http://google.com, pub-7771400403364887, DIRECT, f08c47fec0942fa0 Climatic elements and their global distribution

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Climatic elements and their global distribution

Introduction

In this lesson, the characteristics and global distribution of three fundamental climatic elements, namely temperature, evaporation and precipitation will be discussed. In Physical Geography II, climate was defined as the average conditions of the atmosphere near the earth’s surface over a period of years. It was generally described in meteorological conditions such as temperature, humidity, precipitation, wind, radiation, sunshine and cloudiness. The climate of a region has profound influences its on soil, vegetation and landforms. It determines the suitability of a region for agriculture and it influences the processes that shape the landscape. Therefore, understanding the global distribution of the earth’s climatic regions is the key to understanding many of the characteristics of those regions. Before understanding the overall global distribution of climate it is important to understand the characteristics and distribution of specific climatic elements.

Temperature

Temperature provides a measure of the intensity or degree of hotness of a given object or body. It is therefore the condition that determines the flow of heat energy from one substance to another with the flow always being from high to low temperature. Thus by studying the temperature characteristics of the earth- atmosphere system, one is in fact studying the heat energy of the system and how it is distributed over the earth’s surface.

Vertical distribution of temperature

In the troposphere, temperature decreases with increase in elevation, what is commonly referred to as Environmental Lapse Rate (ELR). This has been calculated for many world areas to average 6.50C/km. The vertical distribution of temperature is influenced by the nature of the underlying surface. For example, temperature decreases most rapidly with altitude over continental areas than water masses in summer.


Another effect of altitude on temperature is the difference it causes on diurnal range of temperature, such that this range is greater at a higher elevation than at an equivalent climate at sea level. The main difference occurs during the night when the escape of terrestrial energy takes place readily because of the lower density of gases at higher elevation. The decrease of pressure with altitude also modifies the meaning of given values on temperature scales. The reduced pressure, for example, means that molecules of water vapour escape more easily from a water surface. Thus at sea level, water boils at a temperature of 1000C, at an elevation of about 3000m, water will boil at 900C, at 6000m water boils at about 700C.


The decrease of temperature with increasing elevation is periodically interrupted, and temperature increase with altitude may occur. This is referred to as temperature inversion. Inversion can occur at ground level when associated with radiation cooling or above the ground level because of subsidence.


Objectives
 minimum, with the range generally increasing with latitude.

Surface properties

The solar energy received on the earth surface depends on the type of surface the energy is striking. Of particular note is the surface reflectivity or albedo. Surfaces with high albedo absorb less incident radiation, with the result that the total energy available is diminished. Thus the polar ice caps are maintained because as much as 80% of the solar radiation falling on them is reflected.


As shown in Table 1, different surfaces have different specific heat capacity. Specific heat capacity is defined as the amount of heat in calories required to raise the temperature of 1g of a substance through 10C. For example the specific heat capacity of water is five times greater than that of rock and the land surface in general. This means that the amount of heat required to raise the temperature of water through 10C is five times greater than that required for the same temperature increase on land. The same amount of energy applied to land surface would result in the lands becoming much hotter than the water.


The difference is also heightened by the difference in heat conductivity of the two materials (Table 1). Loose dry soil is a very poor conductor of heat and only a superficial layer will experience a rise in temperature following energy input. Water has only affair conductivity but its general mobility and transparency permit heat to circulate below surface layers. A natural undisturbed soil with a vegetation cover may have daily temperature changes recorded to 1 metre, while a quiet stand or pool of water has daily temperature variations that can be measured to a depth of about 6 metres. When such differences are applied to the global scale it is evident that land surfaces are rapidly and intensely heated by the sun’s rays whereas water surfaces are only slowly and moderately heated. Again land surfaces cool off more rapidly and reach much lower temperatures than water surfaces when solar radiation is cut off.

Table 1. Thermal properties of air and various surfaces

Substance

Heat capacity



Temperature contrasts are therefore greater over land areas, but only moderate over water areas. It is further true that the larger the mass of land, the greater is the seasonal temperature contrasts. Because the heating of ground and water surfaces controls heating of the atmosphere above, the same observations apply to air temperature as to surface temperature.

Aspect and Topography

The combined influences of steepness and direction faced by a slope determine its aspect. The importance of aspect is best seen in differences that occur on north-facing and south- facing slopes in the northern hemisphere. A north-facing slope may still have snow lying on it while a south-facing slope is quite clear. The north-facing slope gets less intense radiation and, as the sun gets lower in the sky, it will be in shadow long before the south-facing slope. The influence of aspect is seen in many ways; for example, the height of the level of permanent snow and ice on mountains will vary from one slope to another, while vegetation levels (e.g. the tree line) will also be affected. Similarly the depth of snow and frost are found to differ on north-facing and south-facing slopes.


Topography also plays an important role in the nature of climates of neighbouring lowlands. On a continental scale, mountain ranges that run north-south have a different effect from those that run east- west. Thus the lack of any extensive east-west barrier in the United States permits polar and tropical air to penetrate great distances into the continent. One result of this unobstructed flow of air is the high incidences of tornadoes in the United States. The relief alignment of continents also affects the flow of air masses into the interior.

Dynamic factors

The imbalance of energy between the tropics and poles means that an exchange of air must occur through a dynamic process. The mechanisms for the exchange involve the transfers of latent heat (LE), sensible heat (H), and the heat that is stored within the water of the ocean (S).


Table 2 provides the theoretical planetary temperature for sea level, assuming the atmosphere is at rest and the observed mean annual temperature for every 100 C of latitude. The greatest differences are at the equator and for those latitudes above 600. Tropical latitudes are cooler than the theoretical value, while high latitudes are warmer. The differences between the actual and theoretical values result from the transport of energy over the globe by air masses and ocean currents. Every storm system, circulation pattern and evaporation/precipitation event contributes towards the redistribution of temperature regimes that prevail over the earth’s surface.





Planetary Temperatures

33

32

28

22

14

3

-11

-24

-32

The global distribution of temperature near the earth’s surface shows that in the northern hemisphere winter and southern hemisphere summer, the highest temperatures lie in a belt close to the equator over the oceans and somewhat south of it over the land masses. Maximum temperatures exceed 300C over portions of these land areas. Minimum temperatures occur over Polar Regions, with the lowest values below –300C in north central Asia. There are sharp temperature contrasts between land and sea, particularly on the western sides of continents where at a given latitude the northern hemisphere land is colder and the southern hemisphere land warmer than the adjacent ocean. A similar phenomenon, with the hemispheres reversed occurs in the northern hemisphere summer and southern hemisphere winter. In this season Antarctica is the coldest region and the warmest areas are those continental areas just north of the equator. A close comparison of the conditions in the two seasons clearly indicates that seasonal changes in ocean surface temperature are relatively minor, but that mid- latitude continental interiors suffer a much greater range.

Evaporation

Water is brought into the air as vapour by evaporation. It is a physical process by which vapour escapes from any free liquid water surface or wet surface at a temperature below the boiling point of water. In addition to loss by evaporation from soil, water is also lost by transpiration from vegetation covering the soil or water surface. This combined loss is known as evapotranspiration.


Water vapour is the principle participant in the many energy exchanges taking place in the atmosphere. The energy exchanges are responsible for the weather phenomena, which serve as important links connecting the various phases of the hydrological cycle.


Measurement of evaporation and evapotranspiration is of importance in many scientific fields. It is one of the main components of the water budget, knowledge of which is indispensable for the solution of numerous water management problems. Reliable evaporation data are required for planning, designing and operating reservoirs, ponds, shipping canals, irrigation and drainage systems. Evaporation is especially important in arid zones where water must be used in the most efficient way. Knowledge of the water requirement of crops depends partly on the accurate determination of the loss of water by evapotranspiration from cultivated fields.


Evaporation is measured using the following instruments:

Atmometers such as Piche evaporimeter, Livingston atmometer, Bellani plate atnometer, Diag’o type evaporimeter and Hirata type evaporimeter.

Pan or Tank Evaporimeters such as class 4 pan, circular tank, Kenya evaporation pan, rectangular tank and sunken tank.

Lysimeters (soil filled tanks) have also been used to measure evapotranspiration. They are operated under natural conditions and measure the actual evapotranspiration. If they are supplied daily with water by sprinkling or sub-irrigation, they can provide values for potential evapotranspiration

Global Distribution of evaporation

The relative sparseness of observations together with the difficulty of reconciling various estimation methods makes it very difficult to present reliable global maps of evaporation. Maximum rates of actual evaporation occur over subtropical oceans with a general decrease in amount poleward. Land values are lower than oceanic ones, the isopleths making a sharp break at the coasts. Generally on a global scale evapotranspiration from continents is about 470 mm per year, while from the ocean it is about 1300 mm per year. Average evapotranspiration from the continents varies a great deal through time and space. The major variables are the amounts of water and energy available. In tropical areas where there is ample water and energy, evapotranspiration rates are very high. In the lower Amazon valley and the central Congo River Basin the rates of 1200 mm per year, very nearly approach that of evaporation over the open ocean. In parts of the Atlantic and Gulf Coastal plain of United States, the amount is almost as high. Evapotranspiration is probably greatest in the Sudd and in the Chad basin in Sub-Saharan Africa. Here the rate may reach 2400 mm per year far in excess of local rainfall. These two extensive areas of swamp and shallow lakes are supplied by rivers. The White Nile feeds the Sudd, and Lake Chad is fed by a series of rivers from the south. Solar radiation is intense and the air dry, factors that enhance evapotranspiration. On the other hand, where temperatures are lower such as in Northern Europe, evapotranspiration rates drop to as little as 200 mm per year.

Precipitation

The type and size of precipitation leaving the cloud base depends on the conditions within the cloud, but the precipitation that actually reaches the ground is modified by conditions in the air layer between the cloud and the ground. In general, the temperature structure determines whether the precipitation will arrive as frozen or as liquid water, while the humidity of the layer determines the amount of evaporation that will occur and hence the ultimate size of the precipitation particles. In both cases the fall velocity will dictate the time over which the processes can act and hence how completely they will be.


The intensity and duration of precipitation is determined largely by the type of cloud system involved. This in turn is intimately connected with the cloud formation processes considered (i.e. Coalescence model and Bergeron-Findeisen model). In general cumulus vertical motions give large drops and intense precipitation for a short period. Usually their influence is restricted to a fairly small geographical area. Stratus and altostratus, in contrast involve more persistent and less vigorous vertical motions over a much wider area. Hence prolonged, steadier and usually less intense precipitation results. The difference in intensity decreases as the duration increases. World rainfall statistics suggest that intensity is approximately proportional to the inverse square root of the duration, but that there are many regional variations. Types of precipitation include convectional, orographic and frontal already discussed in prerequisite units of Physical Geography I and II.

Global precipitation distribution

We have noted that both the intensity and duration, and thus the amount of precipitation in an individual event depend on the processes acting to create the precipitating clouds, and that the real extent of precipitation depends on the same factors. Since particular processes tend to dominate particular areas of the globe, we can make several pertinent generalisations about the global precipitation on an annual basis.


The area of maximum annual precipitation over 2000mm per year extends in a band through the equatorial regions. The subtropical deserts and the Polar Regions have values below 250mm. The mid-latitude regions have intermediate values, being in general about 1000mm per year.


Mountain ranges play a significant role in the spatial distribution of precipitation. The windward slopes of mountains receive the greatest amount of precipitation. In the leeward side of the mountain ranges the precipitation decreases markedly to give rain shadow effects.

Tropical precipitation

Precipitation in much of the tropics is associated with convective activity. Strong vertical motions occur in a fluctuating band near the equator. This release the abundant water vapour, which creates a regime of intense, short-lived storms from cumulus clouds. Rainfall rates in excess of 100 mm per hour are not uncommon. Although the location of the storms is partly controlled by local topographic features, storms tend to recur sporadically, so that precipitation does not occur at a particular place every day even though there may be a storm in the area each day.


More widespread uplift is associated with monsoonal circulations. Such circulations are particularly well developed over tropical Asia. Although this is a strongly seasonal precipitation regime, the effects of convective uplift, dynamical uplift and topographic forcing combine to produce high annual rainfall totals. Locally rainfall rates may be very high but generally the monsoonal condition is characterised by longer lasting, less intense precipitation.

Mid- latitude precipitation

In mid-latitude much of the precipitation production is associated with depressions and fronts. The result is widespread uplift giving extended periods of gentle rain over a broad area. Rainfall rates vary greatly, although 1-2 mm per hour, which is regarded as a typical value. The intensity is partly controlled by the amount of water vapour available, which in turn depends on the source of the air, which is being uplifted. Air derived directly from the subtropical oceans, where evaporation rates are high is likely to lead to higher precipitation rates. If the source is the tropical deserts, the air is likely to be much drier and it is not uncommon in these conditions for dust and sand particles to form the condensation nuclei and hence to be deposited in large quantities with the rain. Convective activity in the mid- latitudes is primarily a summer phenomena. It can be intense, but is usually less regular, than in the tropics.

Low precipitation areas

The regions of low precipitation in the sub-tropics result mainly from a lack of mechanisms for creating uplift and bringing the air to saturation. Certainly over the oceans and to a large extent over the land deserts as well, there is lack of moisture in the atmosphere in the atmosphere. In contrast, over the Polar Regions the low precipitation totals are as much associated with a lack of atmospheric uplift mechanisms.

Rain days

The spatial distribution of rainfall can also be viewed in terms of the number of rain days per year. A rainy day is usually starting at 0900 GMT, during which 0.2 mm or more of precipitation falls. The climatic average of rain days varies from over 180 per year in humid coastal regions to less than one per annum in very arid regions. In general there is a close relationship between the number of rain days and the total precipitation. However, seasonality can influence the relationship between rainfall totals and rain day numbers. Places that experience distinct wet and dry seasons may have high rainfall totals but few numbers of rain days.


The relationship between rainfall totals and rain days therefore depends strongly on the climatic regime and on the nature of the precipitation producing systems. For many purposes the total rainfall in a given period is the most useful measure of precipitation, but in some cases the number of rain days is more appropriate.

Summary 

In this lesson we discussed the characteristics and global distribution of three key climatic elements namely; temperature, evaporation and precipitation. It was noted that temperature conditions vary both vertically and horizontally, and that the horizontal distribution of temperature is affected by two broad factors, namely locational and dynamic factors. The locational factors are influenced by latitude of a place, surface properties, aspect and topography. The global atmospheric and oceanic circulation processes that transfer energy from one place to another control the dynamic factors.  The importance and measurement of evaporation as a process and a climatic element was highlighted and it global distribution discussed. The same applied to precipitation, apart from a discussion on the comparison between tropical and mid-latitude precipitation characteristics.  Finally the importance of rain days as a measure of precipitation effectiveness was highlighted.

Definitions of Key Words

Aspect is the combined influences of steepness and direction faced by a slope

Evaporation is a physical process by which vapour escapes from any free liquid water surface or wet surface at a temperature below the boiling point of water

Evapotranspiration is a combined loss of water from the soil (evaporation) and the vegetation covering the soil (transpiration)

Precipitation is the liquid or solid water that falls from the atmosphere to the earth surface. It can also be defined as an atmospheric process in which water droplets in cloud grow into raindrops

Rain day refers to a day starting at 0900 GMT during which 0.2 mm or more of precipitation falls

Surface Albedo or Reflectivity is the percentage of incident radiation reflected from a surface

Specific Heat Capacity is amount of heat in calories required to raise the temperature of one gram of a substance through one degree Celsius 

Temperature Inversion refers to the unusual increase of temperature with altitude in the troposphere caused by either radiational cooling at the ground surface or subsidence of cool dense air on the surface.

Revision Questions

Draw the map of the distribution of global surface temperature in Oliver J. E. and Hidore J. J. (Climatology) pages 61-63 or in Henderson-Sellers and Robinson P. J. (Contemporary Climatology page 84)

Explain how temperature, evaporation and precipitation are measured in a standard weather station.

How does temperature inversion relate to atmospheric stability?

Examine the factors that determine the global distribution of each of the following elements; temperature, evaporation and precipitation

Further Reading

Barry, R.G. and Chorley, R. J. 1976. Atmosphere Weather and Climate. Methuen and Co. Ltd. 

Chritchfield, H.J. 1966. General Climatology, Prentice hall Inc.

Dann, W.L. 1975. Meteorology, McGraw-Hill Co. New Jersey.

Henderson-Sellers A. and Robinson P. J. Contemporary Climatology 

Knapp, B.J. 1981. Practical Foundations of Physical Geography, George Allen and Unwin, London

Lake, P. 1961. Physical Geography, Cambridge University Press

Mather, J.R. 1974. Climatology, Fundamentals and Applications, McGraw-Hill Co.

Muller; R.A. and Oberlander, T.M. 1984. Physical geography Today: A portrait of a Planet.

Oliver, J.E. and Hidore, J.J. 1984. Climatology: An Introduction. Bell and Howell Co. Toronto



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