8.1 - Physics of the Atmosphere

Question 1

State the different regions of the atmosphere and how temperature
changes within each region as a function of altitude

Answer 1

Troposphere - 2^C decrease per 1000 ft

Tropopause - constant -55 to -60 ^C

Stratosphere - Gradual rise in temperature

Ionosphere - Mesosphere and Thermosphere

Question 2

Why is there no weather in the Stratosphere?

Answer 2

The water vapour content in the stratosphere is zero

Question 3

What is the adiabatic lapse rate?

Answer 3

Adiabatic lapse rate is the 2^C decrease in temperature per 1000 ft of altitude

Question 4

Calculate the outside air temperature at 25,000ft assuming the
temperature at sea level is 10°C

Answer 4

-40 ^C

( 25,000 / 1000 = 25)

(25 * 2^C = 50 ^c

10 - 50 => -40^C

Question 5

What is the percentage of oxygen at 5,000ft?

Answer 5

21% Oxygen

Question 6

What layer of the atmosphere is used to bounce radio waves across
the surface of the earth?

Answer 6

Ionosphere

Mesosphere and Thermosphere

Question 7

At sea level on a standard day what is the pressure in Mb, HPa,
Lbs. per sq. in., Hg and mm of Hg?

Answer 7

Mb = Millibars = 1013.2 Mb

HPa = Hecto Pascals = 1013.2 Hp

psi = pounds per sq inch = 14.7 psi

Hg = inches of mercury = 29.92 Hg

mm Hg = mm of mercury = 760 mm Hg

Question 8

What is the name of the layer of atmosphere dividing the
troposphere from the stratosphere?

Answer 8

Tropopause

Question 9

What instruments are used to measure atmospheric pressure?

Answer 9

Aneroid altimeter measures Pressure altitude /atmospheric pressure

Barometer

Question 10

What will happen to density when pressure increases?

Answer 10

Air density varies in direct proportion with the pressure

(density increases as pressure increases)

Question 11

Density varies inversely with temperature. What does this mean?

Answer 11

Air Density varies inversely with temperature

(Density decreases as temperature increases)

Question 12

What is the air temperature at sea level on a standard day?
Give in °C, °F and °K

Answer 12

15 ^C

59^F

288K

Question 13

What is humidity?

Answer 13

Humidity is a condition of moisture or dampness

The higher amount of water vapour that the air can hold depends entirely on the temperature; the higher the temperature of the air, the more water vapour it can absorb.

Question 14

14. If the temperature and pressure remain the same, the density of the
    air varies with the humidity. What does this statement mean?

Answer 14

By itself, water vapour weighs approximately five-eights as much as an equal volume of perfectly dry air. (Hydrogen is extremely light, H₂0)

On damp days the density is less than it is on dry days

Question 15

What instrument is used to measure humidity?

Answer 15

Hygrometer is used to measure the aount of water vapour in air.

Question 16

Why is the air lighter when there is moisture in it?

Answer 16

By itself, water vapour weighs approximately five-eights as much as an equal volume of perfectly dry air.

Therefore, when air contains 5 parts of water vapour and 95 parts of perfectly still dry air, it is not as heavy as air containing no moisture. This is because water is composed of hydrogen ( an extremely light gas) and oxygen. Air is composed principally of nitrogen, which is almost as heavy as oxygen.

Question 17

How is relative humidity expressed?

Answer 17

As a percentage

This measurement is the ratio of the amount of water air will hold at this particular temperature.

Question 18

Explain how density is affected by temperature, and pressure, versus altitude?

Answer 18

As altitude increases, both temperature and pressure decrease, the former tending to increase air density and the latter to decrease it. One effect does not exactly compensate for the other because the pressure drop affects density more than the temperature effect.

The net result is a gradual decrease in air density with increasing altitude

Question 19

What is density measured in?

Answer 19

Kg / m³ (metric)

Ib / ft³ (imperial)

Question 20

What effect will a drop in density have on the operation of an
aircraft?

Answer 20

If air density drops there will be a reduction in: -

1. The lift produced by wings
2. The thrust produced by propellers / Jet Engines
3. The power or torque produced by piston engines.

(The last two of these is significant when checking engine operation on the ground)

Question 21

What is pressure altitude?

Answer 21

Pressure altitude is the altitude in the standard atmosphere corresponding to a particular value of air pressure.

(With the altimeter of an aircraft set at 29.92 in Hg, the dial will indicate the number of feet above or below a level where 29.92 in Hg exist, not necessarily above or below sea level, unless standard day conditions exist.

Question 22

What is density altitude?

Answer 22

Density altitude is pressure altitude (Standard day) corrected for differences in temperature.

The density altitude will differ from the pressure altitude when the temperature is not standard.

Question 23

In the troposphere, there is a decrease of temperature with an increase of altitude approximately ___^C per ____ ft (Lapse rate)

Answer 23

2^C

1000ft

Question 24

The temperature in the Tropopause remains reasonably constant at -__^C to -__^C

Answer 24

-55^C to -60^C

Question 25

The air temperature on a standard day at sea level is defined as ___^C

Answer 25

15^C

Question 26

The amount of water vapour in the air may be measured by the use of an instrument called a _____

Answer 26

Hygrometer

Question 27

What effects air density more than the other?

Pressure

Temperature

Answer 27

Pressure drop affects density more than the temperature effect. The result is a gradual fall in air density with increasing altitude.

Question 28

ICAO stands for?

Answer 28

International Civil Aviation Organisation

Question 29

The standard atmosphere approximates the average existing at ___^ latitude ____,

Answer 29

40

latitude North

Question 30

The standard sea level conditions are : ( 40^ Latitude North)

Pressure at 0 altitude (P₀) =

Temperature at 0 altitude (T₀) =

Answer 30

Pressure at 0 altitude (P₀) = 29.92 inches of mercury (Hg)

Temperature at 0 altitude (T₀) = 15^C

Question 31

The speed of sound differs with _____

Answer 31

Temperature

Question 32

ExplainDew point

Answer 32

Dew Point is the temperature to which a body of air must be lowered before the water vapour condenses out and becomes liquid water. (fog)