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Obstacles to flight
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Obstacles to flight

Obstacles to the flight: wind shear, squall, tornado, storm, lightning, rain

 

Windshear - a change its speed and (or) the directions in space. This change may either horizontally (horizontal CB) and vertical (SV vertical) directions. The term "wind shear" describes a wide range of state of the atmosphere.

Wind shear generate different weather events: thunderstorm, rain, virga (rain stream that evaporates before reaching the ground), downdrafts of cold air updrafts, temperature inversion shift of jet streams, squalls, etc. Rain, rain and virga causing micro -poryvy wind - the main cause of the SW [60].

Observations show that approximately 5% of thunderstorms accompanied by micro-impulses. The related descending air currents are distributed in the zone ranging in size from 500 meters to several kilometers. When this flow reaches the ground, it spreads in the surface layer of air in the horizontal plane, sometimes with the formation of vortex rings on the Limits of the zone spreading. The formation of micro-impulse vertical downdraft (symmetrical microburst).

The zone of vortex rings reaches a height of 500 m above the ground and covers an area of ​​up to 2 4 km in diameter. Micro-impulses can arise and reach the surface of the earth without rain in the case of Virga. The resulting adjustment to rain will evaporate, thereby causing the cooling air and, as a consequence, the downward flow of the wind.

Storm clouds are on the move, and a micro-impulse caused them to become asymmetrical shape. The life cycle of the micro-impulse - 15-20 min. The maximum intensity of wind shear reaches approximately 5 minutes after contact with the ground downstream.

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Strong HR (especially at low altitude) can be caused by a flurry of a few kilometers from the area of ​​NE. Its mechanism - lateral spreading of horizontal flow speed reaches 150-185 km / h. It is often a flurry of thunderstorm that develops, accompanied by other types of cloud that prevents identification.

Temperature inversion wind shear often causes changes in the speed and (or) the wind direction in a small surface layer, if the warm air mass moves over a cold air mass - mainly in the coastal areas or the foothills airports. Here, the upward flow is cooled during lifting, but as fast heated when flowing down from the top downwards. As a result, heating the cold air inversion over the foothills of the valley increases. This temperature gradient is extremely increased. This occurs due to the friction NE slow-moving surface currents and warm air, which quickly moves on them.

Summer storm near the windy slopes also create significant CB under appropriate conditions. These thunderstorms are characterized by relatively high cloud-based, often at altitudes 2500-3000 m (sometimes - a) above the ground, in the air masses that generate high temperatures on the surface of the earth (38.40 ° C), but a relatively low dew point (on-6 up + 3 ° C). Rain falling from cumulonimbus clouds at high altitude can completely evaporate before it reaches the ground.

Cumulonimbus clouds depending on their development are divided into vnutrimassovye and front and rain and hail. Clouds are not only the intensity and types of precipitation, but the mechanism of occurrence and development.

In the case of mass-averaged weak cumulonimbus multiple drops resulting from condensation and coagulation, reaching a maximum critical dimensions begin to fall from the clouds to meet the rising air currents. When the effect of the falling drops of exceeds the effect of lifting the air, the storm subsides.

The strong front cumulonimbus clouds under the influence of NE in the middle and upper atmosphere can bend upward flow. In that case the inhibiting effect of precipitation decreases, since they are outside of inclination of the rising air stream. In the formation of tilted upward flow of relatively flat terrain sometimes there are rotational movements, resulting in dramatically increased vertical speed, and with it the speed of rotation.

Research severe thunderstorms in 1990-2005 years. They have shown that they are linked to the thermodynamic instability caused by overheating of the surface layer of air, or uneven distribution of the height of the advection of heat and cold, as well as the convergence and divergence of air flow. This cumulonimbus clouds with hail, squalls and tornadoes (tornadoes) emerge and develop when the top of the troposphere observed jet stream. In general, studies suggest that, instead of the general picture of the distribution of air flow under the cumulonimbus clouds have a description of some of its fragments, and quite contradictory.

Flurry - A sharp increase in short-term wind change its direction. Squalls associated with cumulonimbus clouds, and often occur during storms. For flurry characteristic swirling movement of the air with a horizontal axis in the clouds or below them. The wind speed during a flurry of often more than 20 m / s; the duration of the phenomenon is usually several minutes; Sometimes there are repeated gusts of the squall.

Tornado - Strong vortex for small clouds with an almost vertical axis of rotation. It looks like a dark cloud column (diameter of up to several hundred meters), one part of which falls funnel narrowing from a low base of cumulonimbus cloud, and towards the surface of the Earth can rise from the second part of splashes and dust, which is connected to the first. The narrowest part of the post - in the middle. The wind speed in a tornado reaches 50- 100 m / s with a strong upward component. The air in a tornado and is rotated right and left. Twister can cause catastrophic damage and loss of human life in the way of their movement, although it is close to an almost complete standstill. The tornado over land sometimes called a thrombus, and in the US - a tornado.

 

Thunderstorms and electric discharges

Storm - A complex atmospheric phenomenon, which is characterized by the appearance of intense convective clouds and is accompanied by considerable turbulence, gusts, tornadoes, wind shear, precipitation as rain, snow, hail, frequent electrical discharges and thunder.

Thunderstorms are divided into 6 levels. Thunderstorms level 1 (weak) and level 2 (mild) characterized by weak or moderate atmospheric turbulence and lightning; Thunderstorm level 3 and 4 (strong and very strong) - considerable turbulence, lightning, precipitation in the form of heavy rain; Thunderstorm level 5 - severe turbulence, lightning, sharp gusts of wind, hail possible; Thunderstorm level 6 - considerable turbulence, heavy hail, lightning and numerous lengthy wind. The main sign of a thunderstorm - lightning.

At any given time in the world at the same time there are about 180 individual thunderstorms when lightning strikes on average every 20 with.

Lightning - A giant spark atmospheric electricity between clouds between clouds and the ground, as well as intra-cloud several times.

If the ground facilities represent the main danger level of cloud land, the objects in the atmosphere, dangerous lightning all kinds.

There are several types of lightning. Summer Lightning (heat lightning) - lightning or glowing cloud caused by lightning is not accompanied by thunder (because of the large distance to the observer). For banded lightning illuminated her characteristic banded clouds. Rocket-shaped zipper called long discharge in the atmosphere, which creates the impression of a slow discharge development along the canal. Band formed by lightning in the event that during the time between pulses discharge channel cloud-to-ground moves (perhaps wind). The pulses are separated in such a flash in the horizontal direction, although the eye captures all tapes simultaneously. Lightning has a correct shape, the channel is broken (or seems to be broken) on the glowing fragments of a length of several tens of meters. Ball lightning is called a mobile luminous sphere with a diameter up to 20 see, her life expectancy a few seconds.

 

Linear lightning - It refers to the so-called no electrode discharge. Its length is several kilometers and can reach even 20 km. From the main channel has several branches in length 2-3 km. which increases the probability of hitting the plane. The average speed of lightning - 150 km / s, the current strength in its channel reaches 200 000 A and the temperature of the plasma in the channel exceeds the 10 000 ° C.

The source of the lightning storm clouds are (usually cumulonimbus) and electric charges in them. Power storm clouds are usually small, which is typical of the subtropics, but can reach large sizes in a huge thunderstorm clouds extending to a height of more than 20 km. The height of a typical storm clouds - 8-12 km (upper limit) and 0,5-2 km (lower limit). Their height is determined only by geographical area.

The development process begins lightning in the atmosphere under certain conditions. In particular, it is necessary that the electric field intensity exceeded a certain limit. For storm zones 0,4 MV / m <E cr <1 MV / m.

The length of the individual storm centers horizontally no more than 10 km, so the size of the main areas in which the charges do not exceed a few kilometers. Volumes with a maximum density of charges, which are initiated by lightning, should be even less so on. E. Their linear dimensions are a few hundred meters.

Thus, we can calculate the density of the space charge, sufficient for the formation of breakdown voltage: E = 106 V / m. It is about 45 C / m3 that one or two orders of magnitude higher than the average charge density in cumulonimbus clouds and thunderstorms.

The energy of the electric field generated by lightning tolerated by warm air, which rises up in a cloud.

In a typical storm cloud chaotic wind, water and ice are in a gravitational field, and the field gradient of temperatures and pressures.

These driving force distribution and accumulation of electric charge ultimately leads to the formation of the electrically active region of the atmosphere.

The mechanism of formation of electrical charges in the clouds is not completely clear, but it is believed that it is related to the strong upward movement of air in the center of the cloud formation and collisions with supercooled droplets of water with ice crystals.

Climbing up, moist air cools to the dew point, and its excess water vapor condenses into water droplets, forming clouds. Upon further upward movement (up 20 km) the temperature is reduced to -40 ° C. Water vapor in it turns into ice crystals, which grow together in the small, pretty heavy hailstones. Recently, falling through the cloud, supercooled water droplets collected. Small pieces of ice light go up, carrying a positive charge, leaving the heavier hailstones with a negative charge. Vertical air flow are pieces of ice in the upper part of the cloud, where the positive charges are accumulated, and at the base of the cloud to create a center of negative charge.

As shown by the analysis of the physical processes associated with electrical breakdown of air and the formation of lightning in the atmosphere, sources of lightning can be only zones atmospheric heterogeneity (in particular, clouds), which contain excess electrical charge and create a pretty intense and extended electric field. Such zones are the lightning-hazardous. Such atmospheric formations have their own sources and dynamic characteristics.

The most likely source of the lightning-prone areas - intense convective motion of air currents in thunderstorm cells. This leads to the distribution and makes similarly charged droplets. Other sources Lightning danger zone - dust storms, active volcanoes emissions and nuclear explosions.

Analysis of the mechanisms of distribution and removal of the charge in the atmosphere is quite complex. So far there is no generally accepted theory of such processes to these sources. Such a theory would allow to calculate some of the characteristics of lightning-hazardous area - the maximum electric field intensity distribution of the electric field in the space, the rate of generation of electric charges and a zone of their localization.

The electric charge of the aircraft can be formed in cumulonimbus, cumulus, Altostratus and Nimbostratus. For this it is necessary that in the cloud, there inhomogeneous electric field. The heterogeneity of the cloud on the phase composition, the less it will be a uniform electric field. Charge Q plane that rainfall in the area (under the cloud) is very small, sharply increases at the entrance to the cloud. The main reason for the accumulation of the electric charge of the aircraft is its interaction with cloud particles. Most electrification of the aircraft takes place in sub-zero temperatures (up to -15 ° C). This significantly affects the probability of being struck by lightning. According to NASA aircraft affected area with lightning in the clouds is mainly concentrated in the region bounded by isotherms 0, -10 ° C. The electrification of the aircraft affects the trajectory of any lightning and the likelihood of their generation.

Electrostatic Discharge sun affects flight safety, not only due to the increased likelihood of defeat of his lightning. It also causes effects that degrade the quality of radio reception on board, reduces the accuracy of the radio compass and the quality of the on-board REO as a whole. The electric charge of the aircraft sometimes even affects the aerodynamics of flight. Furthermore, the charge can cause an explosion when fuel refilling sun in flight. The important thing is that the clouds of all types (except alto) aircraft icing electrified stronger. In particular, nimbostratus charge value Q plane Frostscale twice higher than the net plane. In order to ensure safety in the event of a large aircraft electrification recommended by agreement with the controller to change altitude.

 

Storm rainfall

Intense rainfalls (PLD) vnutrimassovyh and fall out of the front of cumulonimbus clouds.

Cumulonimbus clouds - one of the 10 kinds of clouds on the international classification. Their international title - Cumulonimbus (Cb). In temperate latitudes SMILING reach a height 12- 14 km, and in the tropics - 15-16 km. One cloud can cover an area up to 50-100 km2. These clouds often form a front strip lengthseveral thousand kilometers. They are characterized by pronounced vertical air flow, turbulence, electric field. However zones, dangerous to fly in GL, relatively small in size, and in the frontal clouds almost always exist intervals sufficient for the safe flight of aircraft.

There are three stages in the life of CL. In the first stage of development (cumulus) prevails updraft (10-15 minutes from the time when the cloud detected by radar). The second stage - the period of maturity (15-30 min), which is characterized by the presence of ascending and descending air currents, precipitation, occurrence of lightning. In the third stage (over 30 min) cloud breaks, reduced the intensity of precipitation, decreasing the electrical activity and turbulence.

In meteorology, precipitation intensity taken to determine the height of the water column, which fell on a horizontal surface for some time. For example, the intensity of rain 100 mm / h is pretty strong, but the water content in the air in this case is 2-3 g / m3. Measurements for one hour, allowed substantially averaged values.

In the city Yukonvill (North Dakota, United States) 4 1956 July, was recorded highest in the world 1870 precipitation intensity mm / h for over 1 min. In 1962 was conducted measuring the water content in the air during a thunderstorm on a specially equipped airplane F-100. Although ground-based measurements give the moderate values ​​of rainfall intensity (37 mm / h) and water content (1,1 g / m3), measurement taken from the plane showed the average water content 8,4 r / m3 and maximum - to44 g / m3. In the former USSR the maximum intensity of rainfall recorded was about 1000 mm / h.

Regarding the influence of PLD on the aerodynamic characteristics of the Armed Forces of interest such numerical characteristics, the intensity of rainfall, water availability.

 

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