CLIMATOLOGY

C S TomarSc ‘C’

Satellite Application Unit National Satellite Meteorological Centre

Today’s Talk

Distribution of Solar Radiation over Earth Surface Distribution of Temperature over Earth Surface Distribution of Pressure over Earth Surface

SEASONAL AND LATITUDINAL VARIATION OF INSOLATION

The sun is highly condensed mass of very hot gases surrounded by a solar atmosphere of rarified gases. The diameter of the sun is 1.4 million km which is more than 100 times the diameter of the earth.

Although the temperature in the interior of the sun exceeds million degrees Celsius, the temp at the surface is 6000ºC.

The sun radiates approx 56 x 1026 cal/min of energy from its surface in different directions.

Intensity of the short wave radiations (0.2µm to 4.0µm) emitted by the Sun ( at about 6000K temp) peaks near 0.5µm (which lies in the visible part of the spectrum).

The average temp of earth is about 294K, so can emit long wave radiations in the range 4µm-80µm

The maximum intensity of Terrestrial radiation is around 11µm.

Solar Constant (S) The solar radiation intercepted by a planet varies inversely with the square

of its distance from the sun. At the mean distance of the earth from the sun (150 million km) a surface oriented perpendicular to the sun’s rays receives solar energy of 2.0 calories /cm2/min or 2Langly (energy received per unit area= 1Langly or 1ly = cal/cm²) per min. This quantity is known as the Solar constant for the earth.

In other words, the solar constant is defined as the flux of solar radiation at the outer boundary of the earth’s atmosphere that is received on a surface held perpendicular to the direction of sun at the mean distance between the earth and the sun. Flux refers to the total radiant energy of all wavelengths crossing the unit area.

Energy received at top of the Earth Atm If the surface, oriented perpendicular to the sun’s ray, is thought of a

circular non-rotation disc with a radius r (equal to radius of earth) the sun facing side of the disc will intercept the same amount of solar radiation as does the spherical shaped earth.

Since the area of a sphere is four time the area of the one side of the disc the amount of solar energy received at the top of the atmosphere will be 0.5 Cal/cm2/min or 0.5ly/min or one quarter of the solar constant.

Distribution of Solar Radiationover Earth Surface

The unequal distribution of solar radiation over the earth is the primary cause of weather and climate. The rate of receipt of solar energy varies with latitudes, seasons and time of the day i.e. the angle of sun’s rays with the surface of the earth.

Distribution of Solar Radiation without Atmosphere(At the top of the atmosphere)

In the low latitudes between two tropics, the intensity of solar radiation remains quite high throughout the year with little seasonal variation. The noon rays are vertical twice a year at all places situated between two tropics. As a result, the solar radiation curve shows two maxima and minima for the low latitudes.

Middle latitude curve (40º), which is broadly representative of the belts lying between 23½ º and 66½ º in each hemisphere, show a single strong maximum and a single minimum, both of which coincides with Solstices. The curve shows large seasonal variation.

The high latitude curve (80º), which represents belts poleward of Arctic and Antarctic circles, resembles that of the middle latitudes with one maximum and one minimum coinciding with solstices. The only difference in this curve is that it reaches zero during winter when there is no solar radiation.

Distribution of Solar Radiation with Atmosphere (without clouds)

Solar beam while passing through the atmosphere (without clouds), is depleted by scattering, reflection and absorption. Thus the amount of radiation reaching the surface of the earth is less than that received at the outer limits of the atmosphere.

The depletion is max at high latitudes due to higher obliquity of the solar beam resulting in the path of the beam passing through much greater thickness of the atmosphere than in the lower latitudes.

At the time of equinoxes the latitudinal distribution of solar radiation is symmetrically about equator with the amount decreasing to zero at the each pole.

During summer solstice the more nearly vertical solar beam and longer days combined together produce a broad maximum in middle latitudes of northern hemisphere and the same feature is observe in the middle latitude of southern hemisphere during the winter solstice.

Latitudinal variation of radiation is small in summer hemisphere compared to winter hemisphere.

Distribution of Solar Radiation with Atmosphere and Clouds

Clouds reflect a large amount of solar radiation reducing the amount of it reaching the surface of the earth.

The maximum total annual radiation is found not at the equator but rather at the latitude 20º N and 25º S.

The max solar radiation of about 220kcal/cm²/year is received in the eastern Sahara of North Africa (approx. 15-30N/15-35E).

The lesser amount near the equator in the southern hemisphere is due to great cloudiness and more Ocean surface in the Southern Hemisphere.

In the equatorial belt, area of least solar radiation coincides with the warm continents, where convective clouds are in abundant. Maximum solar radiation received in the sub-tropics which are relatively less cloudy region. In the high latitude the lowest annual radiation values are over ocean because of abundant cloudiness.

Global Energy Balance

Trenberth et al, BAMS 2009Trenberth et al, BAMS 2009

Distribution of Temperature over the Earth’ Surface

Factor influencing the distribution of temperature over the Earth’ surface are:

1. Latitude 2. Properties of Earth’s surface (reflectivity, heat capacity & conductivity).

3. Slop & topography4. Dynamic Factors

i) Latitude: The receipt of solar energy on the earth’s surface varies with latitudes. Angle of suns beam reaching a location and the length of the day are the major determinant factors of the receipt of solar energy. These two factors are depending on latitude.

ii) Properties of earth’s surface : The temp of earth’s surface is also influenced by following factors which depend on the

characteristics of surface.

a) Reflectivity: Surfaces with high reflectivity absorb less solar radiation diminishing the quantity of available energy and thus cause lower temperature.

b) Heat Capacity: With the same amount of available solar energy two different surfaces may have different temperatures. This is because different heat capacities of surfaces. The heat capacity of water is much higher than that of rock and the land surface. As a result land becomes hotter than water surface with equal amount of radiation. The soil moisture is also completely controlling the heat capacity.

c) Heat Conductivity: Some materials are good conductor of heat and some are not. Loose dry soil is poor conductor of heat with the result that superficial layer will experience rise in temperature. Whereas, water which is a fair conductor, permits the heat energy to penetrate to a greater depths there by causing less rise of temperature. When we apply these differences in the surface properties on a continental scale, it is evident that the land masses will heat up rapidly than the oceans in the summer. In winter reverse will follow. As heat storage o landmasses are less it will cool more rapidly than the oceans in the winter. As a result land masses tend to experience extreme temperature, while water bodies shows less change of temperature.

iii) Slopes & Topography Temperature of surface is depends on its slope too. A south facing slope at higher latitude in the

northern hemisphere receives more solar energy due to increase in inclination of solar beam than a north facing slope or plane land at the same latitude. Consequently, the south facing will experience higher temp than north facing slopes. Topography also plays an important role in influencing the temp of a place. An east-west oriented mountain range prevents northward movements of warm air from the lower latitudes during summer and the southward movements of colder air to the lower latitudes during winter in the northern hemisphere. The Himalayan ranges an east-west oriented mountain range in the northern boundary of India, prevents the penetration of very cold winds from the north during the winter season.

iv) Dynamic Factors The uneven distribution of energy between the tropical regions and the poles causes an exchange

of air through dynamic processes. Oceans too transport warm water pole wards and cold water equator wards through circulation known as ocean currents. Both atmosphere and ocean transports excess energy from the equatorial belt to higher latitudes through circulation.

Temperature over the Globe as a Function of Latitude

The broad feature of air temp over the earth’s surface may be considered as a function of latitude. The temp will be max in the equatorial region, decreasing gradually polewards and attaining minimum temp in the polar region.

During January, the hottest latitude is near 15°S and about 20°N in July. Thus the mean annual temp is distributed fairly symmetrically not w.r.t. the geographical equator but w.r.t. the meteorological or heat equator, 5°N.

During January the decrease in temperature with latitude in southern hemisphere is small because summer prevails in the southern hemisphere. Also the hemispheric mean temp in southern hemisphere is greater than the northern hemisphere.

Mean temp of Lat circles during Jan & July N S

Variation of temp with latitude is less in both hemispheres in lower latitudes than that of higher latitudes. Its variation from equator to pole is more in winter than summer. The annual mean temp is more in northern hemisphere than the annual mean temp of the southern hemisphere. In fact each latitudes of the northern hemisphere are warmer than the corresponding latitudes of southern hemisphere.

Annual Range: The difference between the mean temp of the warmest and coldest month is termed as annual range. The annual range of temp is more in northern hemisphere than southern because of the presence of more land mass in the northern hemisphere.

Daily Range: Daily range of temp is the difference between max and min temp. Daily range is much larger in clear days than on cloudy days. Small diurnal ranges are also characteristics of oceans and windward coasts, while stations in the interior of continents have large diurnal ranges. Over land the type of soil also influence the daily rang of temp. A sandy loosely packed soil, which inhibits the conduction of heat into or out of soil which gets heated up quickly during day and also cools quickly at night.

Mean Temperature Distribution over the Surface of the Earth:

(January and July)

The temp gradually decreases polewards from the equatorial region in both the months of January and July.

However temperature distribution is appreciably modified by the distribution of land and sea. We know that for several reasons a land surface heat and cools more rapidly than water surfaces. The land areas become hotter in summer and colder in winter than the sea areas at same latitude. The summer in northern hemisphere is somewhat warmer than southern hemispheric summer.

During January as well as July the isotherms run almost parallel to latitudes over the large ocean of the Southern Hemisphere. Elsewhere the isotherms are distorted in passing from land to see and vice-versa.

In July the hottest area is around Lat. 20°N. In the month of January the 30°C isotherm encloses only small area around Lat.

20°S over South Africa & Western Australia . In the month of January the lowest temperature is observed over north eastern parts

of the Asian continent and not in the polar region. Less than -45°C temperatures prevail over the Siberian Region. Such a low temperature region is absent in Southern Hemisphere during July because of the absence of large continents there. The lowest temperature in the Southern Hemisphere is observed over Polar region

MEAN PRESSURE DISTRIBUTION OVER THE SURFACE OF THE EARTH

Generally we find that the equalatorial latitudes are occupied by low pressure area and subtropical latitudes are occupied by high pressure area, which is known as subtropical High region. In the higher latitudes, the pressure again decreases giving rise to a low pressure region between 40° and 60° latitudes known as Sub polar Low Regions.

This general pressure pattern oscillates north-south with the N-S oscillation of the position of the Sun. Land and sea distribution produce High and Low pressure cells in these regions.

In winter the land areas are occupied by low pressure belts and High Pressure Cells prevail over major oceanic areas.

The atmospheric pressure over the entire hemisphere is greater in the winter hemisphere than in the summer hemisphere. Also, the pressure gradient in the winter hemisphere is greater than the summer hemisphere.

The subtropical belts of high pressure are more continuous in winter hemisphere. They are weakened over the heated continents in summer and are best developed over the oceanic regions. In the winter hemisphere, these subtropical highs become very prominent, extend to higher latitudes and occupy huge areas over the continents.

The characteristic pressure distribution of January month is the intense Siberian High around Lat. 45° and Long. 110°E; the Aleutian low over the Pacific and the Icelandic low over the Atlantic region.

In the Southern Hemisphere the low pressure regions lie over South America, South Africa and northern parts of Australia and the high pressure cells lie over the Pacific ocean, the Atlantic ocean and the Indian ocean around latitude 30°S.

July : The Siberian high is replaced by a low pressure region covering the entire southern parts of Asia and northern parts of Africa with the intense low pressure region over North India, South Pakistan, Southern parts of Iran and Arabian Peninsula. The Aleutian Low region over the Pacific is replaced by a high pressure cell and the Icelandic low weakens considerably.

Jan: In the winter, Southern Hemisphere the subtropical high pressure runs roughly along Lat.30°S with high pressure cells, one over Australia, second over the Indian Ocean and third one over the Atlantic off west coast of South America.

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