Humidity

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 understand the concept of Humidity which is one of the important topic.

HUMIDITY

1. Water vapor is always present in the air to a greater or lesser extent, in the troposphere.

2. Water evaporates into the air from oceans, lakes, vegetation etc. It ascends and forms clouds which cause precipitation.

3. Water exists in three phases: the gas (water vapor), liquid (rain, drizzle, shower) and solid(snow, hail).

4. The capacity of air to hold water vapor depends largely on temperature and to some extent on pressure. Higher the temperature, higher is the capacity of air to hold water vapor.

5. Dry Air: Air that contains no water vapor. Exists in upper troposphere or stratosphere.

6. Moist Air: The normal air that we breathe. It is also called unsaturated air.

7. Saturated Air: When air holds maximum water vapor, it is called saturated air.

8. Vapor Pressure: The partial pressure exerted by water vapor in the air. If p is the total pressure of air and e is the vapor pressure, then p-e is the pressure of dry air.

9. Saturated Vapor Pressure: It is the pressure exerted by water vapor when air is saturated.

10. Absolute Humidity: It is the actual amount of water vapor contained in a given volume of air at a given temperature. It is expressed as g/m3.

11. Humidity Mixing Ratio: It is defined as the mass of water vapor contained in a given mass of air. It is expressed as g/kg.

12. Humidity Mixing Ratio for Saturated Air: It is defined as the maximum mass of water vapor that can be contained in a given mass of air at a particular temp and pressure. It is expressed as g/kg.

13. Relative Humidity: It is defined as the ratio, in percentage, of the actual water vapor present in the air to the maximum it can hold at the same temperature and pressure.

14. RH= HMR x 100/HMR for saturated air.

15. RH=Vapor pressure of Air x 100/Saturation vapour pressure of Air.

16. Wet Bulb Temperature (Tw Tw): It is the lowest temperature which air would attain by evaporating water into it to saturate it. Desert Coolers work on this principle.

17. Dew Point Temperature (Td Td): It is the lowest temperature to which air should be cooled at constant pressure to saturate it with respect to water. Cooling below dew point causes condensation.

18. Frost Point: It is the temperature to which air must be cooled to reach saturation with respect to ice. Cooling below the frost point causes formation of hoar frost.

Important Points

1. As the temperature of the air increases, the amount of water vapour required to saturate it also increases.

2. At subzero temperatures water molecules have more energy and greater degree of freedom than ice, consequently the saturation vapour pressure over water drops is more than that over ice particles.

3. If water drops and ice particles co-exist, water drops will evaporate and condense on the ice particles. This explains rainfall from clouds which extend above 0oC and have both super cooled water drops and ice crystals co-existing.

4. Small water drops can exist in super cooled state up to -40o C.

5. For saturated Air( in Fog, Rain ) Air Temp(TT)=TwTw=TdTd

6. For unsaturated air: Dry Bulb Temp>Wet Bulb Temp>Dew Point