Temperature

For a PPL, CPL & ATPL aspirants who wants to become a good pilot, during flying prior to departure or enroute, earth atmosphere plays a vital role for safety of the aircraft. To ensure this, pilot has to be well versant with earth atmosphere & other topics related to aviation meteorology. Study Kwik eLearning module of Aviation Meteorology helps the PPL, CPL & ATPL aspirants to go through the one of the important topic temperature in a detailed manner.

TEMPERATURE

1. Temperature is a measure of heat.

2. It is measured by means of thermometer in degrees Celsius or Fahrenheit. These scales are arbitrarily fixed with reference to the melting point of ice and the boiling point of pure water at normal pressure. On the Celsius scale these are respectively as 0⁰ C and 100⁰ C and in Fahrenheit scale as 32⁰ F and 212⁰ F. Celsius scale is used internationally, in aviation and science. Use of Fahrenheit scale is confined to English- speaking countries only.

3. Heat is a form of energy. As heat is extracted from a substance, its internal energy is reduced and the random motion of its molecules slows down. The molecules get arranged in a more orderly pattern than before. As more heat is extracted the cooling and orderliness increases. Finally a state is reached when no more heat can be extracted and the molecules reach their maximum orderliness and the molecular motion almost ceases. The lowest temperature is reached. This minimum temperature is the same for all substances, and is accordingly called the absolute zero (K). 1K=-273.16 ⁰ C.

Conversion to C, F & K

1. F= 9C/5 + 32
2. C= 5/9 (F-32)
3. K= C+ 273
4. Note that -40 ⁰ C = -40 ⁰ F

Instruments for Measurement

1. Dry Bulb Thermometer, Wet Bulb Thermometer (the bulb is covered with a muslin cloth which is kept moist), Maximum Thermometer (like doctors Thermometer), Minimum Thermometer and Thermograph, are used for measuring temperature.

2. Mercury is used in thermometers except in Minimum Thermometer in which alcohol is used instead of mercury.

3. In Thermographs and in Upper Air temperature measurement, bi- metallic strips are used.

4. A minimum thermometer has dumbbell shaped iron index, which permits alcohol to pass through it when the temperature rises. When temperature falls the alcohol, which has a conclave meniscus, drags the index back to indicate minimum temperature.

5. Surface temperature is recorded at a height of 4 ft(1.25m) from the ground in shade(inside a Stevenson’s Screen).

Dry Air & Moist Air

1. Air which has no water vapour content is called dry air. Such air may exist at very high levels. For all practical purposes unsaturated air is called dry air.

2. With water vapour <4 % (RH <100%) air is called unsaturated or dry.

3. With water vapour >4 %(RH 100%) air is called saturated air.

Heat & Temperature

1. Specific Heat: It is defined as the heat required to raise the temperature of unit mass of a substance by 1⁰C. The specific heat of water, regarded as the highest is 1, that of ice 0.5 and of soil 0.2.

2. Latent Heat: It is defined as the amount of heat absorbed or released during change of phase from/to solid/liquid/vapour. It is absorbed during change from solid to liquid and liquid to gas and released during change of phase from vapour to liquid and liquid to solid.

3. When water changes to vapour, a certain quantity of heat is supplied. To change boiling water into vapour, more than five times as much heat is required as is needed to bring the temperature of ice cold water to a boil. Once the boiling begins, the temperature remains constant and the heat released is stored as latent heat. It is released as latent heat when the water vapour condenses to water.

Heat Transfer Processes

1. Heat is transferred from one place to the other by conduction, convection and radiation.

2. Conduction: In this Process heat is physically transferred by the molecules by contact. Conduction is important process of heat transfer very close to the ground.
3. Convection: In this process heat is bodily transferred to the colder part of the fluid. As more than 70% of the earth is covered with water, hence the importance of convection. In the atmosphere free Convection is triggered by intense solar heating and the forced Convection by topography.

4. Radiation: Everybody radiates heat at its temperature. In this process of heat transfer the medium is neither affected nor required. The solar radiation directly heats up the earth without affecting the atmosphere.

5. Other Methods: Advection, Latent Heat release, Turbulence, Up and downward motion of air are the other methods of heat transfer.

Diurnal Variation of Surface Temperature

1. Diurnal variation of temperature is more over land areas than over sea/coastal areas.

2. Sea Surface temperature shows an average variation from day to night of less than 1⁰C, whereas over land the variation may average as much as 20⁰C.

3. Variation is max when wind is calm.

4. Due to nocturnal cooling the surface temperature continues to fall even after sunrise till a balance is reached between the incoming and outgoing radiation. The balance occurs a little after sunrise.

5. Cloud cover can hamper both cooling and heating.

6. Cloudy nights are warmer than clear nights

7. Cloudy days are less warm than clear days.