Temperature is a relative term implying a degree of molecular activity or heat of a substance. It is a measure of intensity of heat. Primary source of heat for earth and its atmosphere is the sun. One important consequence of radiation exchange is the development of a distinct temperature structure in the atmosphere.
Energy (Heat) Transfer:
Heat energy is transferred by three mechanisms:
(1) Conduction:
It is the heat transfer process when two bodies of unequal temperature are in contact. In this mechanism, heat passes from point to point by means of transfer of adjacent molecules motion. Since air is poor conductor, this type of heat transfer is virtually neglected in the atmosphere, but is important in the ground.
(2) Convection:
Heat transfer is through movement of particles (part of mass) in fluids and gases. These are able to circulate internally and distribute heated parts of the mass. The low viscosity of air and its consequent ease of motion makes this the chief method of atmospheric transfer.
(3) Radiation:
It is the processes of transmission of energy by electro-magnetic waves between two bodies without the necessary aid of an intervening material medium. This is so with solar energy through space, where the earth atmosphere only allows the passage of radiation at certain wave lengths and restricts at the others.
Other Heat Transfer Mechanisms:
Latent Heat:
Transfer of heat through a physical change in the medium such as water-to-ice or water-to-steam or steam-to-water or ice-to-water involves significant energy and is exploited in many ways; steam engine, refrigerator etc.
Heat Pipes:
Using latent heat and capillary action to move heat, heat pipes can carry many times as much heat as a similar-sized copper rod. Originally invented for use in satellites, they are starting to have applications in personal computers.
The distribution of temperature over earth’s surface depends on:
1. Incidence of solar radiation, which decreases from equator to the poles gradually.
2. Exchange of air between the upper and lower layers of atmosphere.
3. Relative distance of land and water surfaces.
4. Nearness to warm ocean currents.
Horizontal Air Temperature Distribution:
Horizontal variation in temperature is commonly shown by isotherms, the lines connecting points of equal temperature. The effectiveness of insolation on heating the earth surface is largely determined by latitude.
General decrease in temperature from equator towards poles is one of the most fundamental facts of climatology. The irregular distribution of land and water on earth surface tends to break up the latitude variation in temperature.
Land areas warm and cool more rapidly than do bodies of water leading to greater annual temperature ranges over land. Horizontal transport (advection of heat) occurs in the form of both latent heat (water vapour which subsequently condenses) and sensible heat (warm air masses). It varies in intensity according to latitude and season.
The general effect of the contrast in heating of land and water areas is to produce cooler winters and warmer summers in the center of continents than along coasts and over oceans. Coastal or marine climates tend to be moderate, experiencing no great extremes in either daily or annual temperature changes.
Through horizontal transport of ocean water in the form of currents and drifts, heat is carried from one part of the earth to another leading to variation in temperatures than would normally be expected for the latitude.
Neither the oceans nor ocean currents can influence temperature, unless the prevailing winds blow from the water to the land. Mountain barriers influence horizontal temperature distribution. Barrier effect of mountain ranges tends to restrict movement of air masses. On a local scale, topographic relief exerts an influence on temperature distribution.
Vertical Air Temperature Distribution:
Permanent snow caps on high mountains, even in tropics indicate the decrease of temperature with altitude. Temperature observations in upper air have shown fairly regular decrease in temperature with an increase in altitude which extends upwards to the troposphere. The average rate of temperature decrease is about 6°C km-1 between 4 and 6 km and 7°C km-1 between 6 and 8 km.
This vertical temperature gradient is referred to as normal lapse rate. It represents average of many observations at different places and times and should not be confused with actual lapse rate, indicated by a single observation over a given location.
Actual lapse rate does not necessarily show a decrease of temperature with altitude at lower levels. When there is no change with altitude, the lapse rate is termed isothermal. The rate at which the temperature changes as air rises or falls is called adiabatic lapse rate which is constant for dry air. The dry adiabatic lapse rate is about 10°C km-1.
Certain special conditions in lower troposphere may produce a reversal of the normal lapse rate so that the temperature actually increases with increase in altitude. This is known as temperature inversion and lapse rate is said to be inverted.
Temperature inversion near ground surface may be due to:
1. Radiation of heat from earth’s surface on a clear night or from surface in high latitudes results in increasingly cooler surface temperatures and consequently cooling of the lower layers of air.
2. Cold air from hill tops and slopes because of greater density tends to collect in valley bottoms, creating an inverted lapse rate up the slopes and free air over the valley floor.
3. When two air masses of different temperatures come together, the colder air being more dense tends to push underneath the warm air and replace.
Measurement of Air Temperature:
Temperature is a measure of hotness or coldness of an object. It is a relative term implying a degree of molecular activity or heat of a substance.
History:
Many methods have been developed for measuring temperature. Most of these rely on measuring some physical property of a working material that varies with temperature. One of the most common devices for measuring temperature is the glass thermometer. This consists of a glass tube filled with mercury or some other liquid, which acts as the working fluid.
Temperature increases cause the fluid to expand, so the temperature can be determined by measuring the volume of the fluid. Such thermometers are usually calibrated, so that one can read the temperature, simply by observing the level of the fluid in the thermometer. Another type of thermometer that is not really used much in practice, but is important from a theoretical standpoint is the gas thermometer.
Other important devices for measuring temperature include:
1. Thermocouples.
2. Thermistors.
3. Resistance temperature detector (RTD).
4. Pyrometers.
5. Langmuir probes (for electron temperature of a plasma).
6. Infrared.
7. Other thermometers.
Theoretical basis for thermometers is the zeroth law of thermodynamics which postulates that if you have three bodies, A, B and C, if A and B are at the same temperature and B and C are at the same temperature then A and C are at the same temperature. B, of course, is the thermometer.
Atmospheric/air temperature is measured by means of thermometers housed in a special wooden box called Stevenson screen (Fig. 2.14) fixed at about 1.22 m above ground level. It is a wooden rectangular box of length 56 cm, width 30 cm and height 40 cm with a double roof having lowered sides.
The screen is painted white and mounted on four wooden supports with the bottom of the screen at 1.22 m above ground level. The screen is set up with its door-facing north (opening downward) to minimise the sunlight entry at the time of recording observations.
Stevenson screen protects the thermometers from direct heating from ground and nearby objects and from losing heat by radiation during night. It allows free air circulation besides protecting the thermometers from rain.
Maximum and minimum thermometers are placed in horizontal position on the upper and lower sides of wooden box, respectively and bulb end rest at an angle of 2° to horizontal plane. Dry and wet bulb thermometers are kept vertical in the wooden box on the left and right sides, respectively.
Meteorological thermometers are:
Maximum Temperature:
Mercury in glass thermometer called maximum thermometer (Fig. 2.14) is used for measuring maximum temperature. The temperature range is from -35° to +55° C. A constriction in the capillary of glass tube, below the lowest graduation, allows the mercury to be forced with rising temperature but restricts its movement back with falling temperature. Hence, maximum temperature can be noted at a later time.
The thermometer should be set at 0700 hours (LMT). Reading of the maximum thermometer after setting should agree with that of dry bulb within 0.3° C. The setting is by giving mechanical jerks by holding the thermometer with hand.
Minimum Temperature:
Minimum temperature of air (lowest temperature of air during last 24 hours) is measured with a sprit-in-glass minimum thermometer (Fig. 2.14) ranging from – 40° to + 50°C. Upper end of the index in the spirit column, held by surface tension, gives the minimum temperature. The instrument is set at 1400 hours by tilting the bulb upwards. Reading of the minimum thermometer after setting should agree with that of dry bulb thermometer within 0.6°C.
Dry Bulb Temperature:
Mercury in glass thermometer called dry bulb thermometer (Fig. 2.14) is used for measuring dry bulb temperature (air temperature). The temperature range is from – 35° to + 55°C. Least count is 0.5°C. However, observation can be recorded up to 0.1°. This temperature is used for calculating humidity, vapour pressure and dew point.
Wet Bulb Temperature:
Temperature on cool air (wet bulb temperature) is measured with wet bulb thermometer (Fig. 2.14). It is similar to dry bulb thermometer, but the bulb of the thermometer acts as evaporating surface. The bulb of the thermometer is continuously kept moist by muslin cloth covering the bulb. Four strands of cotton thread placed in a small container with distilled water keeps the muslin cloth covered bulb continuously wet.
The temperature readings of both dry and wet bulb thermometers will be same under saturated conditions. However, when the air becomes dry, the difference would increase. The difference is known as wet bulb depression, which is used to know dew point, vapour pressure and humidity.
Thermograph:
The thermograph placed in Stevenson screen, is an automatic self-recording instrument which mark the prevailing temperature continuously on a graph paper wounded round a drum. The drum makes one revolution in a day, marking temperature changes. The graph paper is changed every day at 0830 hours IST.
Bimetallic thermograph (Fig. 2.14) consists of a sensitive element of two strips of different metals (brass and iron), welded together along the flat surfaces and bent into as arc. One end of the arc is fixed to the base of the instrument and the other connected to bend arm, which traces the changes on graph paper. Changes in temperature causes the two metals to expand or contract differentially so that they bend or unbend accordingly.
True surface temperatures can be obtained from satellite observations of upwelling wind in selected channels in the atmospheric winds. The results, after the correction for atmospheric transmission losses and surface emissivity are true surface temperatures, integrated over the field of view of the sensor.
Units of Temperature Measurement:
Temperature measurement, using modern scientific thermometers and temperature scales, goes back at least as far as the early 18th century, when Gabriel Fahrenheit adapted a thermometer (switching to mercury) and a scale both developed by Ole Christensen Romer. Fahrenheit’s scale is still in use in the USA, with the Celsius scale in use in the rest of the world.
The basic unit of temperature (T) in the International System of Units (SI) is the kelvin (K). Absolute zero is defined as being precisely 0 K and -273.15°C. Absolute zero is where all kinetic motion in the particles comprising matter ceases and they are at complete rest in the “classic” (non-quantum mechanical) sense. At absolute zero, matter contains no thermal energy. Also, the triple point of water is defined as being precisely 273.16 K and 0.01°C.
This definition does three things:
(i) It fixes the magnitude of the Kelvin unit as being precisely 1 part in 273.16 parts the difference between absolute zero and the triple point of water.
(ii) It establishes that one Kelvin has precisely the same magnitude as a one degree increment on the Celsius scale.
(iii) It establishes the difference between the two scales’ null points as being precisely -273.15 kelvins (0 K = -273.15°C and 273.16 K = 0.01°C).
For everyday applications, it is very often convenient to use the Celsius scale, in which 0°C corresponds to the temperature at which water freezes and 100°C corresponds to the boiling point of water at sea level. In this scale, a temperature difference of 1.0 degree is the same as a 1 K temperature difference, so the scale is essentially the same as the Kelvin scale, but offset by the temperature at which water freezes (273.15 K).
1. Fahrenheit scale fixes the boiling point of water at 212° and melting point of ice at 32°.
2. Centigrade scale fixes the boiling point of water at 100° and melting point of ice at 0°.
3. The above two scales indicate the same temperature at – 40° F.
4. The value of each degree on the Kelvin (K) or absolute zero scale equals that of centigrade degree but absolute zero – 273° C
5. Centigrade temperature may thus be converted to absolute, simply by adding 273 (20° C = 20° + 273° = 293° K).
Conversion of Scales:
Conversion of one scale into another can be with the following formula:
Meteorological observatories measure temperature and humidity of air near the surface of the earth usually using thermometers placed in a Stevenson screen. The true daily mean, obtained from a thermograph, is approximated by the mean of 24 hourly readings (which is not the same as the mean of the daily minimum and maximum readings).
Comparison of the three temperature scales in use is given in Table 2.11:
Normal human body temperature is 36.8 ± 0.7°C, or 98.2 ± 1.3°F. The commonly given value 98.6°F is simply the exact conversion of the nineteenth-century German standard of 37°C. Since it does not list an acceptable range, it could therefore be said to have excess (invalid) precision).
Temperature and Photosynthesis:
Effect of temperature on net photosynthesis is of vital concern in crop production. The rate of this complex physiological process is usually measured with popularly known temperature coefficient (Q10)- It is defined as the ratio of rate of a process at a certain temperature to that at a temperature 10°C lower.
It is expressed as:
Q10 = K2/K1
where, K2 = rate at T°C
K1 = rate at T°C -10°C.
Photosynthesis consists of light reaction and dark reaction. The latter has a higher Q10 than the former which has Q10 of about 1.0. At lower temperature range, dark reaction is limited while at higher ranges light reaction is likely to be limited. Thus, the rate of photosynthesis rises fast from 5° to 40°C, has Q10 of about 1.0 at 20°C and tends to level off after 25°C.
Above 30°C, the rate usually drops due to inadequate CO2 and heat convection of enzymatic reactions in dark phase. On the other hand, respiration increases with temperature having Q10 of 3.0 and the increase is maintained in the range in which true photosynthesis begins to level off and decline.
As such, higher temperatures have more adverse influence on net photosynthesis than lower temperatures, leading to decreased production of photosynthates above a certain temperature.
Lower temperature from panicle initiation to flowering leads to formation of more number of grains per plant, mainly due to prolonged duration of the period. For many grain crops, active leaves present at flowering are associated with production of photosynthates for grain formation. Temperature is a factor affecting net assimilation rate, the rate of production of dry matter per unit leaf area.
Hence, higher grain yields are possible only when maximum leaf area index (leaf area per unit land area) and net assimilation rate occurs by flowering stage. In many grain crops, considerable fraction of dry matter in the grains comes from photosynthesis in the ears. As such, influence of temperatures during flowering assume importance. Thus, optimum temperature at each growth phase leads to realisation of yield potentials of crops.
Relationship between Temperature and Plant Development:
The relationship between the amount of heat and plant development was first quantified by Reaumur in 1935 by summing up mean daily temperature during each plant development phase. This did not prove practically significant, but laid foundation for the concept of day degree, growing degree days, degree days, heat unit or heat sums, which is a measure of relative warmth of growing season of a given length.
A heat unit is the departure from the mean daily temperature above the minimum threshold temperature. The minimum threshold temperature is the temperature below which no growth takes place. It usually ranges from 4.5 to 12.5°C for different crops, though the most commonly used value is 6.0°C.
A degree day is obtained by subtracting the threshold temperature from daily mean temperature. Summation of the daily values over the growth period gives degree days for the crop. Though the concept of degree days is simple it has limitations.
1. It assumes linear relationship between temperature and growth which is not correct.
2. No allowance is made for threshold temperatures at different phases of crop growth.
3. The concept does not allow phenological requirements and occurrence of low or high temperature.
4. It does not take the effect of light on growth, probably the better index.
It has been found that photothermal units obtained by multiplying the day degrees with photoperiod (photothermal units = day degrees x average hours of day length) are more stable than day degrees.
In spite of limitations in the concept of day degrees, it is popular because of its simplicity in the fields of agriculture for guiding the farmers in the areas of:
1. Forecast of planting and harvesting dates and yield.
2. Potential areas for a crop and a cultivar can be known even without experimentation.
3. Wherever possible, micro-climate can be modified to produce nearly optimum conditions for increasing the yields.
4. Incidence of diseases and insect pests can also be forecast.
Temperature Variations Across the Country:
The mean annual temperature across the country varies from less than 10°C in extreme north to more than 28°C. As such, temperature is not a limiting factor for crop production over a major part of the cultivated area.
Temperature starts to increase over the country from March onwards and reaches a peak in May and June. It increases from 20 days in March to 30 days in May. By May, many parts of the country record mean daily maximum above 40°C. On individual days, temperatures can be over 46 to 55°C as in 2003.
During the active monsoon months (July-September), the mean temperatures over the country remain around 28 to 30°C. Tamil Nadu in south, however, is characterised by high temperatures during this period clue to low monsoon activity. In winter season, low temperatures are in January with mean around 14°C in northern parts of India and around 27°C in southern parts.
During winter, northern states like Bihar, Uttar Pradesh, Punjab, Himachal Pradesh, Rajasthan, northern Madhya Pradesh experience minimum temperatures less than 10°C. These regions are prone to occasional frost during rabi. Lower minimum (up to -60°C) are recorded in Ladakh region of Dras, which is one of the coldest regions of the world in winter.