- Weather is what you actually experience anytime you step outside. Weather consists of the short-term changes within the Earth’s atmosphere. There are multiple facets to weather: sleet, rain, freezing rain, sunshine, thunderstorms, and hail just to name a few. When people think of weather, they tend to think in terms of temperature, humidity, precipitation, cloudiness, visibility, and wind. Weather can change minute-to-minute, hour-to-hour, day-to-day, and season-to-season.
- Climate, however, is what you expect to happen when you step outside. Climate is the study of long-term patterns of weather within a particular area. Climate studies consist of the averages of precipitation, temperature, humidity, wind velocity, and other measurements. The study of climate and the changes that come with it are truly important. A slight change within a region’s climate can have varying affects on human health, animals, and many types of ecosystems.
As you probably have learned in school, warm air rises. This is because heat causes air molecules to spread apart. Consequently, cool heavy air tends to sink to replace the warm lighter air. The differences in air density caused by these changes in temperature result in the change of atmospheric pressure. This, in turn, creates perpetual motion within the atmosphere, both vertically and horizontally, thus ultimately creating wind and wind currents. It’s these constant air movements that create the chain reactions for what we eventually refer to as weather.
Why is this important? Because a basic knowledge and understanding of the forces that create weather can help a pilot make calculated decisions during flight planning after s/he receives a weather briefing.
The atmosphere (Fig 1) is made of 5 distinct layers that contain gases which envelope the Earth and help protect life from radiation and the vacuum of space.
- The Troposphere - This is the lowest part of the atmosphere (the part we live in). It contains most of our weather and contains about 75% of all the oxygen in the atmosphere, and almost all of the water vapour. When air expands it cools: so air higher up is cooler than air lower down.
Why is this important? Because turbulence is generated by thermals rising from the Earth’s surface. The turbulence created by these thermals help to redistribute the heat and moisture created by the Sun.
The Stratosphere - This layer contains much of the ozone in the atmosphere. The absorption of ultraviolet (UV) radiation from the Sun occurs within this layer and helps to protect us from skin cancer and other health damages.
The Mesosphere - This is the coldest atmospheric layer surrounding the Earth. It burns up most meteors and asteroids before they are able to reach the Earth’s surface. In the lower mesosphere the zonal winds are blow from the north to the south, whilst in the upper mesosphere zonal winds are blown from east to west. This is also the layer from which one of the coolest things on earth happens: ‘Sprites’. (Fig 2)
The Thermosphere and Ionosphere - The Thermosphere layer assists to absorb energetic ultraviolet and X-Ray radiation from the Sun. The Ionosphere is a region within the Thermosphere that is caused when energetic solar radiation knocks electrons off of molecules and atoms thus turning them into ions with a positive charge.
Why is this important? Flights can lose contact with most geosynchronous satellites and must rely on “old-fashioned” radio communications, especially when flying over the poles. Solar and geomagnetic storms that unsettle the Ionosphere can cause GPS position errors as large as 100 metres. Luckily for us, NAIRAS (https://sol.spacenvironment.net/nairas/Dose_Rates.html) helps us to determine these ion fluxes.
- The Exosphere and Magnetosphere - This is the outermost layer of the Earth’s atmosphere. The molecular densities are so far apart that they follow “ballistic” trajectories under the influence of gravity. Ultimately, this layer no longer behaves like a gas and these particles constantly escape into space. The Exosphere and Magnetosphere contain the hydrogen geocorona and the Van Allen radiation belts.
Why is this important? Because the Van Allen radiation belts (Fig 3) are the magnetic fields which deflect energetic particles and help to protect the entire atmosphere from destruction. The Van Allen radiation belts can have impacts on the operations of satellites. Particles and currents from the Magnetosphere can heat the upper atmosphere and result in satellite drag that can affect the orbits of low-altitude Earth orbiting satellites. Additionally, influences from the Magnetosphere on the ionosphere can also affect communication and navigation systems.
ATMOSPHERIC PRESSURE AND THE AFFECTS OF ALTITUDE
At higher altitudes, the atmospheric pressure decreases. In turn, this increases take-off and landing distances, while climb rates decrease. Due to the thin air, engine performance are less efficient in producing the speed necessary for lift: more speed is required to obtain enough lift for take-off; causing a longer ground run. (Fig 4)
Why is this important? Because atmospheric pressure and altitude affect an aircraft’s performance, especially during take-off, ROC (Vx and Vy), and landings. (Vx is the indicated forward airspeed for best angle of climb, and Vy is the indicated airspeed for best rate of climb.)
THE WIND'S PATTERNS AND CURRENTS
As you can probably surmise by now, wind currents are vital to the basic fundamentals of flight. More importantly, it is specifically the wind that creates and determines the varying weather conditions you as a pilot would be encountering. The Coriolis force, atmospheric pressure, topography and temperature all combine to create two types of atmospheric motion: convective currents (vertical motion) and the wind itself (horizontal motion).
- Patterns - Anti-Cyclonic Circulation is the movement of air flowing from areas of a high to low atmospheric pressure system. It is forced to the right to produce a clockwise circulation around an area of high pressure. In areas of low atmospheric pressure, the air flow is forced toward itself to produce an anti-clockwise circulation and is referred to as Cyclonic Circulation. Conversely, air movement is opposite when in the Southern Hemisphere. (Fig 5)
Why is this important? Because a basic understanding of what to expect based on prevailing areas of low and high atmospheric pressure systems will help you during your flight planning phase. Good weather is typically associated with a high atmospheric pressure system; whereas bad weather is typically associated with a low atmospheric pressure system. These anti-cyclonic and cyclonic air movements are what give you favourable winds near a high pressure atmospheric system. (Fig 6)
- Currents - Due to the uneven heating of the Earth described above, small pockets of local air circulation cause what is referred to as convective currents. These specific types of currents can occur anytime there is an uneven heating of the Earth’s surface. When at low altitude flight, and dependant upon the type of surface, these warmer temperatures can cause updraughts and downdraughts. These currents can be very noticeable in areas with land masses near a large body of water (Fig 7). During the day, the change in air density causes an onshore wind referred to sea breeze. Alternatively, during the night, the change in air density causes an offshore wind referred to land breeze (Fig 8). Typically, flying at a higher altitude can cause these turbulent winds to cease.
Why is this important? Because these convective currents can inhibit a pilot’s ability to fully control the aircraft. Particularly when landing, these drastic changes in air densities can cause a pilot to over-shoot or be just shy of the runway. (Fig 9)
- Shears - Low-level wind shear is a drastic and sudden change in the speed and direction of the wind within a consolidated area. The most severe type of low-level wind shear is a micro-burst (Fig 10).
As you can imagine, wind shear is extremely dangerous due to the violent updraughts/downdraughts, and horizontal movement to which the aircraft is subjected. These micro-bursts can happen to any pilot, at any point during flight, and at any altitude. Luckily, there are alerting systems know as the LLWAS-NE (Fig 11), the TDWR (Fig 12) and the ASR-9 WSP (Fig 13) which collectively can help to detect these shears with an accuracy of up to 90%.
Why is this important? Because these abrupt and severe changes to the direction of the wind can cause extremely dangerous situations for you during approach and take-off, with gains and loses of up to 90 knots. Typically, should you inadvertently take-off into a micro-burst, you will initially experience a performance-enhancing headwind, followed by a performance-decreasing downdraught, followed by a rapidly increasing tailwind (Fig 14). The same sequence of events occurs if a micro-burst is encountered during approach which could force the aircraft to the ground.
THE FLIGHT SERVICE STATION (FSS) & WEATHER BRIEFINGS
The FSS is the primary source for preflight weather information for pilots. It is available 24-hours a day, 7 days a week. They provide services such as: Telephone Information Briefing Service (TIBS), Hazardous In-flight Weather Advisory Services (HIWAS) and Transcribed Weather Broadcast (TWEB) (both only for Alaska). All of these services offered are to help pilots determine if it is safe or hazardous to fly.
There are mainly 3 types of weather briefings:
- Standard Briefing - This provides the most complete information and a more complete weather picture. This briefing should always be obtained prior to any flight and should be constantly referred to and updated in-flight.
- Abbreviated Briefing - This is a shortened version of the Standard Briefing.
- Outlook Briefing - This is a briefing that is obtained when a known departure is 6 or more hours away. I personally use it as a go/no-go determining factor if I planned on flying that particular day.
|STANDARD WEATHER BRIEFINGS INCLUDE:|
- Adverse Conditions - Information that may influence a decision to cancel or alter a route
- VFR Flight Not Recommended - If weather is below VFR minimums
- Synopsis - An overview of weather systems that affect the general area
- Current Conditions - Current ceilings, visibility, winds and temperatures
- En Route Forecast - Summary of weather forecast for during ETE proposed route
- Destination Forecast - Summary of expected weather for Dest ICAO at the ETA
- Forecast Winds/Temps Aloft - A forecast of winds at specific ALT
- ATC Delays - This NEVER happens (thanks to @Tyler_Shelton 😉)
- Other Information - A FSS Specialist assigns a specific frequencies to open a FP and directs you to contact EFAS
AVIATION ROUTINE WEATHER REPORT (METAR)
PILOT WEATHER REPORTS (PIREPs)
|AVIATION ROUTINE WEATHER REPORT (METAR)|
A METAR is an observation of current surface weather reports formatted in an international standard.
|ALL STANDARD METARs REPORT THE FOLLOWING (SEQUENTIALLY):|
- Type of report - ROUTINE or SPECI (this is a special report that can be updated anytime)
- Station Identifier issuing report - The four-letter ICAO
- Date & Time of report - ALWAYS depicted in a six-digit group: the first two digits are the date, the last four digits are the time of the METAR/SPECI, in UTC (ex: 151830Z).
- Modifier - Indicates that the METAR/SPECI came from an automated source or that it was corrected. Sometimes you’ll see a COR within the METAR, this just means it’s a corrected version of a previously sent out version.
- Wind - Reported with five digits (ex: 13121KT) . Unless the speed is >99KT, then it’s reported with six digits. The first three digits indicate the direction the true wind is blowing from in tens of degrees. If winds are variable, they are indicated by ‘VRB’. If winds are gusting, they are indicated with the letter ‘G’ and precedes the speed (ex: G30KT)
- Visibility - The prevailing visibility in statute miles is reported and denoted by the letters ‘SM’. They are reported by both miles and fractions of miles (ex: ¼SM).
- Weather - These are divided into two different categories: qualifiers and weather phenomenon (Fig 15). Weather groups are constructed by considering the conditions and phenomenon in sequences: intensity, descriptor and the phenomenon.
- Sky Condition - This provides a report, in the sequence of amount, height and type or indefinite ceiling (Fig 16). The heights or the cloud bases are reported with a three-digit number in hundreds of feet AGL
- Temperature and dew-point - When the temperature is below 0°C, the letter ‘M’ is used to indicate minus.
- Altimeter - Reported in inches of Hg (Mercury) ALWAYS preceded by the letter ‘A’ and in a four-digit number group. (ex: A2690) Rising or falling pressure can also be displayed as ‘PRESRR’ or ‘PRESFR’.
IF YOU EVER WANT TO SEE REAL-TIME METARs OR PIREPs, GO TO:
CLICK ON TAB LABELED ‘OBSERVATIONS’, SCROLL DOWN TO ‘AIRCRAFT REPS’ OR ‘METARs’
Why is this important? Because you NEED to know how to read and decipher a METAR properly! It can save your life in real-world aviation. Here is a practise example I would like you to decipher (and no cheating):
EGLL 262320Z AUTO 33009KT 290V010 9999 FEW030 OVC037 11/06
And last, but most certainly not least…one of the most important reports a pilot can make:
|PILOT WEATHER REPORTS (PIREPs):|
These reports provide invaluable information to FSS, ATC and of course other pilots. PIREPs relay real-time valuable SITREPs which cannot be gathered from any other sources. Pilots who provide these PIREPs can confirm the actual height of bases of clouds, locations of dangerous wind shears, turbulence and even of in-flight icing occurrences. PIREPs are standardised and extremely self-explanatory. (Fig 17). These reports can save lives!
Figure 1 - The Earth's Atmosphere
Figure 2 - Sprites
Figure 3 - Van Allen radiation belts
Figure 4 - ROC based on altitude & pressure
Figure 5 - Anti-Cyclonic & Cyclonic Circulation
Figure 6 - High pressure system producing favourable winds
Figure 7 - Low-altitude convective current turbulence
Figure 8 - Land Breezes & Sea Breezes
Figure 9 - Varying surfaces and effects of convection currents
Figure 10 - A Micro-burst
Figure 11 - Low Level Wind Shear Alert System
Figure 12 - Terminal Doppler Weather Radar
Figure 13 - Airport Surveillance Radar system
Figure 14 - Micro-burst effects during take-off
Figure 15 - Descriptors and weather phenomena in typical METARs
Figure 16 - Sky Contractions Table
Figure 17 - PIREP
As always, thank you for taking the time to read this. It is my sincere wish that I helped to provided some insight into the beautifully complex weather systems this planet can throw at us. Moreover, I sincerely hope you might have learned something new.
If you missed some of my other informational essays, I’ll list some of them below for your reference: