Temperature is a measure of heat. Temperature along with pressure is one of the most important variables that affect the atmosphere. The temperature changes that occur on the earth’s surface initiate both air movement and cloud development .
Apart from its importance in the generation of weather phenomena, temperature has a direct bearing on aviation. Engine performance and cooling systems depend on the variation of temperature in the atmosphere. The performance of an aircraft (both piston and jet type) is affected by the density of air, which in turn is inversely proportional to the temperature at constant pressure. High temperature implies lower density and so has an adverse effect on engine performance. This effect is usually greatest during take off, but it should also be considered at other stages of flight, especially for jet aircrafts.
Genesis of Atmospheric Heating
The sun converts hydrogen into helium, this chemical reaction releases large amount of energy which radiates into the universe. Part of this energy strikes the earth and is changed into heat leading to the formation of other forms of energy. The earth experiences some of this energy as air and weather instability.
Solar Radiation consists of em waves. All substances having a temperature above -273º C emit some kind of radiation. All em waves travel at same speed, that is speed of light, however, the wavelength and frequency may be different.
Solar Radiation is mainly short waves. The white light consists of various wavelength colours (VIBGYOR), in which the longest wavelength is of Red and the shortest of Violet. In addition to visible light the solar radiation consists ofwavelengths too short to be seen by eye. The wavelength beyond the violet spectrum is called UV. and those on the other end beyond red are called Infra Red (IR). UV and IR, is responsible for all the heat that Earth receives fromsun as short waves. IR is about 46% and Visible light 45 % and UV 9%.
Laws of Radiation
Stefan’s Law It states that the total amount of energy radiated by a black body is proportional to the fourth power of its absolute temperature. Hence intense radiation is emitted by hot bodies like sun.
E ∝ T4
Wien’s Law The wavelength of most intense radiation is inversely proportional to the absolute temperature. Therefore, sun radiates in short waves and earth radiates in long waves.
A substance which absorbs all em radiation falling on it and then emits max radiation is called a black body.
A black body in thermal equilibrium has two notable properties:
- It is an ideal emitter: at every frequency, it emits as much energy as, or more energy than, any other body at the same temperature.
- It is a diffuse emitter: the energy is radiated isotropically independent of direction.
The sun and the earth are assumed to behave like black bodies.
The percentage of reflected radiation is called albedo. Albedo is the fraction of solar energy (shortwave radiation) reflected from the Earth back into space. It is a measure of the reflectivity of the earth’s surface. Ice, especially with snow on top of it, has a high albedo, most sunlight hitting the surface bounces back towards space.
Wavelengths are measured in micrometers (micron) and radiations greater that 1 micron is called long wave radiation, while less than 1 micron radiations are called short wave radiation.
50% of the solar radiation received by the earth and its atmosphere is reflected while the remaining 45% is converted to heat.
The ability of any body to absorb radiation from another source depends upon the nature of the body as well as the wavelength of the radiation. Except for ozone, the other constituents of air cannot appreciably absorb the radiation received from the Sun. During daytime this radiation passes through the atmosphere without heating it and reaches the surface of the earth where it is absorbed. The earth re-radiates this energy in longer wavelengths. The atmosphere is capable of absorbing this radiation in longer wavelengths. Thus, although the Sun is the primary source of heat, the source from which the atmosphere gets heat is the earth. The earth being the secondary source of heat, temperature in the lower atmosphere decreases with distance from earth, i.e. with height.
The above is only a simplified explanation. In actual fact dry air is transparent even to the longwave radiation from the earth. On the other hand, water vapour absorbs a part of this longwave radiation. However, it is found that the actual rate of decrease of temperature with height in the atmosphere is smaller than the one which would result from a process of heat transfer from the earth by longwave radiation only. Other important methods of heat transfer from the earth to the atmosphere are:
- Latent heat of condensation of water vapour, which has been transported upwards
Measurement of Temperature
Scales of measuring temperature
The scales commonly used in meteorology to measure the temperature are the Celsius, Fahrenheit, and Absolute Scales, such as the Kelvin and Rankine Scales. The Celsius Scale is used in Canada and Europe for all meteorological temperature readings. In the US, temperatures are given in Fahrenheit and Celsius scales.
- Celsius scale (0°C – 100 °C)
- Fahrenheit scale (32 °F – 212 °F)
- Absolute scale (273 A – 373 A)
Relation between °C and °F
- C=5(F-32)/9 and F=9C/5+32
Relation between °C and A
- C=A-273 and A=C+273
P.S. -40º C = -40ºF
The Celsius and the Fahrenheit scales are equal at -40º
As heat is extracted from a substance, its internal energy is reduced and the random motions of its molecules slow down. The molecules get arranged in a more orderly pattern than before. More the extraction of heat more is the cooling and so more is the orderliness. Finally a state is reached when the molecules attain their maximum orderliness and the molecular motion almost ceases. After this state is reached, no more heat can be extracted and the temperature of the object reaches its lowest value. This minimum temperature is same for all substances, and is called Absolute Zero (K) = -273.16ºC
Measurement of Temperature
Thermometer is the instrument used to measure the temperature
Types of thermometers
- Dry bulb thermometers – Air temp
- Wet bulb thermometers – Temp attained by evaporation
- Maximum thermometers – Max temp of the day
- Minimum thermometers – Min temp of the day
Air temperature is the temperature of free air in a shade (where there is no direct sunlight) at a height of 4 feet (1.25 M) above ground level, so that there is no influence of ground temperature..
Thermometers are installed inside the Stevenson’s screen to get the above mentioned conditions.
Temperature Variations in the Atmosphere and the Surface
Differences in amount of solar energy received by the various regions of the earth throughout a day or year cause temperature variations that power our dynamic atmosphere. The sources of variations are:
Diurnal Variation. During the day, solar radiation exceeds terrestrial radiation and the surface of the earth becomes warmer. At night, solar radiation ceases but terrestrial radiation continues and cools the surface. Warming and cooling of the atmosphere occur as a result of this diurnal imbalance.
Seasonal Variation. Because the axis of the earth is tilted to the plane of its orbit, the angle at which the solar radiation strikes the each varies from season to season. The Northern Hemisphere receives more solar energy in June, July and August and is therefore warmer and receives less in December, January and February and is therefore cooler.
Latitude. The sun is more directly overhead in equatorial regions than at higher latitudes. The tropics consequently receive the most radiant energy and are warmer than the polar regions where the slanting rays of the sun deliver less energy over a given area.
Topography. Land surfaces absorb more solar radiation than do water surfaces and radiate it more readily. Land surfaces therefore warm up more rapidly during the day and cool more rapidly at night. All land surfaces do not, however, absorb radiation at a uniform rate. There is great variation in radiation absorption by varying types of land surface. Wet soil, such as is found in swamps and marshes, is almost as effective as water in suppressing temperature changes. Heavy vegetation acts as an insulation against heat transfer. The greatest temperature changes occur over arid, barren surfaces such as deserts and rocky plains.
Some of the solar radiation is reflected back out to space by the earth’s surface and is not absorbed at all. Some of this reflection is due to the angle at which the radiation strikes the surface but the principal cause of reflection is the type of surface. A snow surface, for example, can reflect 90% of the radiation.
Clouds greatly affect temperature. A layer of clouds will reflect a high percentage of the incoming solar radiation back out to space, drastically reducing the amount of energy reaching the earth to warm it. On a cloudy night, the clouds absorb then outgoing terrestrial radiation and radiate a considerable pan of it back to earth, hindering the escape of heat.
Maximum and Minimum Temperatures
There is a cycle to the temperature during the day, The lowest temperature occurs 1/2 hrs after dawn after the earth has been cooled all night. By 1400 hrs Local Time, the temperature stabilises and this is the maximum temperature time. (varies from 1400- 1600 Hrs).
Temperature decreases with height in the troposphere being lowest at tropopause. Tropopause is higher over the equator (16-18 km) and lower over the poles (8-10 km). Above tropopause, in the stratopause temperature increases with height being heighest at stratopause (In order of 0ºC). Further above in the mesosphere, temperature again decreases with height being lowest over mesopause at about 85 km where the lowest temperature in the atmosphere is found (ABOUT – 90ºC).
In Troposphere the temperature decreases ast the altitude increases. This is known as the Lapse Rate. The average temperature decrease (or av Lapse Rate) in troposphere is usually given as 2º C/ 100 ft (1.98º C).
Concept of ISA Deviation
This is a concept where we compare the actual temperature at any level in the atmosphere with the theoretical ISA value at that level.
The Difference is then called ISA deviation.
If the actual temperature is more than the ISA value than the ISA Deviation is positive, and if Actual temperature is less than the ISA value than the ISA Deviation is negative.
Altitude = 31000 ft
Actual Temp = -37ºC. Find ISA Deviation
ISA temp at 31000 ft = 15 – (2x 31) = -47º C, whereas the actual temp is -37ºC, i.e. 10ºc above standard
Therefore, The flight is being operated in ISA + 10