L1 Properties of the Atmosphere

Flashcards on Properties of the Atmosphere

Meteorology

The study of the atmosphere and its phenomena.

Purpose of Meteorology in Pilot Training

Equips pilots to interpret weather information for safe planning decisions.

Atmosphere

A mixture of gases held to the Earth by gravity.

Percentage of Nitrogen in the Atmosphere

Approximately 78%.

Percentage of Oxygen in the Atmosphere

Approximately 21%.

Maximum Percentage of Water Vapor in the Atmosphere

Up to approximately 4% in hot, humid regions.

Gases with Greatest Influence on Weather

Water Vapor, Carbon Dioxide, and Ozone.

Troposphere

The layer closest to the Earth, where most weather occurs.

Tropopause

The region at the top of the troposphere.

Seasonal Variation of Tropopause Height

Higher in summer than in winter.

Global Variation of Troposphere Depth

Most shallow over the poles and deepest over the equator.

Stratosphere

The layer above the tropopause.

Main Properties of a Gas

Temperature, pressure, and density.

Temperature

A measure of the degree of heat present.

ICAO Standard Unit of Temperature

Degrees Celsius (°C).

Atmospheric Pressure

The force exerted by the weight of the atmosphere.

ICAO Unit for Pressure

Hectopascal (hPa).

Unit for Pressure Used by UK Met Office

Millibars (Mb).

Boyle's Law

A gas's pressure and volume are inversely proportional.

Density

The amount of matter in a given volume.

Unit for Air Density

Grams per cubic meter (g/m³).

Relationship between Air Density, Pressure, and Temperature

Air density is directly proportional to pressure and indirectly proportional to temperature.

Convection

Vertical movement in the atmosphere.

Effect of Temperature on Air Movement

Warmer air rises, colder air sinks.

Temperature Variation with Altitude

Decreases with increasing altitude under normal circumstances.

Rate of Temperature Change with Altitude

Approximately 2°C per 1000 ft.

Lapse Rate

The rate of temperature change through the atmosphere.

Temperature in the Stratosphere

Assumed to remain constant with altitude.

Dominant Factor Affecting Density at Altitude

The change in pressure has a more marked effect.

Celsius to Fahrenheit Conversion

Divide by 5, multiply by 9, and add 32.

Fahrenheit to Celsius Conversion

Subtract 32, divide by 9, and multiply by 5.

International Standard Atmosphere (ISA)

An 'average' set of atmospheric conditions.

Mean Altitude of Tropopause in ISA

11 km or 35,000 ft.

ISA Sea-Level Air Temperature

+15°C.

ISA Sea-Level Pressure

1013.25 mb/hPa.

ISA Temperature Lapse Rate

1.98°C/1000 ft.

ISA Sea-Level Density

1.225 kg/cubic meter.

ISA Deviation

The difference between actual air temperature and ISA temperature.

Effect of Reduced Density on Aircraft Performance

Reduces aircraft performance.

Cause of Temperature Inversion

Widespread descending air warms, trapping cooler air below.

Isothermal Layer

Temperature remains constant with altitude.

Temperature Inversion

Temperature rises with altitude.

Root Cause of Weather Accidents

Ignoring or failing to account for forecast bad weather.

Water Vapor

The invisible gas in the atmosphere.

Extent of the Troposphere

From the surface upwards to the tropopause.

Measurements of Thermo-hygometer

Temperature, pressure, and humidity.

Humidity

The amount of water vapor in the atmosphere.

Moist Air

The air the contains water vapor.

Air Density and Temperature

Air which is warmer than its surroundings will be less dense and so will rise.

Altitude

The height above the earth's surface.

OAT Warmer than ISA

ISA + 7°C

OAT Limitation

Flight in conditions beyond ISA +30°C is not permitted

ISA Deviation Calculation

the difference between the calculated ISA value for temperature at a specific height and the actual OAT.

widespread Descending Air

May trap cooler denser air below.

Mixing Temperature of Warm Air and Cold Air

As warm air and cold air mix we can find ourselves in a situation where as we climb we may enter a layer of the atmosphere where the temperature remains constant

Layer where the temperature rises as you climb

This would be a temperature Inversion

Meteorological section of the PPL course

Interpret weather information and so make safe planning decisions based on it

Ability gained from meteorological section of PPL course

Assess the actual weather occurring, and make sound decisions based on what you see

Weather Systems

Complex interactions between many elements

Weather

Major factor in a large proportion of general aviation accidents.

Taking it on or having a go

Is merely inviting the inevitable conclusion to a one-sided contest

Weather accidents

The root cause of 'weather' accidents is nearly always lack of judgement

Form of water vapor

invisible gas

Most weather

Clouds, fog, mist, rain, snow etc

Pilots interest of the Stratosphere

Certain types of wind

Thermo-Hygrometer

Measure temperature, pressure and humidity

Atmospheric pressure

Defined as the pressure exerted by the weight of the atmosphere

Hectopascal and Millibars

The two units are equal i.e 1 hPa = 1 Mb

Dry

Air density is greater than moist air

Relationship

Is crucial to the development of vertical movement in the atmosphere

Collectively

These individual molecular forces results in atmospheric pressure

From the suface of the earth

The further away from the surface of the Earth the lower the air pressure

Troposphere

Heated by the earth from below

Lapse Rate

No lapse rate in the stratosphere

10°C 50°F

Thereafter allow 9°F for each 5°C

Graphs and conversion tables

Gives a better chance of arriving at an accurate answer

Who needs a constant calibration to work to

Aircraft designers, instrument manufacturers

ISA temperature

Can be calculated at any altitude up to 36 090 ft

Increased altitude

Reduced density, which reduces aircraft performance

Height or altitude

Convert to 1000's of feet if necessary and then the 2°C per thousand feet approximate lapse rate applied

39 000 ft

ISA difference is therefore '11.5°C' BUT the OAT is warmer than the ISA (-45°C is 'warmer' than - 56.5 °C) thus the ISA deviation is + 11.5 °C

Normally

The temperature of the atmosphere falls by 1.98 (approximately 2) degrees per 1000 ft as we climb

The better you understand meteorology

The better your judgement will be

Required understanding

These basics that are covered in the first few lessons of this subject

Sombre Note

The weather is a powerful and ruthless force when it turns against you

Approximate

The proportions of gases in a dry atmosphere

Relatively small amounts of Water Vapor (H2O), Carbon Dioxide (CO2) and Ozone (03)

Have the greatest influence over the weather

Stratosphere

The tropopause effectively acts as a lid

Temperature

Is a measure of the degree of heat present

Pressure, Temperature and Density

A fixed relationship between the properties of the gases present in our atmosphere

Image

Indicates the relationship of reducing air pressure with increasing altitude

Temperature

Indicates the 'standard' temperatures at various altitudes

Throughout this book

Temperature is referred to in terms of degrees Celsius

Overcome problems

ISA has been established

Marked

Performance tables for an aircraft may be marked ISA, ISA +10°C, ISA =10°C

Given

Altitude or flight level

Widespread descending air will warm up as it descends

This may trap cooler denser air below

If you appreciate the inherent unpredictability

Treat weather forecasts with a reasonable degree of caution

Dealing with bad weather is all about

Pilot judgement

Tropopause

The temperature is assumed to remain constant regardless of altitude