The theory subject "Meteorology" deals with the weather and all related aspects and their influence on aviation. It deals with the atmosphere, fog and clouds, rain and wind, icing, thunderstorms, larger climatic contexts, altitude measurement in aviation and how to prepare for the weather accordingly.
The atmosphere
The Earth's atmosphere, a gaseous envelope consisting mainly of nitrogen and oxygen, is crucial for flight and life on Earth. Despite its relative thinness compared to the Earth's circumference, it enables aerodynamic effects such as the lift required for flight. The atmosphere is divided into different layers, the temperatures of which significantly influence the weather.
The troposphere, the lowest layer, is relevant for weather patterns. Here, the temperature decreases with altitude until it reaches the tropopause, where it remains constant or even increases. The height of the tropopause varies depending on latitude and season.
Air pressure, another important component, decreases with altitude. It is determined by the weight of the air masses over a surface and is measured using a barometer. The barometric altitude, which indicates the difference in altitude for a change in pressure, is important for aviation.
The air temperature is a measure of the kinetic energy of the air molecules and varies with factors such as time of day, latitude, season and cloud cover. Heat is generated by sunlight and the ground, which warms the neighbouring layers of air.
Air density, defined as the ratio of mass to volume, is dependent on air pressure and temperature. It decreases with increasing altitude.
Humidity, an important parameter for weather conditions, is the proportion of water vapour in the air. The air's ability to absorb moisture increases with the temperature. Relative humidity is the ratio of the current humidity to the maximum possible humidity at a certain temperature. The dew point is the temperature at which the air is saturated and condensation begins. Weather phenomena such as fog formation are closely related to the difference between the current temperature and the dew point.
To summarise, the atmosphere is a complex system whose understanding is of fundamental importance for aviation. It influences important factors such as lift, weather and flight conditions.
After the general explanation of the atmosphere, the standard atmosphere defined by the ICAO is then discussed. The ICAO Standard Atmosphere (ISA) is a reference model for typical atmospheric conditions that is used for flight performance evaluation and instrument calibration. ISA conditions are defined as follows:
- An air pressure of 1013.25 hPa at sea level (MSL).
- A temperature of 15 °C at sea level.
- An air density of 1.225 kg/m³ at sea level.
- A relative humidity of 0%, which means pure air without water vapour.
The ISA defines an average temperature decrease in the troposphere of 2°C per 1,000 feet increase in altitude up to an altitude of 11 km (36,000 feet), where the tropopause is located. Above the tropopause, the temperature remains constant at -56.5°C.
Assuming a relative humidity of 0% in ISA simplifies the model, as humidity in the real world can vary greatly both in time and location. It is important to note that high temperatures and humidity in the real world can reduce air density, which in turn affects flight performance.
In summary, ISA provides a standardised altitude reference system for aviation based on global averages and simplifications such as assuming a humidity of 0%. Deviations from these standard conditions can have a significant impact on flight performance and must therefore be taken into account during flight preparation.
Fog and clouds
In the troposphere, water vapour is crucial for weather phenomena such as cloud formation, fog and reduced visibility. Visibility conditions are relevant for take-off, landing and visual flight rules. In addition to water vapour, dust particles can also affect visibility.
In aviation meteorology, the distinction between different views is essential, as they directly influence the execution of visual flights. While general visibility describes the recognisability of objects up to a defined distance, flight visibility refers to the estimated view from the cockpit in the direction of flight and is crucial for avoiding collisions. Ground visibility, which is measured on the ground, and runway visibility, which is determined from the cockpit along the runway, are particularly relevant for approaches and departures.
Fog, a major factor in visual impairment, can take different forms. Radiation fog, the most common type in Central Europe, is caused by nocturnal radiation and the subsequent cooling of the air layers near the ground. Advection fog forms when warm, moist air is transported over a colder surface, typically in coastal regions. Mixing fog occurs when warm, moist air mixes with colder air, often in connection with fronts. Evaporation fog occurs when cold air flows over warm, moist surfaces, which leads to an increase in humidity and thus to fog formation.
These different types of fog and visibility conditions require pilots to have a deep understanding and careful consideration when planning flights. Knowledge of the formation, characteristics and possible duration of fog is crucial for safe navigation and dealing with sudden changes in visibility, especially during approaches and departures.
Fog can also simply be regarded as a special type of cloud. In aviation meteorology, air movements and the resulting temperature changes play a decisive role in cloud formation. Understanding the physical processes that lead to different forms of cloud cover is essential for assessing weather-related hazards in flight operations.
The atmosphere consists of different layers of air, with the troposphere being particularly important for weather conditions. Here, the temperature decreases on average with altitude, but there are also areas with temperature increases (inversions) or constant temperatures (isotherms). The stratification of the atmosphere influences the stability and lability of the air masses, which in turn influences cloud formation and weather patterns. Stable air masses tend to return to their original state, while unstable air masses intensify a certain effect.
Adiabatic processes describe temperature changes in air masses during vertical movements without heat exchange with the environment. During ascent, the air cools down, whereby the cooling rate depends on whether condensation takes place or not. Dry adiabatic ascent occurs without condensation with a cooling rate of around 1°C per 100 metres, while wet adiabatic ascent with condensation has a lower cooling rate of 0.6°C per 100 metres on average.
Clouds are formed when rising air cools down to its dew point and the moisture it contains condenses. This can be triggered by various mechanisms:
- Thermal formation: Air masses heated by solar radiation rise and cool down, which can cause cumulus clouds to form. The height at which condensation sets in and clouds form (cumulus condensation level) can be estimated by multiplying the spread (temperature difference between temperature and dew point) by certain factors.
- Formation at air mass boundaries: When different air masses meet, as in the case of warm or cold fronts, air masses are forced to rise, which can lead to the formation of clouds.
- Orographic formation: Air is forced to rise by mountains, which leads to dust clouds on the windward side and often to foehn weather with clear skies on the leeward side. Foehn is a warm downslope wind that is caused by the different cooling rates during ascent (moist adiabatic) and descent (dry adiabatic).
In aviation meteorology, understanding different cloud types and classifications is essential for careful flight planning. Clouds cannot always be clearly categorised due to their uniqueness, but they do have certain similarities that are relevant to flight safety, such as icing and turbulence risks and precipitation patterns.
Clouds are initially divided into cumulus and stratus clouds according to their formation. Swelling clouds, characterised by a uniform lower boundary and uneven upper boundary, are formed by convection in unstable weather conditions or forced lifting at orographic obstacles. They are known as cumulus or cumulo. Layered clouds, caused by uniform lifting or cooling of a stable air mass, are usually visually recognisable as an extended layer without internal structure and are given the partial designation stratus or strato.
Clouds can be categorised into three levels according to their height and vertical extent: lower, middle and upper levels. In the lower storey, clouds are almost exclusively made of water, in the middle storey there are mixed clouds of water and ice, and in the upper storey clouds consist mainly of ice. The storeys are defined by the lower boundary of the clouds. Multi-level clouds, such as cumulonimbus or nimbostratus, are often associated with heavy precipitation and other weather phenomena.
Visibility conditions in the vicinity of clouds are particularly important for flight planning. Visibility can be impaired by increasing humidity near the cloud base and by precipitation. Low temperatures in the vicinity of clouds can also harbour the risk of icing. Source clouds can cause turbulence, whereas the lateral areas next to them are usually free of such hazards.
Overall, cloud science in aviation meteorology covers a wide range of aspects, from their formation and classification to the associated weather conditions, which are important for flight planning and execution.
Rain and wind
This chapter of aviation meteorology explains the different types of precipitation and the associated hazards for pilots. There are two main types of precipitation: showers that fall from convective clouds and surface precipitation from stratus clouds. Showers are usually strong but short-lived and occur in cumulus clouds, which are characterised by strong vertical movements of the air. These clouds are often localised and can be associated with strong updraughts and downdraughts and reduced visibility. Surface precipitation, on the other hand, is longer lasting and falls from stratiform clouds. They are often associated with low visibility and low cloud bases, which makes flying more difficult.
Special forms of precipitation such as hail, freezing rain and snow harbour additional dangers. Hail forms in high-reaching cumulus clouds and can cause damage if it hits an aircraft. Freezing rain, which freezes immediately on contact with the aircraft surface, is one of the greatest meteorological hazards as it leads to heavy ice accumulation. Snow usually occurs in winter and can severely reduce visibility, but is less dangerous in the air than freezing rain.
Precipitation can also occur on the ground in the form of dew or frost, which occurs mainly in the morning hours as the air cools down. This can affect the aerodynamic properties of an aircraft and must be removed before take-off.
Knowledge of these types of precipitation and the associated hazards is of crucial importance for flight planning and the safe execution of flights.
The chapter also deals with the formation and properties of wind and its significance for flight operations. Wind is caused by horizontal air movements that equalise pressure differences in the atmosphere. These pressure differences can be caused by differences in the heating of the earth's surface or by large-scale movements of air masses.
The most important forces that influence the wind are the pressure gradient force, the Coriolis force and the frictional force. The pressure gradient force accelerates the air from the high to the low pressure area and is stronger the greater the pressure difference. The Coriolis force, which arises due to the Earth's rotation, deflects moving air masses to the right in the northern hemisphere and to the left in the southern hemisphere. Near the ground, this equilibrium is disturbed by frictional forces, which leads to a deceleration and change in direction of the air flow.
The wind speeds are given in knots (kt) or, for gliding, also in kilometres per hour. The wind generally blows parallel to the isobars. Close to the ground, however, the wind deviates from the isobars by about 30° in the direction of the low pressure area. At an altitude of approx. 3,000 feet, the wind turns 30° to the right compared to the ground wind and doubles its speed.
The surface winds mentioned in weather reports are average values over a period of 10 minutes. They can fluctuate greatly, especially at high wind speeds.
Turbulence, defined as the noticeable response of an aircraft to air movement, can be caused by various factors such as ground surface conditions, other air traffic or thermals. Light turbulence includes short-term impacts such as gusts, while wind shear refers to long-term changes in wind direction and/or wind speed over short distances or altitude intervals.
Finally, typical regional winds such as land and sea winds as well as mountain and valley winds are discussed. Land and sea winds are caused by differences in the heating of land and water, with land winds blowing at night and sea winds during the day. Mountain and valley winds, on the other hand, are a result of the diurnal cycle of solar radiation in the mountains, with mountain winds blowing at night and valley winds blowing during the day.
Overall, it is crucial for the safe execution of a flight to have an understanding of the formation of wind and its various forms in order to be able to react appropriately to the associated challenges.
The climate
The weather in Central Europe is characterised by alternating calm phases and areas of precipitation. This dynamic results from the encounter between polar cold air and subtropical warm air, which leads to large-scale lifting and precipitation. Understanding these air mass boundaries is essential for pilots, as they often cannot be flown around or over and have a significant influence on weather patterns.
Air masses are classified according to temperature, humidity and origin. Polar air masses come from the north and are cold, subtropical air masses come from the south and are warm. Maritime air is moist and originates from oceans, while continental air is dry and characterised by land masses. The advection process describes the horizontal movement of these air masses, which can change the temperature and humidity in the atmosphere.
Climate refers to long-term atmospheric patterns defined by average conditions over decades. Weather, on the other hand, is the short-term state of the atmosphere. The intensity of solar radiation, which varies according to the angle of incidence, influences global circulation and leads to different weather conditions.
In low-pressure areas that form along polar fronts, where cold and warm air masses meet, typical weather phenomena such as warm and cold fronts occur. Warm fronts bring long-lasting rain and cloudy weather, while cold fronts cause rapid deterioration in the weather with showers and thunderstorms. The area between a warm front and a cold front is known as the warm sector and has moderate to good visibility. Occlusions occur when a cold front catches up with a warm front, lifting the warm air off the ground. They can be either warm or cold fronts, depending on the temperature differences between the air masses involved.
The weather patterns in Central Europe are also specifically addressed. Both high and low pressure areas play a central role here. These pressure areas are defined relative to their surroundings, whereby the absolute pressure is less decisive.
Dynamic high-pressure areas, also known as warm highs, are created by large-scale sinking processes in the atmosphere. In summer, they lead to warming and the formation of an inversion layer, resulting in clear weather. In winter, however, these high pressure areas can lead to fog or high fog.
Thermal high pressure areas, also known as cold highs, are formed by strong cooling processes, especially in winter over continental regions at high latitudes. Intermediate highs form between two areas of low pressure when cold air causes an influx of mass and thus an increase in pressure.
High pressure bridges, troughs and cold air drops are other phenomena that influence the weather in Central Europe. They each bring their own characteristic weather conditions, such as showers and thunderstorms with cold air drops.
The climate in Central Europe is characterised by the westerly wind zone of the temperate latitudes, with the main direction of the pressure patterns running from west to east. The weather alternates between maritime and continental influences, which is typical of a temperate climate zone.
Regional winds such as the Föhn in the foothills of the Alps, the Mistral in the Rhone Valley, the Bora on the Adriatic coast and the Scirocco in the northern Mediterranean also influence the weather in specific areas of Central Europe. They are caused by the special interaction of high and low pressure areas with the local geography and lead to unique weather phenomena, such as strong winds or significant amounts of precipitation.
Ice
Icing of aircraft parts is one of the greatest dangers during flight. It is important to understand icing processes, recognise icing conditions and take preventive measures. Icing often occurs in clouds and can affect various parts of the aircraft, in particular the wings, elevator, cockpit windows, propeller blades and antennas. Visual flights are less frequently affected than instrument flights, as they should take place outside of clouds.
Icing depends on temperature, humidity and droplet size. It is particularly dangerous at temperatures between 0°C and -12°C, where supercooled water freezes on aircraft surfaces. Higher altitudes with temperatures below -30°C pose a lower risk of icing, as there are mainly ice crystals that do not adhere to the aircraft surface.
There are different types of ice formation: Rough ice is formed by small ice crystals that are deposited as a milky white coating, especially on the front surfaces of the aircraft. Clear ice forms when supercooled water hits the surface and freezes, resulting in a clear, compact layer of ice that adds weight and impairs aerodynamic properties. Rime can form on the ground and requires thorough removal before flight. Carburettor icing, another hazard, can also occur at temperatures above 0°C.
Icing has a significant impact on flight physics as it increases the mass of the aircraft, shifts the centre of gravity and changes the aerodynamics. This increases the stall speed and can be particularly dangerous during take-off and landing. Ice formation can also impair visibility and block rudders or aerials.
Careful weather advice and analysis is necessary to prevent icing. Ice must be removed from the ground before flying, and areas at risk of icing should be avoided when flying. If you accidentally fly into icing, it is crucial to leave the icing zone as quickly as possible. Some aircraft have de-icing systems, but the general rule is: icing must be avoided at all costs during visual flights.
Thunderstorm
Thunderstorms are one of the most impressive and dangerous weather phenomena, and understanding them is essential for pilots to recognise and avoid dangers in flight. Thunderclouds derive their energy from warm air and condensing moisture. They require a high-reaching, moisture-labile atmospheric stratification, high humidity and a lifting mechanism to form.
Thunderstorms go through a life cycle of around 1.5 to 2 hours, consisting of a build-up, maturity and dissipation stage. The build-up stage is characterised by strong updrafts and a vertical expansion, without precipitation. In the ripening stage, the typical thunderstorm anvil forms and lightning, thunder and heavy precipitation occur. In the dissipation stage, the winds weaken and the precipitation decreases.
Thunderstorm hazards include electrical discharges, strong turbulence, heavy precipitation and icing. Air mass thunderstorms, such as heat thunderstorms, occur within a homogeneous air mass due to strong warming or uplift on mountains. Frontal thunderstorms occur at air mass boundaries, especially at cold fronts, and are often associated with intense thunderstorm lines.
It is important for pilots to avoid thunderstorms. They should be able to recognise the development of thunderstorms and take appropriate action, such as flying around or waiting out thunderstorms. Individual warm thunderstorms can be flown around, while flying around thunderstorm fronts is often impossible. Frontal thunderstorms are particularly dangerous and should be taken into account when planning a flight.
Height measurement
The altimeter is an essential instrument in aviation, based on the principle of air pressure measurement. It essentially works like a barometer, but its display is specially calibrated for altitude measurement and is calibrated according to the ICAO standard atmosphere. The displayed altitude is correct if the actual atmospheric pressure and temperature conditions as well as the air pressure at sea level correspond to the ICAO standard values. It is important to distinguish between the displayed altitude and the true altitude, especially when preparing for a flight.
The altimeter displays a relative altitude in relation to an adjustable pressure surface. Deviations from the standard conditions require an adjustment of the altimeter display with regard to pressure and temperature. There are different altitude indications: Height (QFE), Altitude (QNH), QFF and Flight Level (FL, QNE). Height (QFE) indicates the altitude above the aerodrome, Altitude (QNH) the altitude above sea level, while FL (QNE) is used for flight levels in higher air layers. QFF is the actual air pressure reduced to sea level.
The correct altitude is determined by pressure and temperature corrections, as deviations from the standard atmosphere affect the altimeter display. If the air pressure is lower than the standard value, the true altitude is lower than displayed. At higher temperatures than the standard value, the true altitude is higher than displayed. The density altitude influences flight performance - higher density altitude leads to poorer performance, lower density altitude to better performance.
It is important to carefully check and adjust the altimeter display to ensure correct altitude readings during the flight and, in particular, to guarantee safety during the flight.
Meteorological flight preparation
Meteorological flight preparation is part of all flight planning and is required by law. It includes obtaining up-to-date weather information, especially for cross-country flights, and is essential even in good weather at the take-off airfield. Various weather charts and reports, such as synoptic weather charts, forecast charts, ground weather charts, altitude weather charts and significant weather charts, provide detailed information about the weather situation. In addition, specific reports such as GAFOR, GAMET, AIRMET or SIGMET are used. METAR and TAF are important reports for aerodrome weather forecasts. During the flight, current weather can be called up by radio via VOLMET, and ATIS provides landing information at the destination airport. The inclusion of all this weather information is crucial to ensure the safety and efficiency of the flight.