Weather Hazards

 Low Level Windshear: The invisible Enemy. 



The heat radiation produced by the sun 

and received by the many different surfaces on earth including, forest, cities, water and inhabited areas across the globe is the main reason of the constant changes in the atmosphere. The oval shape of the planet, wider by the equator and narrower by the poles receive surface radiation differently, thus it’s cooling down process at these many areas are also different. The product of this surface cooling (land and water) of warm, warmer and cool air elevates up to the atmosphere and further cools down, depending from where the cooling is occurring the different masses of air meet in the atmosphere producing rotational movement from different air pressures, this produces high and low pressure with late enables bigger air masses to travel across earth as fronts, these front could bring serious climate change in the form of tornados, tropical storms, thunderstorms, hail, among other weather conditions that meteorologist battle to forecast daily to anticipate disasters. 


This Mother Nature process occurs daily 

and as earth rotates around the sun and around its axis climate changes continues to occur throughout the year in the form of seasons. Meanwhile, the study of this weather phenomena is conducted by experts meteorologists with the aids of technology such as radar. In aviation, beyond the fuel planing to reach a destination weather is the number one discussion topic, because once in the air the only alternative is to get back on the ground safely and without risk complications. 


The weather associated with low pressure 

systems are thunderstorms which is the most severe of all, and regardless of its size, the crew members of an aircraft should never fly thru one, and stay as far as 20-50 miles if possible to avoid heavy turbulence and even hail. Because this is the most dangerous is also the most avoidable and meteorologists “paint” the picture on the map showing the areas to avoid. As a rule of thumb, aircraft remain outside 20 miles from cumulonimbus clouds and thunderstorms cell. In contrast, the product of thunderstorms, surface cooling, low pressure systems, high temperature and moisture (humidity) in the air creates rapid changes of velocity and direction in the air called windshear, this invisible yet, dangerous condition could occur at any altitude and flying thru an area of high level windshear may just feel like a light to moderate turbulence. However, low level windshear around the approach path of a runway and the airport environment could easily lead to sudden loss of control, sometimes the shear may not be forgiving and strong enough to cause an airplane to crash while landing. 


To understand further the effects of low 

level windshear is important to mention the preconditions in the atmosphere for windshear to form. The most common come from thunderstorms, during the cumulus and mature stage the formation of updrafts and suddenly downdrafts (dissipating stage) are produce, this phenomenon is key for winshear to form. Another way of windshear formation is at nighttime inversions, which happens when the surface is cooling overnight rising warm air up to the atmosphere until it meet the colder air which is a calm air. This threshold of interference between the warm and cold air sets the changes in wind and depending the altitude of where this occurs windshear can form and noted as low or high, strong or weak.


An approaching aircraft landing into an airport 

travels at a much slower speed, further it’s also producing more form drag (from the landing head, flaps, slats...) the closer it gets to the touchdown point on the runway the more steady constant speed is required, stall speed at this point is closer. If windshear was either reported at the airport, or was just encountered unforescasted the suddenly loss or gain of air speed will impact directly the aerodynamics of the aircraft. The quick reaction and sometimes effective response of the pilots is required to Go Around and unfortunately just to cause a catastrophic accident. 


Since windshear is “invisible” yet, 

sometimes predicted by the airport, others reported by other pilots that already encounter these conditions and were able to land safely, the best recommendations to crew are to anticipate the possibility of low level windshear associated with thunderstorms in the vicinity, and low level temperature inversion. This pre planing will enable the crew to fly at a higher approach speed and prevent a sudden loss or gain of air speed and altitude to land safe and secure.




References. 


Windshear. (2011). International Geophysics. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/wind-shear


Low Level Windshear. (N. d.). Weather.Gov. https://www.weather.gov/media/zhu/ZHU_Training_Page/llws/LLWS/llws.pdf


Comments