per 166 m or 6°C per 1,000 metres of altitude. There are, however some places where certain special conditions in the lower atmosphere produce a reversal of the normal lapse rate so that temperature actually increases with an increase in altitude. This is known as temperature inversion.

It is generally experienced in hollows and valleys on calm clear nights when radiation has caused cooling and cold air has sunk down. At the same time on the mountain slopes the air is warmer. Temperature inversion is normally found in the middle latitude and for this reason fruit growers in these regions prefer gentle slopes to valley bottom for orchard sites.

The Torrid zone will have a higher range of temperature because it lies between temperate zone and equator. The equatorial region is warm throughout the year but as we move northward or southward the range of temperature i.e. difference between highest and lowest temperature, goes on increasing.

Factors that affect the insolation over the Earth's surface are:

1. Latitude
2. Altitude
3. Winds
4. Clouds and rainfall
5. Slope of the land
6. Vegetation

The slanting rays are less effective than the vertical rays in heating the atmosphere because slanting rays travel a longer distance through the atmosphere where much of its heat is absorbed by clouds, water vapour and dust particles while vertical rays travel through a shorter distance in the atmosphere and the loss of heat is less.

Winds blowing from hotter area increase the temperature while winds blowing from cold areas decrease the temperature. For example, a cold wind blowing from the interior of the continent during winter further reduces the temperature of the places along their path, e.g., China. Generally, the winds blowing from the lower latitudes to higher ones increase the temperature of the higher latitudes.

With the increase of height, temperature decreases at the rate of 1°C for every 166 metres. This is known as Normal Lapse Rate.

The slope of the land affects the temperature in three ways:

1. Presence of mountains acting as a barrier — For example, Himalayas prevent the monsoon winds from leaving India and also prevent the cold winds from China to enter India.
2. Inversion of temperature — Steep slopes experience more changes in temperature than a gentle slope because of reversal of Normal Lapse Rate of temperature.
3. Direction of slope of the land — Slopes facing south in the northern hemisphere are warmer than those facing north because the sun rays strike the south facing slopes at a steeper angle than they do at the north facing slopes which are also called Sheltered slopes.

The temperature of a place in the atmosphere generally decreases with altitude because the gases absorb very little of the incoming solar radiation but the ground absorbs this radiation and then heats the tropospheric air by convection and radiation. Therefore, atmosphere is hotter near the surface and get cooler as the height increases.

Equable climate refers to a climate with low range of annual and diurnal temperature. It is experienced in areas situated close to sea. The sea has a moderating effect on the climate of coastal areas which causes the equable climate.

The land absorbs more solar radiation and reflects a little while water reflects more solar radiation and absorbs a little. The differential heating of land and water is caused because of different rates at which the land and water are heated up. Land gets heated faster than the water. Therefore, during day land gets hotter than water. This results in wind blowing from sea to land during day. During night the land cools down faster and becomes cooler than the water bodies. This results in wind blowing from land towards the sea.

The solar radiation intercepted by the Earth is known as insolation.

Transfer of heat from an areas of more heat to areas of less heat by direct contact or directly through the matter is called conduction.

The normal lapse rate refers to decrease in temperature at the rate of 1°C per 166 metres increase in altitude.

Reversal of Normal Lapse Rate of temperature in special conditions where temperature actually increases with an increase in altitude, is called temperature inversion.

Land surfaces absorb much more solar radiation than water. Therefore, it gets heated faster than the sea.

The continental interiors are away from any water bodies so they cannot experience the moderating effect of the water bodies. Therefore, the climate of continental interiors is of comparatively extreme type.

The sea has a moderating effect on the climate of coastal areas due to sea and land breezes. Coastal areas generally have more moderate temperatures than inland areas because of the heat capacity of the ocean. Therefore, coastal climates are equable.

Slopes facing south in the northern hemisphere are warmer than those facing north because the sun rays strike the south facing slopes at a steeper angle than they do at the north facing slopes.

The amount of insolation received is maximum at the equator. It goes on reducing with higher latitudes and is minimum at the poles. This is because in low latitudes the Sun's rays are direct and have to travel a lesser extent through the atmosphere. Hence, the heat of these rays is more intense. But in higher latitudes, the Sun's rays are slanting and have to pass through a greater extent of atmosphere. Hence, these rays lose heat on the way and minimum insolation occurs at the poles.

With the increase of height, temperature decreases. For every 166 metres the temperature falls by 1°C. This is known as Normal Lapse Rate. This is the reason why hill stations are cooler even in the summer.

Below diagram shows the Effect of Latitude on Solar Insolation:

Conduction

All the above

All the above

The areas in the high latitude are not very hot as compared to the equatorial regions.

The vegetation cover checks the radiation from the upper layer of the soil.

All the above

Winter

6000°c

Mumbai is nearer to the coast than Nagpur.

99.9%

This is mainly due to the phenomenon of temperature inversion. In mountainous regions at night, the heavier cold air sinks down into the valley, while the lighter warm air moves up the slopes. This causes the valley floor to become much cooler, often leading to foggy and damp conditions. To avoid these colder, less comfortable conditions, houses, farms, and resorts are usually built on the warmer upper slopes.

The temperature difference between Srinagar and Agra is mainly due to the variation in their altitudes. According to the normal lapse rate, temperature decreases with increasing elevation. Since Srinagar is located at a much higher altitude than Agra, it experiences cooler temperatures, even during the summer.

This difference in vegetation is due to the variation in sunlight and rainfall received by the slopes. The south-facing slope of the Himalayas receives direct sunlight for longer periods and gets abundant rainfall, creating favorable conditions for dense vegetation. In contrast, the north-facing slope lies in the rain shadow region, receives less sunlight at a lower angle, and gets limited rainfall. As a result, vegetation is much sparser on the north-facing slope.

A is true but R is false.

Explanation — The assertion is true: Mangalore has an equitable (moderate) climate, while Nagpur experiences an extreme climate. This is because Mangalore is located on the western coast of India and is influenced by the moderating effect of the sea, which keeps temperatures fairly stable throughout the year. On the other hand, the reason given is false—Nagpur is located inland, far from the coast, which is why it experiences greater temperature variations. Hence, A is true but R is false.

Both A and R are true and R is the correct explanation of A.

Explanation — Equator that is located at 0° latitude and other low latitudinal regions receive direct rays of the Sun. As Sun's rays have to travel a lesser extent through the atmosphere, the heat of these rays is more intense, compared to higher latitudinal regions.