25 meters a second strong wind. Storms, squalls, hurricanes, their characteristics, damaging factors

Meteorological hazards are natural processes and phenomena that occur in the atmosphere under the influence of various natural factors or their combinations that have or may have a damaging effect on people, farm animals and plants, economic objects and the natural environment.

Wind - this is the movement of air parallel to the earth's surface, resulting from the uneven distribution of heat and atmospheric pressure and directed from a high pressure zone to a low pressure zone.

The wind is characterized by:
1. Wind direction - determined by the azimuth of the side of the horizon, from where
it blows, and is measured in degrees.
2. Wind speed - measured in meters per second (m/s; km/h; miles/hour)
(1 mile = 1609 km; 1 nautical mile = 1853 km).
3. Wind force - measured by the pressure that it exerts on 1 m2 of surface. The strength of the wind varies almost proportional to the speed,
therefore, the strength of the wind is often estimated not by pressure, but by speed, which simplifies the perception and understanding of these quantities.

Many words are used to indicate the movement of the wind: tornado, storm, hurricane, storm, typhoon, cyclone and many local names. To systematize them, all over the world use Beaufort scale, which allows you to very accurately estimate the strength of the wind in points (from 0 to 12) according to its effect on ground objects or on waves in the sea. This scale is also convenient in that it allows, according to the signs described in it, to fairly accurately determine the wind speed without instruments.

Beaufort scale (Table 1)

Breeze (light to strong breeze) sailors refer to the wind as having a speed of 4 to 31 miles per hour. In terms of kilometers (factor 1.6) it will be 6.4-50 km/h

Wind speed and direction determine weather and climate.

Strong winds, significant changes in atmospheric pressure and a large number of precipitation causes dangerous atmospheric whirlwinds (cyclones, storms, squalls, hurricanes) that can cause destruction and loss of life.

Cyclone - common name eddies with reduced pressure in the center.

An anticyclone is an area of ​​high pressure in the atmosphere with a maximum in the center. In the Northern Hemisphere, the winds in the anticyclone blow counterclockwise, and in the Southern Hemisphere - clockwise, in the cyclone the wind movement is reversed.

Hurricane - wind of destructive force and considerable duration, the speed of which is equal to or exceeds 32.7 m/s (12 points on the Beaufort scale), which is equivalent to 117 km/h (Table 1).
In half of the cases, the wind speed during a hurricane exceeds 35 m/s, reaching up to 40-60 m/s, and sometimes up to 100 m/s.

Hurricanes are classified into three types based on wind speed:
- Hurricane (32 m/s and more),
- strong hurricane (39.2 m/s or more)
- fierce hurricane (48.6 m/s and more).

Cause of these hurricane winds is the occurrence, as a rule, on the line of collision of the fronts of warm and cold air masses, powerful cyclones with a sharp pressure drop from the periphery to the center and with the creation of a vortex air flow moving in the lower layers (3-5 km) in a spiral towards the middle and up, in the northern hemisphere, counterclockwise.

Such cyclones, depending on the place of their occurrence and structure, are usually divided into:
- tropical cyclones found over warm tropical oceans, usually moves westward during formation, and curves poleward after formation.
A tropical cyclone that reaches unusual strength is called hurricane if he is born in Atlantic Ocean and adjacent seas; typhoon - V pacific ocean or its seas; cyclone - in the region indian ocean.
mid-latitude cyclones can form both over land and over water. They usually move from west to east. A characteristic feature of such cyclones is their great "dryness". The amount of precipitation during their passage is much less than in the zone of tropical cyclones.
The European continent is affected by both tropical hurricanes that originate in the central Atlantic and cyclones of temperate latitudes.
Storm – a type of hurricane, but has a lower wind speed 15-31
m/sec.

The duration of storms is from several hours to several days, the width is from tens to several hundreds of kilometers.
Storms are divided into:

2. Stream storms – These are local phenomena of small distribution. They are weaker than whirlwinds. They are subdivided:
- stock - the air flow moves down the slope from top to bottom.
- Jet - characterized by the fact that the air flow moves horizontally or up the slope.
Stream storms pass most often between chains of mountains connecting valleys.
Depending on the color of the particles involved in the movement, black, red, yellow-red and white storms are distinguished.
Depending on the wind speed, storms are classified:
- storm 20 m/s and more
- strong storm 26 m/s and more
- severe storm of 30.5 m/s and more.

Squall – a sharp short-term increase in wind up to 20–30 m/s and higher, accompanied by a change in its direction associated with convective processes. Despite the short duration of squalls, they can lead to catastrophic consequences. Squalls in most cases are associated with cumulonimbus (thunderstorm) clouds, either local convection or a cold front. A squall is usually associated with heavy rainfall and thunderstorms, sometimes with hail. Atmospheric pressure during a squall rises sharply due to the rapid precipitation, and then falls again.

If possible, limit the area of ​​impact, all of the listed natural disasters are classified as non-localized.

Dangerous consequences of hurricanes and storms.

Hurricanes are one of the most powerful forces of the elements and, in their detrimental effect, are not inferior to such terrible natural disasters like earthquakes. This is due to the fact that hurricanes carry enormous energy. Its amount released by a hurricane of average power during 1 hour is equal to the energy of a nuclear explosion of 36 Mt. In one day, the amount of energy that would be enough to provide electricity to a country like the United States is released. And in two weeks (the average duration of the existence of a hurricane), such a hurricane releases energy equal to the energy of the Bratsk hydroelectric power station, which it can generate in 26 thousand years. The pressure in the hurricane zone is also very high. It reaches several hundred kilograms per square meter of a fixed surface located perpendicular to the direction of wind movement.

The hurricane destroys strong and demolishes light buildings, devastates sown fields, breaks wires and knocks down poles of power transmission and communication lines, damages highways and bridges, breaks and uproots trees, damages and sinks ships, causes accidents on utility networks, in production. There are cases when hurricane winds destroyed dams and dams, which led to large floods, threw trains off the rails, tore bridges off their supports, knocked down factory pipes, and threw ships onto land. Hurricanes are often accompanied by heavy downpours, which are more dangerous than the hurricane itself, as they cause mudflows and landslides.

Hurricanes vary in size. Usually, the width of the zone of catastrophic destruction is taken as the width of the hurricane. Often, the area of ​​storm force winds with relatively little damage is added to this zone. Then the width of the hurricane is measured in hundreds of kilometers, sometimes reaching 1000 km. For typhoons, the destruction zone is usually 15-45 km. The average duration of a hurricane is 9-12 days. Hurricanes occur at any time of the year, but most often from July to October. In the remaining 8 months they are rare, their paths are short.

The damage caused by a hurricane is determined by a whole complex of various factors, including the terrain, the degree of development and the strength of buildings, the nature of vegetation, the presence of population and animals in its zone of action, the time of year, preventive measures taken and a number of other circumstances, the main of which is velocity head of the air flow q, proportional to the product of the atmospheric air density and the square of the air flow velocity q = 0.5 pv 2.

According to building codes and regulations, the maximum normative value of wind pressure is q = 0.85 kPa, which, at an air density of r = 1.22 kg/m3, corresponds to wind speed.

For comparison, we can cite the calculated values ​​of the velocity head used for the design of nuclear power plants for the region Caribbean: for buildings of category I - 3.44 kPa, II and III - 1.75 kPa and for open installations - 1.15 kPa.

Every year, about a hundred powerful hurricanes march through the globe, causing destruction and often claiming human lives (Table 2). June 23, 1997 over for the most part A hurricane swept through the Brest and Minsk regions, as a result of which 4 people died, 50 were injured. IN Brest region was de-energized 229 settlements, 1071 substations were put out of action, roofs were torn off from 10-80% of residential buildings in more than 100 settlements, up to 60% of buildings of agricultural production were destroyed. In the Minsk region, 1,410 settlements were de-energized, hundreds of houses were damaged. Broken and uprooted trees in forests and forest parks. At the end of December 1999 from hurricane wind swept through Europe, Belarus also suffered. Power lines were cut, many settlements were de-energized. In total, 70 districts and more than 1,500 settlements were affected by the hurricane. Only in the Grodno region, 325 transformer substations failed, in the Mogilev region even more - 665.

table 2
Impact of some hurricanes

Tornado (tornado)- whirlwind movement of air, propagating in the form of a giant black column with a diameter of up to hundreds of meters, inside which there is a rarefaction of air, where various objects are drawn.

Tornadoes occur both over the water surface and over land, much more often than hurricanes. Very often they are accompanied by thunderstorms, hail and showers. The speed of air rotation in the dust column reaches 50-300 m/s and more. During its existence, it can travel up to 600 km - along a strip of terrain several hundred meters wide, and sometimes up to several kilometers, where destruction occurs. The air in the column rises in a spiral and draws in dust, water, objects, people.
Dangerous factors: buildings caught in a tornado due to a vacuum in the air column are destroyed from the pressure of air from the inside. It uproots trees, overturns cars, trains, lifts houses into the air, etc.

Tornadoes in Belarus occurred in 1859, 1927 and 1956.

Many people ask the question: at what wind speed do planes not fly? Indeed, there are certain speed limits. Compared to the speed of an aircraft, which reaches 250 m/s, even strong wind at a speed of 20 m / s will not interfere with the aircraft during flight. However, a crosswind can interfere with an airliner when it is moving at a slower speed, namely at the time of takeoff or landing. Therefore, under such conditions, planes do not take off. Air currents affect the speed of the aircraft, the direction of movement, as well as the length of the roll and takeoff run. In the atmosphere, these streams are present at all altitudes. This movement of air in relation to a flying airliner is a portable movement. If a strong wind is blowing, the direction of movement of the airliner in relation to the ground does not coincide with the longitudinal axis of the aircraft. Strong air currents can blow the plane off course.

Airliners always land and take off against the direction of the wind. In the case of takeoff or landing with a tailwind, the length of the takeoff run and run increases significantly. When taking off or landing, an airliner penetrates the lower atmosphere so quickly that the pilot does not have time to respond to a change in the wind. If he does not know about a sharp increase or, conversely, a weakening of air flows in the lower layers of the atmosphere, this is fraught with a plane crash.

During takeoff, when an airliner is gaining altitude, it enters a zone of strong headwind. As the aircraft climbs, the lift force of the aircraft increases. Moreover, the increase occurs faster than the pilot can control it. The flight path in this case may be higher than the calculated one. If there is a sharp increase in wind, this can cause the airliner to fall into a supercritical angle of attack. This can lead to airflow stall and collision with the ground.

Generally, the allowable maximum wind force is determined for each aircraft individually, depending on the specifics of its specific characteristics and technical capabilities. Sets the maximum wind speed at which takeoff or landing can be made, by the manufacturer of the airliner. More precisely, the manufacturer sets two maximum speeds: passing and lateral. Tail speed for most modern airliners is the same. During takeoff and landing, the tail speed must not exceed 5 m/s. As for the lateral speed, it is different for each airliner:

• for TU-154 aircraft - 17 m/s;
• for AN-24 - 12 m/s;
• for TU-134 - 20 m/s.

On average, airliners are set to the maximum lateral speed 17 m/s. At more speed the vast majority of aircraft do not take off. If there is a sharp increase in wind in the arrival area, the speed of which exceeds the permissible values, the planes do not land at this airport, but make emergency landing to another runway where conditions allow the aircraft to land safely.

Answering the question in what wind the planes do not fly, it can be said with confidence that at a speed of more than 20 m / s, if the wind blows perpendicular to the runway, takeoff cannot be carried out. Such a strong wind is associated with the passage of powerful cyclones. Below you can watch a video of landing an aircraft in a strong crosswind to see how difficult it is to do even for a professional experienced pilot with a long experience. Of particular danger in this case is the gusty wind in the lower layers of the atmosphere. It can begin to blow at the most inopportune moment, forming a large roll, which poses a great danger to the aircraft.

Crosswind is dangerous because it requires the pilot to take certain actions that are very difficult to perform. In aviation, there is such a thing as a "drift angle". This term refers to the amount of angle an airliner deviates from a given direction due to the wind. The stronger the wind, the larger this angle. Accordingly, the more effort the pilot needs to make to turn the airliner to this angle in reverse side. As long as the aircraft is in flight, even such a strong wind does not cause any problems. But as soon as the plane makes contact with the surface of the runway, the airliner acquires traction and begins to move in a direction parallel to its axis. At this moment, the pilot must abruptly change the direction of the aircraft, which is very difficult.

As for the problem of a strong tailwind, it is easily solved by changing the operating threshold of the runway. However, not every airport has such an opportunity. For example, Sochi and Gelendzhik are deprived of such an opportunity. If a strong wind blows towards the sea, landing can be carried out, but taking off under such conditions is unsafe. That is, landing an aircraft in strong winds is possible, but not in all cases.

Runway condition

Even if the wind speed allows you to take off or land, there are still a number of factors that can affect the final decision. In particular, in addition to weather conditions, visibility, the condition of the runway is taken into account. If it is covered with ice, landing or takeoff cannot be carried out. In aviation, there is such a term as "traction coefficient". If this indicator is below 0.3, this runway strip unsuitable for landing or takeoff and needs to be cleaned. If the decrease in friction coefficient was due to heavy snowfall, in which cleaning is not possible, the entire airport is closed until the weather improves. Such a break in work can last several hours.

How is the decision to take off made?

This decision must be made by the aircraft commander. To do this, first of all, he must familiarize himself with the meteorological data on the air hubs of departure, landing and alternate airports. For this, METAR and TAF forecasts are used. The first forecast is issued for all airports every half hour. The second is given every 3-6 hours. Such forecasts reflect all relevant information that may influence the decision to take off or cancel a flight. In particular, such forecasts contain data on wind speed and its changes.

To make a decision, all flights are conditionally divided into 2-hour and longer ones. If the flight lasts less than two hours, it is enough for the actual weather to be acceptable (above the minimum) for takeoff. If the flight is longer, the TAF forecast must be additionally taken into account. If at the destination weather do not allow landing, in some cases, the decision to take off may be positive. For example, if the weather conditions at the destination are below the minimum, however, there are two airfields in the immediate vicinity with optimal weather conditions. But a positive decision is almost never made in these cases, given the danger of such a flight.

In contact with

Wind(horizontal component of air movement relative to the earth's surface) is characterized by direction and speed.
Wind speed measured in meters per second (m/s), kilometers per hour (km/h), knots or Beaufort (wind force). A knot is a nautical measure of speed, 1 nautical mile per hour, approximately 1 knot equals 0.5 m/s. The Beaufort scale (Francis Beaufort, 1774-1875) was created in 1805.

Direction of the wind(where it blows from) is indicated either in rhumbs (on a 16-rhumb scale, for example, North wind- N, northeast - NE, etc.), or in the corners (relative to the meridian, north - 360 ° or 0 °, east - 90 °, south - 180 °, west - 270 °), fig. 1.

In 1963, the World Meteorological Organization clarified Beaufort scale and it was adopted for an approximate estimate of wind speed by its effect on ground objects or by waves on the high seas. average speed wind is indicated at a standard height of 10 meters above an open flat surface.

The smoke (from the captain's pipe) rises vertically, the leaves of the trees are motionless. Mirror-like sea.

Wind 0 - 0.2m/s

The smoke deviates from the vertical direction, there are light ripples on the sea, there is no foam on the ridges. Wave height up to 0.1m.

The wind is felt in the face, the leaves rustle, the weather vane starts to move, the sea has short waves with a maximum height of up to 0.3 m.

Wind 1.6 - 3.3m/s.

Leaves and thin branches of trees sway, light flags sway, slight excitement on the water, occasionally small lambs form.

The average wave height is 0.6 m. The wind is 3.4 - 5.4 m/s.

The wind raises dust, pieces of paper; thin branches of trees sway, white lambs on the sea are visible in many places.

Maximum wave height up to 1.5 m. Wind 5.5 - 7.9 m/s.

Branches and thin tree trunks sway, the wind is felt by hand, white lambs are visible everywhere.

The maximum wave height is 2.5 m, the average is 2 m. The wind is 8.0 - 10.7 m/s.

In this weather, we tried to leave by Baltic Sea from Darlowo. (Poland) against the wave. In 30 minutes only approx. 10km. and very wet from the splashes. We returned along the way - och. funny.

The thick branches of the trees sway, the thin trees bend, the telephone wires hum, the umbrellas are hardly used; white foamy ridges occupy large areas, water dust is formed. The maximum wave height is up to 4m, the average is 3m. Wind 10.8 - 13.8m/s.

Such weather was caught on boats in front of Rostock. The navigator was afraid to look around, the most valuable thing was stuffed into his pockets, the radio was tied to his vest. Spray from the side waves constantly covered us. For a water-powered fleet, not to mention a simple motorboat, this is probably the maximum ...

Tree trunks sway, large branches bend, it is difficult to go against the wind, the crests of the waves are torn off by the wind. The maximum wave height is up to 5.5m. wind 13.9 - 17.1 m/s.

Thin and dry branches of trees break, it is impossible to speak in the wind, it is very difficult to go against the wind. Strong storm at sea.

The maximum wave height is up to 7.5 m, the average is 5.5 m. The wind is 17.2 - 20.7 m / s.

Large trees are bending, the wind is tearing tiles from the roofs, very strong sea waves, high waves. It is observed very rarely. Accompanied by destruction in large spaces. At sea, there are exceptionally high waves (maximum height - up to 16m, average - 11.5m), small vessels are sometimes hidden from view.

Wind 28.5 - 32.6m/s. Violent storm.

The sea is all covered with strips of foam. The air is filled with foam and spray. Visibility is very poor. Full p ... ts small-sized ships, yachts and other ships - it's better not to get hit.

Wind 32.7 m/s or more...

Absolutely Not summer weather from the point of view of the passenger, it can be only a minor inconvenience for the pilot, while at the same time, quite tolerable weather in the traditional sense can be non-flying. Of course, in the latter case, delays and cancellations of flights cause understandable anger on the part of the passenger. In fact, a number of meteorological phenomena can interfere with the safe operation of a flight. It often happens that flights of some airlines take off and land, while others wait for hours for the weather or are canceled altogether. We have already touched on the topic of weather conditions in, in this article we will talk in more detail about what kind of weather and how it affects aviation activities, what is meteorological minimum and how the crew decides to take off.

So, let's start with the fact that before trying to determine whether the weather is flying or not, you need to establish the appropriate criterion. This criterion is called meteorological minimum, takeoff and landing minima apply to wind speed and direction, visibility, cloud base, runway conditions.

As such, there are no minima for flying along the route, but we must not forget that there are a number of meteorological conditions that are a priori dangerous for aviation, we are talking primarily about thunderstorms and related phenomena, such as hail, lightning, heavy icing, severe turbulence. Of course, most thunderstorms can be bypassed, but when it comes to frontal thunderstorms that stretch for hundreds of kilometers like a solid wall, it is often not possible to bypass them.

As a rule, when talking about minima, we are talking about the minimum visibility on the runway and the decision height (CHL). Decision Height is the height at which the pilot must perform a go-around if he cannot see the runway.

There are three types of minima:

• Aircraft minimum.

  This is the minimum set by the aircraft manufacturer, that is, a list of acceptable weather conditions under which the manufacturer guarantees the safe operation of the aircraft.

• Airfield minimum.

  This is the minimum set at this airport for each particular runway. It depends on the ground radio navigation, lighting and technical equipment installed at the aerodrome and surrounding airport terrain (mainly we are talking about the relief and artificial obstacles).

• Crew minimum.

  The crew minimum is the personal admission of each pilot to perform a flight in certain weather conditions. Pilot minimums are achieved by passing a special training program and confirmed by flight checks.

The basic rule for the application of meteorological minima is that the worst minimum of the three is applied: airplane, airport, and crew.

Let's take an example. The aircraft manufacturer set the minimum visibility on the landing strip for this aircraft at 200 meters, the crew, as a result of checks, confirmed their qualifications and has a landing clearance with a horizontal visibility of 200 meters, however, for the aerodrome on which the flight is performed, a minimum of 800 meters is set. As mentioned above, the worst minimum is selected, that is, in this case, a minimum of 800 meters will be applied. Everything is extremely logical, in this case, despite the excellent equipment of the aircraft and the high qualification of the pilots, the airport has less advanced equipment that will not allow you to perform a landing approach with such high accuracy, so the final minimum will correspond to the minimum of the airfield.

Let's talk in more detail about weather phenomena that limit the activities of aviation.

Visibility.

Probably the most common cause weather delays are limited visibility. This group includes such meteorological phenomena as fog, rain, snow, dust, smoke, in general, everything that somehow reduces visibility. From an aviation point of view, it is not particularly important what the visibility is limited to, the main parameter determining the possibility of taking off and landing is the runway visual range, or RVR (Runway visual range). The second landing minimum parameter is decision height. For example, 60x550, where 60 meters is the decision height and 550 meters is the runway visual range. Sometimes a third parameter is added - the height of the cloud base.

As already mentioned, the minimum airfield depends, among other things, on the radio navigation equipment of the runway, most often on the category of heading and glide path landing system ILS. Most Russian airports have a basic ILS system of the first category, which provides a minimum 60x550, often the airfield is not equipped with a HUD at all, then the landing approach is carried out according to the so-called inaccurate systems and the airfield minimum is much higher. ILS equipment of the second category is currently installed at several airports in the Russian Federation such as Ufa, Vnukovo, Novosibirsk, Krasnoyarsk, the minimum is 30x300 meters. And only three airports have Category IIIA HUD equipment, the minimum for which is 15x200 meters, these are Sheremetyevo, Domodedovo and Pulkovo.

Mountain airfields are a special case, where the minima can be much higher despite the ground equipment installed.

Speaking of minimums aircraft, then the majority of foreign-made aircraft, of which the majority are today, are approved for flights under the category IIIB and IIIC, that is, they can land in automatic mode when visibility is close to zero, but in Russia so far not a single airport has the appropriate equipment, which is not surprising because of its huge cost. As for the pilots, most of them have a landing clearance of at least 15x200, less often you can meet crews with a 60x550 clearance, as a rule, these are those who have only recently performed solo flights.

Airport minimums for takeoff depend mainly on the characteristics of the lighting and technical equipment of the runway and obstacles around the runway and are usually about 150-250 meters.

Wind.

Usually, the wind limits are the limits set by the aircraft manufacturer, very rarely airport regulations require these values ​​to be adjusted upwards. The wind speed is decomposed into two components - lateral and longitudinal. Airplanes are taking off and landing against the wind, or with a small associated component. The reason for this is security, because take-off and landing against the wind can significantly reduce the speed of landing and take-off, and therefore reduce the take-off and run distances. For most modern civil aircraft the maximum tailwind component during takeoff and landing is 5 meters per second, and the lateral component is about 17-18 meters per second.

The wind speed of 11 m/s is decomposed into two components: lateral and tailwind.

side wind is dangerous, because in order to compensate for it, it is necessary to turn the aircraft slightly against the wind, to the so-called drift angle the stronger the wind, the larger this angle. While the plane is flying, drift does not cause problems, but at the moment it touches the runway, the plane acquires grip with its surface and tends to move in a direction parallel to its axis, at this moment the pilot needs to sharply change the direction of movement, which is not always easy. Of particular danger is the gusty wind, which can “blow” at the most inopportune moment, creating a large roll, which is very dangerous in conditions of proximity to the ground.

Landing with a strong side wind.

Recall that we are talking about the wind components decomposed for a particular runway direction, the value of the wind speed itself can be much higher.

Wind that would blow strictly perpendicular to the runway at a speed of about 20 meters per second is an infrequent phenomenon, usually such a strong wind is associated with the passage of powerful cyclones. As for the tailwind, for the vast majority of airports this problem is solved by simply changing the operating threshold of the runway, but there are a number of airports where this is not possible. For example, Sochi and Gelendzhik. These airfields are located in close proximity to the mountains, which excludes the possibility of taking off towards the mountains and landing from the side of the mountains, that is, you need to take off at sea. If the wind blows towards the sea, often the tail component excludes the possibility of a safe takeoff. That is, in fact, you can sit down, but you can’t take off anymore.

Adler Airport in Sochi.

Runway condition.

If the runway is covered with a layer of ice, whatever one may say, it is impossible to take off and land. In aviation, such a concept is used as adhesion coefficient, which is regularly measured by the aerodrome service, if its value falls below 0.3, the runway is not suitable for takeoff and landing. In case there is a side wind, this threshold value is adjusted upwards. A friction coefficient below 0.29 means that the lane is covered with a layer of ice, snow or slush and needs cleaning. Unfavorable weather conditions such as heavy snowfall or freezing precipitation can derail all runway clearance work, causing the airfield to be closed for many hours.

How is the decision to fly made?

The decision to take off is the exclusive right of the aircraft commander. To decide to fly or not to fly, first of all, you need to familiarize yourself with the meteorological information on the aerodromes of departure, destination and alternate. For this, weather reports of the actual weather are used. METAR, which are issued for all airports with a frequency of 30 minutes and forecasts TAF, the frequency of release of which is usually 3 or 6 hours. METAR and TAF reflect in standard form all meteorological information that is somehow significant when flying to a given aerodrome.

As an example, let's take the METAR of Krasnoyarsk Airport:

UNKL 181830Z 00000MPS 4600 BCFG SCT046 BKN240 11/09 Q1012 TEMPO 0500 FG RMK QFE733 29////65

For an uninitiated person, this is just a set of letters and numbers, but one glance is enough for a pilot to understand that the weather is “not very good”. The following information is encoded in the report: at the Krasnoyarsk airfield on the 18th at 18:30 UTC, the following conditions existed: wind - calm, visibility 4600m, fog in places, scattered clouds at 1500 meters, broken at 800 meters, temperature 11 degrees, dew point 9 degrees, fog at times with visibility of 500 meters, pressure 733 millimeters of mercury column, friction coefficient on the runway 0.65.

When deciding on a departure, all flights are conditionally divided into two categories: less than two hours and more than two hours. For flights less than two hours, it is allowed to ignore the forecast and take off if the actual weather in this moment above the minimum. If the flight lasts more than two hours, on the contrary, the actual weather at the aerodrome is not taken into account, and the decision is made based on the TAF forecast. By the way, Russian legislation allows you to make a decision to take off if the weather at the destination airfield is predicted to be below the minimum if there are two alternate airfields with acceptable weather conditions, but this opportunity is rarely used, which is quite reasonable.

Why do some take off and land, while others wait for the weather?

There are many reasons. Let's give examples. For example, fog below the minimum is predicted in Samara, while the actual weather is still above the minimum. Flights from Moscow take off and land, and flights from St. Petersburg are delayed. The fact is that the flight from Moscow lasts less than two hours, and the decision to take off is made based on actual conditions, while the flight from St. Petersburg takes more than two hours, which means that it will be possible to fly only under the predicted improvement.

Some sat down, while others went to the alternate airfield, why? Yet again, different aircraft, different crews. It is possible that the flight that was diverted was operated by a crew with a poor personal minimum, or the aircraft was not allowed to land in these conditions. By the way, even two outwardly identical aircraft one manufacturer may have different restrictions, for example, part of the A320 aircraft is allowed to operate with a tailwind component of 7 meters per second, while the rest have a limit of 5 meters per second.

Often, from passengers waiting for the departure of a flight delayed due to weather conditions, one can hear statements like “I just called my aunt, she said that there is no fog and never was! We are being deceived!" We hasten to assure that no one is deceiving anyone. For some reason, many citizens think that if there is fog in Sheremetyevo, then all of Moscow, exactly along its border, should be covered with fog. Not at all. Many weather events are very local in nature. It happens that visibility on parallel runways differs by several kilometers.