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Climatology - Part 1
1.0 INTRODUCTION
The stabilization of atmosphere in its present form took place in the Cambrian period (about 600 million years ago). The gases of the present atmosphere are evolutionary products of volcanic eruptions, hot springs, chemical breakdown of solid matter and distribution from the biosphere including photosynthesis and human activity.
2.0 The Atmosphere
The atmosphere is made up of gases and vapour, and receives incoming solar energy from the sun giving rise to, what we call, climate. We actually live at the bottom of this indefinite layer of atmosphere where the air is densest. Hither up, the air thins out and it is still a matter of conjecture where the atmosphere ends. One estimate puts this limit at about 600 miles above sea level. The lowest layer, in which the weather is confined, is known as the troposphere.
It extends from the earth's surface for a height of 6 miles, and within it temperature normally falls with increasing altitude. The climatic elements such as temperature, precipitation, clouds, pressure and humidity within the troposphere account for the great variations in local climate and weather that play such a great part in our daily live. From analyses taken in different parts of the globe, it is found that the lower part of the atmosphere contains a consistent proportion of certain gases: 78.084 percent of nitrogen, 20.946 percent of oxygen, 0.93 percent of argon and 0.04 percent of carbon dioxide. Helium and other rare gases constitute the rest.
In addition, it has an unpredictable proportion of water, existing either as a gas like water vapour, a liquid like rain, clouds and sleet or a solid like snow and hailstones, as well as other solid particles like smoke and dust. It is because of the variable water content of the atmosphere that we have such great contrast in weather and climate over different parts of the world. If we were to live in a dry atmosphere, absolutely without water, there would be no weather and not even much climate.
Above the troposphere lies the stratosphere or the upper layer of the atmosphere. It extends upwards for another 50 miles or even more. It is not only very cold, but cloudless, with extremely thin air and without dust, smoke or water vapour but there are marked seasonal temperature changes.
Beyond the stratosphere is the ionosphere which goes several hundred miles up. It has electrically conducting layers which make short-wave radio transmission possible over long distances. Modern artificial satellites, launched in the upper strata of the atmosphere, as well as balloons, are used to transmit back to earth valuable information regarding the conditios of the conditions of the atmosphere.
3.0 HEAT BUDGET OF THE EARTH
The heat budget of the Earth is defined as the balance between incoming and outgoing solar radiation. Incoming solar energy varies at different times of year and for different locations across the globe.
3.1 Insolation
The only source of energy for the earth's atmosphere comes from the sun which has a surface temperature of more than 10,800° F. This energy travels through space for a distance of 93 million miles (9 crore 30 lac miles) and reaches us as solar energy or radiant energy in the process called insolation. This radiation from the sun is made up of three parts - the visible 'white' light that we see when the sun shines and the less visible ultra-violet, and infra-red rays.
The visible 'white' light is the most intense and has the greatest influence on our climate. The ultra -violet rays affect our skin and cause sun-burn when our bare body is exposed to them for too long a period. The infra-red rays can penetrate even dust and fog and are widely used in photography. Only that part of the sun's radiation which reaches the earth is called insolation.
What matters most is the effect of the atmosphere upon the incoming solar radiation. It is estimated that of the total radiation coming to us, 35 percent reaches the atmosphere and is directly reflected back to space by dust, clouds and air molecules. It plays practically no part in heating the earth and its atmosphere.
Another 14 percent is absorbed by the water vapour, carbon dioxide and other gases. Its interception by the air causes it to be 'scattered' and 'diffused' so that the visible rays of the spectrum between the ultra-violet and infra-red give rise to the characteristic blue sky that we see above us. The remaining 51 percent reaches the earth and warms the surface.
In turn the earth warms the layers of air above it by direct contact or conduction, and through the transmission of heat by upward movement of air currents or convection. This radiation of heat by the earth continues during the night, when insolation from the sun cannot replace it. The earth-surface therefore cools at night.
The rate of heating differs between land and water surfaces. Land gets heated up much more quickly than water. Because water is transparent, heat is absorbed more slowly and because it is always in motion, its absorbed heat is distributed over a greater depth and area. Thus any appreciable rise in temperature takes a much longer time. On the other hand, the opaque nature of land allows greater absorption but all the radiant heat is concentrated at the surface, and temperature rises rapidly. Because of these differences between land and water surfaces, land also cools more quickly than water.
3.2 Latitude and Energy balance
The two main features of the Earth's energy balance are:
- There is a net gain of solar energy in the tropical latitudes and a net loss towards the poles
- The tropical latitudes receive more of the Sun's energy than polar regions.
There is a surplus of energy between 35° North and 35° South. In this region, incoming insolation exceeds outgoing radiation.
Tropical areas get more insolation than polar regions. Insolation rises sharply from approximately 50 joules at the poles to 275 joules at the equator. Terrestrial radiation varies less, from 120 joules at the poles to 200 joules at the equator. Through atmospheric circulation, energy is transferred from lower latitude energy surplus areas to higher latitude energy deficit areas. Without this lower latitudes would get hotter and hotter and higher latitudes colder and colder.
4.0 Elements of climate and Factors affecting them
Of the various climatic elements, temperature, precipitation, pressure and winds are the most important because of their far reaching global influences. These elements and their distribution, whether horizontal from equatorial to polar regions, or vertical from ground to atmosphere, are in one way or another affected by some or all of the climatic factors: latitude, altitude, continentality, ocean currents, insolation, prevailing winds, slope and aspect, natural vegetation and soil.
4.1 Temperature
Temperature influences the climate in the following ways:
- Temperature influences the actual amount of water vapour present in the air and thus decides the moisture-carrying capacity of the air.
- It decides the rate of evaporation and condensation, and therefore governs the degree of stability of the atmosphere.
- As relative humidity is directly related to the temperature of the air, it affects the nature and types of cloud formation and precipitation.
4.2 Latitude
One of the most important factors determining the climate is latitude. Between 23.5S and 23.5N latitudes we have the tropics - where high temperatures are the norm, and the sun can beat down from directly overhead once or twice each year. From 23.5N to 66.5N and between 23.5S and 66.5S are the temperate zones, where there are clear spring/summer/fall/winter seasons. From 66.5N to the North Pole we have the Arctic, and from 66.5S to the South Pole, the Antarctic. In these zones the sun is above the horizon at midnight for part or all of the summer and never rises at all during some day(s) in the winter. At the pole, daily motion is parallel to the horizon.
With the meridian diagram, we can find out the altitude of the sun at noon at any season as observed from any place on Earth. This information can be very helpful in planning a garden or a house so that the sun will shine on the areas we want it to. Knowing the sun's altitude at noon in the summer is also useful in figuring out how to shade your windows against the noon summer sun, while allowing the winter sun to shine in and keep you warm in the interior.
4.3 Altitude
Since the atmosphere is mainly heated by conduction from the earth, it can be expected that places nearer to the earth's surface are warmer than those higher up. Thus temperature decreases with increasing height above sea level. This rate of decrease with altitude (lapse rate) is never constant, varying from place to place and from season to season. But for all practical purposes, it may be reckoned that a fall of 1°F occurs with an ascent of 300 feet or 0.6° C. per 100 metres. It is usually more in summer than in winter.
For example in temperate latitudes, in summer, an ascent of only 280 feet will cause the temperature to drop by 1°F, whereas in winter it requires 400 feet. Similarly, the lapse rate is greater by day than at night, greater on elevated highlands than on level plain. In tropical countries where the sea level is 80°F, a town that is located at a height of 4,500 feet will record a mean temperature of 65°F.
4.4 Continentality
Land surfaces are heated more quickly than water surfaces, because of the higher specific heat of water. In other words, it requires only one-third as much energy to raise the temperature of a given volume of land by 1°F as it does for an equal volume of water. This accounts for the warmer summers, colder winters and greater range of temperature of continental interiors as compared with maritime districts.
4.5 Ocean currents and winds
Both ocean currents and winds affect temperature by transporting their heat or coldness into adjacent regions. Ocean currents like the Gulf Stream or the North Atlantic Drift warm the coastal districts of Western Europe keeping their ports ice-free.
Ports located in the same latitude but washed by cold currents, such as the cold currents, such as the cold Labrador current off north-east Canada, are frozen for several months.
Cold currents also lower the summer temperature, particularly when they are carried landwards by on-shore winds. On the other hand on-shore Westerlies, convey much tropical warm air to temperate coasts, especially in winter. The Westerlies that come to Britain and Norway tend to be cool winds in summer and warm winds in winter and are most valuable in moderating the climate.
Local winds, e.g. Fohn, Chinook, Sirocco, Mistral, also produce marked changes in temperature.
4.6 Slope, shelter and aspect
A steep slope experiences a more rapid change in temperature than a gentle one. Mountain ranges that have an east-west alignment like the Alps show a higher temperature on the south-facing 'sunny slope' than the north facing 'sheltered slope'. The greater insolation of the southern slope is better suited for vine cultivation and has a more flourishing vegetative cover. Consequently, there are more settlements and it is better utilized than the 'shady slope'. In hilly areas a hot day followed by calm, cloudless night during which the air cools more rapidly over the higher ground may induce cold, heavy air to flow down the slope and accumulate at the valley bottom pushing the warmer air upwards. The temperature may then be lower in the valley than higher up as the slopes. A reversal of the lapse rate has taken place. This is called a temperature inversion.
4.7 Nature vegetation and soil
There is a definite difference in temperature between forested regions and open ground. The thick foliage of the Amazon jungle cuts off much of the incoming insolation and in many places sunlight never reaches the ground. It is, in fact, cool in the jungle and its shade temperature is a few degrees lower than that of open spaces in corresponding latitudes. During the day trees lose water by evapo-transpiration so that the air above is cooled. Relative humidity increases and mist and fog may form.
Light soils reflect more heat than darker soils which are better absorbers. Such soil differences my give rise to slight variations in the temperature of the region.
As a whole, dry soils like sands are very sensitive to temperature changes, whereas wet soils, like clay, retain much moisture and warm up or cool down more slowly.
5.0 PRECIPITATION
5.1 Types of precipitation
If air is sufficiently cooled below dew-point, tiny drops of water vapour will condense around dust particles. When they float about as masses of minute water droplets or ice crystals at a considerable height above sea level, they form clouds-cirrus, cumulus or stratus. When condensation occurs at ground level without necessarily resulting in rain, haze, mist or fogs are formed. In higher latitudes or altitudes, where condensation of water vapour may take place in the atmosphere at temperatures below freezing-point, snow falls, either as feathery flakes or individual ice crystals. If the moist air ascends rapidly to the cooler layers of the atmosphere, the water droplets freeze into ice pellets and fall to the earth as hail or hailstones.
As more and more super-cooled water drops accumulate around a hailstone, it increases steadily in size; some of them weigh as much as two pounds. In a severe hail-storm the hailstones do great damage to crops and buildings. Very often, the ice-pellets exist as frozen rain-drops, melting and re-freezing on their way down; this forms sleet. It is only when the droplets in clouds coalesce into larger drops between 0.2mm and 6mm, that rain falls.
5.2 Types of rainfall
There are three major types of rainfall.
5.2.1 Convectional rainfall
This type of rainfall is most common in regions that are intensely heated, either during the day, as in the tropics, or in the summer, as in temperate interiors. When the earth's surface is heated by conduction, moisture-laden vapour rises because heated air always expands, and becomes lighter. Air rises in a convection current after a prolonged period of intense heating. In ascending, its water vapour condenses into cumulonimbus clouds with a great vertical extent.
This probably reaches its maximum in the afternoon when the convectional system is well developed. Hot, rising air has great capacity for holding moisture, which is abundant in regions of high relative humidity. As the air rises it cools and when saturation point is reached torrential downpours occur, often accompanied by thunder and lightning. The summer showers in temperate regions are equally heavy with occasional thunderstorms. These downpours may not be entirely useful for agriculture because the rain is so intense that is does not sink into the soil but is drained off almost immediately.
5.2.2 Orographic or relief rain
Unlike convectional rain which is caused by convection currents, Orographic rain is formed wherever moist air is forced to ascend a mountain barrier. It is best developed on the windward slopes of mountains where the prevailing moisture-laden winds come from the sea. The air is compelled to rise, and is thereby cooled by expansion in the higher altitudes and the subsequent decrease in atmospheric pressure.
Further ascent cools the air until the air is completely saturated (relative humidity is 100 percent). Condensation takes place forming clouds and eventually rain. Since it is caused by the relief of the land, it is also known as relief rain. Much of the precipitation experienced on the windward slopes of the north-east of West Malaysia, western New Zealand, western New Zealand, western Scotland and Wales and the Assam hills of the Indian sub-continent, is relief rain.
On descending the leeward slope, a decrease in altitude increases both the pressure and the temperature; the air is compressed and warmed. Consequently, the relative humidity will drop. There is evaporation and little or no precipitation. The area in the lee of the hills is termed the rain shadow area. The effects of rain shadow are felt on the Canterbury Plain of South Island, New Zealand and the western slopes of the Northern and Central Andes and in many other areas.
5.2.3 Cyclonic or frontal rain
This type of rainfall is independent of relief or convection. It is purely associated with cyclonic activity whether in the temperate regions (depressions) or tropical regions (cyclones). Basically it is due to the convergence (meeting) of two different air masses with different temperatures and other physical properties. As cold air is denser, it tends to remain close to the ground. The warm air is lighter and tends to rise over the cold air. In ascent pressure decreases, the air expands and cools, condensation takes place and light showers called cyclonic or frontal rain occur. The heavier and colder air masses eventually push up the warmer and lighter air and the sky is clear again.
6.0 TEMPERATURE AND PRESSURE BELT
The variations in pressure between different regions are shown on maps by means of Isobars. These are lines joining the places having the same barometric pressure.
The rate of change of atmospheric pressure between two points on the earth's surface is called the pressure gradient. It is defined as the decrease in pressure per unit distance in the direction in which the pressure decreases most rapidly. On the weather chart this is indicated by the spacing of isobars. The gradient is steep if the bars are close together and gentle if they are far apart.
6.1 World Pressure Belts
The distribution of atmospheric pressure across the latitudes is termed global horizontal distribution of pressure. On the earth's surface, there are seven pressure belts; the Equatorial Low, the two Sub-tropical Highs, the two Sub-polar Lows, and the two Polar Highs. Except the Equatorial low, the others form matching pairs in the Northern and Southern Hemispheres.
Due to the spherical shape of the Earth different parts of the Earth receive different amounts of heat. Hence, There is a pattern of alternate high and low pressure belts over the earth.
The Equatorial region receives more amount of heat throughout the year. As warm air being light, the air at the Equator rises, creating a low pressure. At the poles the cold heavy air causes high pressure to be created. In the Sub-polar region around latitudes 60° to 65° North and South of the Equator, the rotation of the earth pushes up the bulk of the air towards the Equator, creating a low pressure belt in this region.
Equatorial Low Pressure Belts: This low pressure belt extends from 0 to 5° North and South of Equator. Due to the vertical rays of the sun here, there is intense heating. The air therefore, expands and rises as convection current causing a low pressure to develop here. This low pressure belt is also called as doldrums, because it is a zone of total calm without any breeze.
Sub-tropical High Pressure Belts: At about 30°North and South of Equator lies the area where the ascending equatorial air currents descend. This area is thus an area of high pressure. It is also called as the Horse latitude.
Winds always blow from high pressure to low pressure. Hence the winds from sub tropical region blow towards Equator as Trade winds and another winds blow towards Sub-Polar Low-Pressure as Westerlies.
Circum-polar Low Pressure Belts: These belts are located between 60° and 70° in each hemisphere and are known as Circum-polar Low Pressure Belts. In the Sub-tropical region the descending air gets divided into two parts. One part blows towards the Equatorial Low Pressure Belt. The other part blows towards the Circum- polar Low Pressure Belt. This zone is marked by ascent of warm Sub-tropical air over cold polar air blowing from poles. Due to earth's rotation, the winds surrounding the Polar region blow towards the Equator. Centrifugal forces operating in this region create the low pressure belt appropriately called Circum-polar Low Pressure Belt. This region is marked by violent storms in winter.
Polar High Pressure Areas: At the North and South Poles, between 70° to 90° North and South, the temperatures are always extremely low. The cold descending air gives rise to high pressures over the Poles. These areas of Polar high pressure are known as the Polar Highs. These regions are characterised by permanent Ice Caps.
6.2 Shifting of Pressure Belts
The Earth is inclined at an angle of 23.5 degrees towards the sun. On account of this inclination, differences in heating of the continents, oceans and pressure conditions in January and July vary greatly. January represents the winter season and July, the summer season in the Northern hemisphere. Opposite conditions prevail in the Southern Hemisphere.
When the sun is overhead on the Tropic of Cancer (21 June) the pressure belts shift 5° northward and when it shines vertically overhead on Tropic of Capricorn (22 December), they shift 5° southward from their original position. The shifting of the pressure belts cause seasonal changes in the climate, especially between latitudes 30° and 40° in both hemispheres. In this region the Mediterranean type of climate is experienced because of shifting of permanent belts southwards and northwards with the overhead position of the sun. During winters Westerlies prevail and cause rain. During summers dry Trade Winds blow offshore and are unable to give rainfall in these regions.
When the sun shines vertically over the Equator on 21st March and 23rd September (the Equinoxes), the pressure belts remain balanced in both the hemispheres.
6.3 The world distribution of pressure
In respect to temperature and pressure of the atmosphere the main contrast is between continents and oceans. Since there are large differences in the heating and cooling of land and water, there is a strong contrast in pressure conditions between them. The differences are more marked between the Northern and Southern Hemispheres during the months of January and July.
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