What are the Different Layers of the Earth’s Atmosphere?
The Earth has a number of layers of atmosphere which are listed below.
- Troposphere: 0 to 11 km (0 to 7 miles)
- Stratosphere: 11 to 50 km (7 to 31 miles)
- Mesosphere: 50 to 80 km (31 to 50 miles)
- Thermosphere: 80 to 700 km (50 to 440 miles)
- Exosphere: 700 to 10,000 km (440 to 6,200 miles)
Commercial aircraft typically fly in the Troposphere and Stratosphere.
What is the Troposphere?
The Troposphere is the lowest portion of the atmosphere, and it is where the vast majority of weather is found. The height of the top of the Troposphere varies with latitude (it’s lowest over the poles and highest at the equator) and by season (it’s lower in winter and higher in summer). It can be as high as 20 km (12 miles or 65,000 feet) near the equator, and as low as 7 km (4 miles or 23,000 feet) over the poles in winter. In aviation, ISA stipulates the Troposphere ceases at 11,000 m (36,089ft).
The top of the Troposphere is called the Tropopause which is the boundary between the Troposphere and Stratosphere. Jet streams (very strong narrow corridors of air) are typically found in the Tropopause. The Tropopause is generally an ‘Isothermal’ zone where temperature remains constant with altitude.
What is the Stratosphere?
The Stratosphere is sometimes known as the Ozone Layer. This is where Ultra Violet (UV) light from the sun is absorbed by ozone particles and converted to heat. In the Stratosphere, the temperature increase with height (unlike the tropopause where the temperature reduces with height). Some large birds can fly within the Stratosphere such Swans and Vultures.
Commercial aircraft generally fly in the lower levels of the Stratosphere as it contains little moisture and most of the weather (which can cause turbulence) is found in the Troposphere and Tropopause below. Commercial aircraft can’t fly too high within the Stratosphere as their altitude can be limited by the amount of thrust that can be produced by the engines, which decreases as temperature increases.
What is the Mesosphere?
‘Meso’ means middle. The Mesosphere is responsible for protecting the Earth against asteroids and space debris and is where these objects would tend to burn up. The Mesosphere contains about 0.1% of the Earth’s air mass and still has a mix of gases found in the atmosphere. In this atmospheric layer, temperature decreases with height.
At the top of the Mesosphere, the coldest atmospheric temperatures can be found at around -143 °C / -225 °F. Neither military nor commercial aircraft can fly in the Mesosphere due to the lack of air mass, yet the small amount of air there is would still produce too much drag to allow orbit of the Earth.
What is the Thermosphere / Ionosphere?
The Thermosphere is incredibly hot due to the absorption of solar radiation. Temperatures can reach 2,000 °C / 3,600 °F. The Thermosphere is where the Kármán line can be found which defines where ‘Outer Space’ starts at 100 km / 62 miles above sea level. Some satellites, including the International Space Station (ISS) orbit the Earth in the Thermosphere.
The Thermosphere is also known as the Ionosphere due to the presence of Ions within this atmospheric layer. This occurs due to the photoionization of molecules caused by ultraviolet radiation. Some radio waves are reflected by the Ions which is why commercial aircraft can communication through High Frequency to stations beyond the horizon.
What is the Exosphere?
‘Exo’ means outside. The Exosphere is the highest, most outer layer of the atmosphere where the air is extremely thin. It is the transition between the Earth’s atmosphere and the vacuum of space. It is predominantly made up of Hydrogen and Helium but contains trace amount of other gases. It acts as a zone where the Earth’s gravity starts to be felt.
Temperature in the Exosphere varies significantly (between 0 °C and 1700 °C) and is much hotter in the day than night.
Can Passenger Jets Land in Heavy Rain?
Can passenger jets land in heavy rain?
Yes, generally speaking commercial passenger jets are capable of landing perfectly safely when it is raining heavily. The aircraft’s jet engines are designed to be able to operate flawlessly whilst ingesting huge amounts of water from rain. However, in some circumstances, the aircraft may not be able to land whilst it is raining very heavily. This isn’t because of the rain itself, rather cause and effect of the heavy rain. Very heavy rain can be an indicator of other weather phenomenon which pilots are trained to avoid. Such circumstances include:
Low Visibility Caused by Heavy Rain
For the vast majority of the flight, the pilots are using the aircrafts instruments to fly the plane and only revert to visual references shortly before landing (usually for about the final 500ft). Therefore, moderately reduced visibility due to heavy rain isn’t a problem. However, to land the aircraft manually, the pilots require a horizontal visibility of 550m. If the rain is extremely heavy, visibility may reduce to below this level. This then requires the pilots to carry out an ‘auto-land’ where the aircraft touches down with the autopilot engaged. Some aircraft and airports do not have the facilitates to support an ‘auto-land’ and therefore landing at the affected airport may not be possible.
Rain Caused by Thunderstorms / Cumulonimbus Clouds
Very heavy rain is often caused by a thunderstorm or cumulonimbus clouds. Whilst the heavy rain probably won’t stop the aircraft from landing safely, other hazards associated with a thunderstorm may well do.
Flying directly through or within the vicinity of a thunderstorm can result in windshear. Windshear is the sudden change in direction or velocity of the wind. If an aircraft encounters this, it can result in control difficulties and could even result in the aircraft stalling. Pilots will therefore avoid flying near thunderstorms where possible. This would include delaying the approach and landing phase if a thunderstorm was overhead the airport.
Thunderstorms also produce microbursts which can result in a very strong current of downward air which can push the aircraft towards the ground or result in a very rapid change in the aircraft’s airspeed. Microbursts can be associated with heavy rain showers. This is another reason why pilots avoid flying near thunderstorms.
Contaminated / Flooded Runway Caused by Heavy Rain
If the rain is very heavy and sustained, it can result in standing water building up on the runway. Whilst most aircraft can travel through water on the runway to a certain depth, it can get too deep. If it gets too deep, the aircraft is unable to brake sufficiently to stop the aircraft on landing. If necessary, the pilots would receive regular updates on the state of the runway and delay landing if needed.
Can Pilots Detect Heavy Rain?
Yes, all commercial passenger aircraft are fitted with a weather radar which detects the movement of water droplets. Pilots are therefore able to see exactly where the rain is located and how intense it is. They can therefore plan to avoid the area if they believe the cloud producing the rain is going to result in strong turbulence.
Is Heavy Rain Turbulent?
The rain itself doesn’t result in turbulence, but the cloud producing the heavy rain can produce strong turbulence. Pilots therefore seek to avoid clouds which are producing heavy rain showers. Clouds associated with short sharp showers (towering cumulous and cumulonimbus) tend to be much more turbulent than clouds associated with heavy, but sustained rain (nimbostratus).
What happens if the plane can’t land?
Pilots will be aware of the potential for heavy rain or thunderstorms to occur at the destination and may choose to take some extra fuel if they feel it is needed. Very heavy rain is typically associated with short lived showers and thunderstorms that usually pass through quickly. Extra fuel will allow the pilots to delay the approach and landing through taking up a holding pattern (sometimes referred to as a ‘stack’) away from bad weather such as thunderstorms. When the bad weather is clear they can then make the approach and landing.
If the weather doesn’t clear as quickly as hoped, the pilots may elect to divert the aircraft to another airport where the weather is better. The aircraft is always loaded with enough fuel to be able to fly to another airport in case it can’t land at the destination for whatever reason.
If you found this article of interest, check out our page on ‘Are thunderstorms dangerous?‘.
What’s the strongest wind passenger jets can fly in?
There is no single maximum wind limit as it depends on the direction of wind and phase of flight. A crosswind above about 40mph and tailwind above 10mph can start to cause problems and stop commercial jets taking off and landing.
It can sometimes be too windy to take-off or land. The limitations are in place for the safety of the passengers and crew.
Aircraft & Wind – An Overview
In aviation, speed is measured in Knots (Nautical Miles Per Hour). This is converted to Miles Per Hour (mph) by multiplying it by 1.15.
The maximum wind limits for commercial aircraft depend on the aircraft, airport and the direction of the wind compared to the direction of the take-off or landing.
For take-off and landing, aircraft have different limitations, and these are again split up into dry, wet or contaminated runway limitations. A contaminated runway is where is there is snow, ice or standing water on the runway.
Aircraft maximum wind limits are split up into:
- Crosswind component – the side element of the wind
- Headwind – amount of wind from the front of the aircraft
- Tailwind – amount of wind from behind the aircraft
- Total wind – total speed of the wind
Aircraft want to take off and land into a headwind as this reduces the distance they require to get airborne or distance need to bring the aircraft to a stop. If an aircraft is standing still on the runway, and has a headwind component of 20kts, that’s 20kts of air flowing over the wing and therefore giving the aircraft an airspeed of 20kts, even though it’s not moving. If it has a take-off speed of 140kts, the aircraft’s ground speed would only need to be 120kts to get airborne because it already has 20kts of airspeed from the wind.
In general, commercial aircraft demonstrate a capability to land with a certain crosswind at the aircraft certification stage. This demonstrated limit is not a maximum limit, but is the figure that most operators (airlines) will choose to use as a limit – it’s not sensible to have an uncapped figure. Therefore, the figures provided below for the B737-800 are typical of those dictated by an operator, not the manufacture.
On a dry runway, a Boeing 737-800 has a maximum allowable crosswind component of approximately 33kts. For taking off on a wet runway it’s about 27kts. The actual figure might be slightly above or below this because the airline can choose to set its own more restrictive value if it wishes. If the cross-wind component was greater than this, the aircraft might have an option to choose another runway which is more into wind, but in the case of a single runway airport, it wouldn’t be able to depart.
There is no headwind limitation for most commercial aircraft for take-off, and therefore is no maximum overall limit for take-off (or landing). If there was a 100mph wind, all of which was a headwind component, in theory the aircraft wouldn’t be restricted from taking off. However, the reality is that there are wind limits for opening and closing the aircraft doors (around 45kts) and no pilots would attempt to taxi and depart in such conditions. The airport would have closed in such circumstances anyway!
Tailwind vs Heading
The maximum allowable tailwind value is usually between 10-15kts. Tailwinds are easy to avoid at most airports, as if you simply take off in the other direction, the headwind has now become a tailwind. Tailwind has the opposite effects of a headwind, increasing the runway distance needed to take off and land.
However, at some airfields, it’s actually better to accept a tailwind on one runway rather than a headwind on another because of potential terrain issues. A good example for this is Florence (FLR). It’s actually better to accept a bit of a tailwind on runway 23 than a headwind on RWY 05 because of the very large hill/mountain in very close proximity to RWY05. If you were to lose an engine, you must still be able to meet a climb gradient guaranteed to get you above the terrain on RWY 05, which can mean you are very weight limited even with a headwind. You can actually increase your maximum take-off weight even by accepting a tailwind on RWY 23.
On a dry runway, the Boeing 737-800 crosswind limitation is the same as take-off, 33kts. On a wet runway this reduces to a maximum of 30kts.
The maximum tailwind component for take-off and landing is usually between 10-15kts, but the actual figure that can be used may be performance limited by runway length, aircraft weight etc.
In the event of a contaminated runway, both the maximum allowable crosswind / tailwind limits reduce, depending on the type and depth of the contaminant. Most airlines do not allow a tailwind take-off on a contaminated runway.
Some aircraft specify maximum taxi limits. For example, on the Boeing 737, the maximum taxi speed is 65kts.
There are limits on when the cargo and passenger doors can be opened. If the wind is more than about 45kts, it isn’t deemed safe to open the doors.
Other things that affect the maximum allowable wind:
- The airline might impose tighter restrictions than the manufacture. Therefore, a Boeing 737-800 airline might have different limits to another airline operating the same aircraft.
- Pilots have different limits. Captains can usually operate the aircraft up to the maximum specified limits but Senior and Junior First Officers will have more restrictive limits. The Captain will do the landing if it exceeds their limits.
- Some airports impose restrictions on wind limits. For example, at London City Airport, the maximum cross wind limit is 25kts, whereas some of the aircraft operating into there have a 38kts dry runway limit for take-off and landing. This is because it’s narrower and shorter than other airports.
- Aircraft design. An aircraft with a big vertical stabiliser (the big fin at the tail) is going to be more effected by a crosswind than one with a smaller one. An aircraft with winglets is also more likely to be affected by crosswind.
The airport itself may limit the maximum allowable wind limits. For example, at London City Airport (LCY), the maximum crosswind limit is 25kts for all aircraft because the runway is only 30m wide (compared to a standard width of 45m or sometimes even 60m at larger airfields).
Aircraft Type & Aerodynamics
The maximum wind limits between aircraft differ because of aerodynamic and handling differences. Generally speaking the larger the vertical stabiliser compared to the rest of the aircraft, the more the plane will be affected by a crosswind. Larger aircraft counteract this by having larger control surfaces therefore giving the pilots greater control authority. If you take the Boeing 777 for example, it’s crosswind limit for landing on a dry runway is 40kts – more than a Boeing 737.
Some of the same aircraft types even differ because of their configuration. For example, a standard A320 has a higher crosswind limit than an A320 with Sharklets (Airbus’s version of winglets), as the Sharklets are an additional vertical surface for the wind to act upon.
Is Turbulence Dangerous for Passenger Jets?
Whilst experiencing turbulence can be upsetting for some people, turbulence does not pose a danger to the structural integrity of the aircraft. The plane will keep flying even in severe turbulence and is only dangerous in extreme circumstances, which most pilots will never experience However, turbulence can be dangerous for the passengers if they are not seated with their seatbelt fastened as in very severe cases it can cause occupants to be thrown around the aircraft cabin resulting in injury.
Turbulence is probably the single most common cause of anxiety for airline passengers, yet it rarely causes the pilots any concerns about the safety of the aircraft. Turbulence is very common and is usually experienced to some degree every single flight. In all but the most extreme cases, turbulence is not a danger to the safety of the flight, rather more of an uncomfortable inconvenience. In essence, turbulence is annoying but rarely dangerous and your plane won’t crash because of it.
The best way to see how bad the turbulence is expected to be is to watch the Cabin Crew. If they have stopped the service and taken their seats, you know it’s about to get pretty bumpy!
What is Turbulence?
Turbulence is a bit like driving down a bumpy road in a car, except in this case it’s the air in the sky that’s a bit bump and rough. There are a number of different types of turbulence, some of which can be detected by most commercial aircraft, whilst other types are invisible and very difficult to predict.
Different Types of Turbulence
Convection (rising air) is the process which causes clouds to form. This process can also be responsible for creating turbulence which is associated with clouds, particularly cumulus cloud types. The greater the vertical extent of the cloud (i.e. the taller it is), the greater the up drafts and therefore the worse the turbulence is likely to be. It is particularly bad when flying through Cumulonimbus clouds (which are associated with very heavy rain showers / hail or thunderstorms).
Clear Air Turbulence
Clear Air Turbulence (nicknamed CAT) is caused by jet streams which are very strong corridors of wind found at high altitudes. They can reach in excess of 150mph and whilst this can dramatically reduce the flight time if the wind is behind you, they can also be responsible for strong levels of turbulence.
Jet Streams form between the boundaries of warm and cold air and therefore vary in position throughout the year. Whilst the position of the jet stream isn’t difficult to predict, the turbulence associated with it is very difficult to accurately pinpoint.
Low Level Thermals
This is hot air rising from the ground which is prevalent on hot days at low altitudes, particularly when over land. They are worst when the ground is at its hottest, so typically in the afternoon.
Unlike the other types of turbulence mentioned so far which are weather related, wake turbulence is phenomenon caused by other aircraft.
It is caused by the aerodynamic effects of flight through the interaction between low pressure at the top of the wing and high pressure at the bottom (needed to produce lift). The air moves from the bottom to the top of the wing at the wing tip which causes wing tip vortices. If you fly through another aircraft’s wake vortex, you might experience turbulence from it.
Wake turbulence occurs for a few seconds, and it can be very violent. Air traffic control apply minimum time or distance spacing between aircraft to ensure adequate wake turbulence separation. Very strong wake turbulence has caused aircraft to crash in the past which is why there is now a minimum distance or time separation between aircraft taking off or landing. When it’s windy, the wake turbulence disappears quickly, however when it’s calm, the wake lingers around for longer.
Generally speaking, when closer to the ground, the wind is subject to friction and its flow is effected by the Earth’s surface and anything on it (like buildings etc). Therefore, near the ground, the wind is less laminar (straight and steady) and more turbulent (varying in direction and speed). Rapid variations in direction and speed can cause turbulence at low level, particularly when the aircraft is on final approach coming into land.
Can Pilots Detect Turbulence?
Pilots can detect certain types of turbulence using an onboard weather radar. The radar highlights where there are large quantities of water droplets (rain) on the pilot’s instruments. Generally speaking the bigger the rain drops, the bigger the cloud, and the more turbulent it will be inside that cloud. The pilots make a judgement on if they can fly through the cloud or avoid it by flying around it.
Can Pilots Avoid Turbulence?
Sometimes. There are some types of turbulence which can’t be detected, like Clear Air Turbulence or CAT which is associated with a Jet Steam. Sometimes it’s forecast and the pilots can do their best to avoid it by avoiding certain flight levels, but sometimes the forecasts are wrong and the pilots fly into it without knowing it’s there. That’s why they ALWAYS recommend that passengers keep their seat belts fastened whenever seated, regardless of the status of the fasten belt sign.
Prior and during flight, the pilots will be studying various weather charts which predict where any areas of Clear Air Turbulence is located, or where any thunderstorm (cumulonimbus) clouds mights occur during the flight. It provides the flight crew with a rough location and the altitudes that turbulence may be encountered. The pilots can then take action at the pre-flight planning stage to adjust the planned flight level or routing of the flight if required.
The pilots will often put the fasten seatbelt signs on prior to experiencing any turbulence based on the forecast or following communication with ATC or other aircraft.
Pilot Actions in Turbulence
Passenger comfort is a high priority for pilots so when the aircraft enters turbulence they are almost always doing their best to get out of it. This is not because it’s dangerous, but because it’s uncomfortable for both the passengers and crew. The pilots don’t like it any more than you do. To get out of it, pilots are regularly speaking to air traffic control and other aircraft to see what levels are free of turbulence or where along the route it might subside.
Unfortunately on some days, turbulence is prevalent at all levels and is impossible to avoid. In extremely rare cases where there is severe to extremely turbulence, the pilots may decide to divert the aircraft and land, but this is very, very rare.
On most occasions, what the public would consider to be severe turbulence, is rarely more than moderate from the pilots point of view.
Turbulence is categorised into Light, Moderate and Severe. The official definitions from IATA are as follows:
Light Turbulence Definition:
Light turbulence might cause slight, erratic changes in aircraft altitude and/or attitude (pitch, roll, yaw).
- Liquids are shaking but not splashing out of cups
- Carts can be manoeuvred with little difficulty
- Passengers may feel a light strain against seat belts
Moderate Turbulence Definition:
Moderate turbulence can be defined as slight changes in aircraft altitude and/or attitude and a greater intensity of motion than light turbulence. However, the aircraft remains in control at all times.
- Liquids are splashing out of cups
- Difficulties to walk or stand without balancing or holding on to something. Carts are difficult to manoeuvre
- Passengers feel definite strain against seat belt
Severe Turbulence Definition:
Severe turbulence is defined as the aircraft being subject to large, abrupt changes in altitude and/or attitude. It usually causes large variations in airspeed and the aircraft may at times be out of control.
- Items in the cabin are falling over unsecured objects are tossed about.
- Walking around the aircraft is impossible
- Passengers are forced violently against seat belts
Are Thunderstorms Dangerous For Commercial Passenger Aircraft?
A look at how dangerous thunderstorms are for passenger jets
Can Thunderstorms Be Dangerous to Passenger Jets?
Yes, they can be dangerous to commercial aircraft as they can contain hail, heavy rain, lightning, ice, severe winds and super cooled water droplets, all of which may cause damage to the aircraft in extreme conditions. Pilots therefore do their very best to avoid flying through thunderstorms wherever possible.
What’s in a Thunderstorm?
Thunderstorms (Cumulonimbus clouds or ‘CBs’ as pilots refer to them) contain lightning, precipitation, hail, extreme turbulence, supercooled water droplets, ice, microbursts and violent winds, all of which can be hazardous to commercial passenger jets. They have caused passenger jets to crash before, and could possibly be a contributory part in a crash in the future.
Commercial aircraft usually get struck by lightning a few times a year. The damage the aircraft receives from a lightning strike varies. The structure of the aircraft is designed to dissipate the electric charge overboard, but the entry and exit points can cause damage to the aircraft’s skin. It can also potentially interfere with the aircraft’s electrical systems but it rarely causes any significant problems.
Severe icing is dangerous for commercial aircraft. Ice forming on the aircraft structure increase the weight of the aircraft, which in turn increases the stalling speed. It also disrupts the airflow over the wings, reducing the total lift produced by the wings. Lift created by the wings counteracts the weight of the aircraft which is what gets, and keeps the aircraft airborne. Most commercial aircraft are certified to fly through light to moderate icing but not severe icing. Pilots will therefore always look to avoid areas of severe icing. Thunderstorms can be particuarly dangerous as they may contain ‘Super Cooled Waterdroplets’. This is liquid which remains as a fluid despite it being below the freezing temperature. It freezes as soon as it comes into contact with a hard surface, like an aircraft, which can cause significant performance issues.
Large hail stones found within large thunderstorms cells can cause structural damage to the aircraft and its engines. They have been known to cause damage to the leading edge of the wings and nose cone as well as crack the windshield and damage the engines.
Updrafts / Downdrafts (Wind Shear)
These can disrupt the aircrafts flight path and airspeed, potentially causing the aircraft to overspeed (go to fast) or stall (go to slowly). It can also push the aircraft towards the ground .
A Microburst is a phenomenon found underneath a Cumulonimbus cloud, where a strong downdraft causes a large change in wind direction over a small area. An aircraft flying through a microburst will likely see a large increase in airspeed followed by a dramatic reduction. If the aircraft is close to its landing speed at this point, it’s flying close to its stalling speed. A sudden and dramatic reduction in airspeed is very dangerous to any aircraft.
Microbursts also have the potential to cause significant downdrafts which ‘push’ the aircraft towards the ground. In extreme circumstances, the force at which the aircraft is pushed downwards is more than the thrust force produced by the engines and the rate of descent can’t be arrested.
If you found this article interesting, you might find our page on ‘Can pilots detect thunderstorms?‘ to be worth a read.
Can pilots detect thunderstorms on a commercial aircraft?
A look at the weather radar capabilities on a commercial jet…
Can passenger jets detect thunderstorms?
Yes they can. Commercial Aircraft have weather radars onboard which allows the pilots to measure the size of the water particles in a cloud which in turn allows the flight crew to make an assessment on the type of cloud it is (i.e. a thunderstorm) and whether to avoid it or not. The radar returns are imposed on a map, with varying colours which signify the size of the water droplets.
A cloud with large water droplets signifies that there are large vertical updrafts associated with it, which is a sign that the cloud could be a Cumulonimbus which is potentially hazardous to aviation.
Pilots do their best to avoid thunderstorms as they can be dangerous to passenger jets.
How the weather radar works
A weather radar uses a doppler system, which sends out a beam which is reflected by the water particles. The level of reflection depends on the size of the water droplets.
Pilots should have a good idea before the flight weather they should be on the look out for adverse weather such as thunderstorms. In the preflight briefing stage, the flight crew look through the flight paperwork, which includes weather reports for the departure, destination and en-route phases of flight. One or more diversion (alternate) airfields are also looked at, ensuring the weather at these airports is suitable to land it should the aircraft not be able to land at it’s scheduled destination.
Pilots will actively avoid taking the aircraft into Cumulonimbus type clouds as they can be hazardous to aviation. They can be very turbulent due to the updrafts and downdrafts, contain icing, heavy rain and hail and lightning.
Generally speaking, it’s best not to overly a Cumulonimbus cloud, as it can be in the formation stage where it can potentially out climb an aircraft. They can extend to over 50,000ft over the Equator where troposphere is at its highest.