Weather Hazards

Throughout its history aviation has had an intimate relationship with the weather. Time has brought many improvements--better aircraft, improved air navigation systems and systematized program for pilot training. Despite this weather will continue to exact its toll.


In the aviation world 'weather' tends to be used to mean not only "what's happening now, also "what's going to happen during my flight?" Based on the answers received, a pilot will opt to continue or cancel his flight. In my blog I will discuss specific weather issues/hazards and how they affect flight.



Thunderstorms
No weather encountered by a pilot can be as violent or as threatening as a thunderstorm. Thunderstorms create many hazards to the aviation community and since they are so common in summer time, it is important that pilots understand their nature and know how to deal with them. To produce a thunderstorm several conditions must be in place. These include

• unstable airmass
• moisture in the low levels
• something to trigger them, daytime heating, upper level cooling
• for severe thunderstorms, wind shear

The thunderstorm may cover an area ranging from 5 miles in diamete or as much as 50 miles, usually consists of two or more cells in different stages of their life cycle. The stages of life of the individual cells are: Cumulus Stage, Mature Stage, Dissipating Stage

Thunderstorm Hazards

The enviornment in and around a thunderstorm can be the most hazardous encountered by an aircraft. In addition to the usual risks such as severe turnbulence, severe clear icing, large ahil, heavy precipitation, low visibility and electrical discharges with and near the cell, there are other hazards that occur in the surrounding enviorment.




Gust Front
The gust front is the leading edge of any downburst and can run many miles ahead of the storm. This may occur under relatively clear skies and, can be particularly nasty for the uwary pilot. Aircraft taking off, landing, or operating at low levels can find themselves in rapidly changing fields that quickly threaten the aircraft's ability to remain airborne. In a matter of seconds, the wind direction can change by as much 180 degrees, while at the same time the wind speed can approach 100 knots inthe gusts.





Downburst, Macroburst and Microburst

A downburst is a concentrated, severe downdraft which accompanies a descending column of precipitation underneath the cell. When it hits the ground it induces an outward, horizontal burst of damaging winds. There are two types of downburst, the "macroburst" and the "microburst".


A macroburst is a downdraft of air with an outflow diameter of 2.2 nautical miles or greater, with damaging winds that last from 5 to 20 minutes. Such occurences are common in the but rarely hit town or airports.



Embedded within the downburst, is a violent column of descending air known as a "microburst". Microbursts have an outflow diameter of less than 2.2 nautical miles and peak winds lasting from 2 to 5 minutes. Such winds can force an aircraft into the ground.




Funnel Cloud, Tornado and Waterspout
The most violent thunderstorms draw air into their base with great vigor. The incoming air tends to have some rotating motion and, if it should become concentrated in a small area, forms a rotating vortex in the cloud base in which wind speeds can exceed 200 knots. If the vortex becomes strong enough, it will begin to extend to funnel-shaped cloud downwards from the base. If the cloud does not reach the ground, it is called a funnel cloud. If it reaches the ground, it is called a tornado, and if it touches the water, it is a waterspout.

Any severe thunderstorm should be avoided by a wide margin as all extremely hazardous to an aircraft.

Cold Weather Operations




Operating an aircraft in extremely cold weather conditions can bring on a unique set of potential problems.

Temperature Inversion and Cold Air Outbreaks
Low level inversions are common in most areas during the fall and winter due to very cold outbreaks and strong radiation cooling.When cold air moves out over the open water, it becomes very unstable. Cloud can be seen to almost be “boiling” off the waters surface and forming vortices that rotate upwards. Such a condition can be very turbulent and there is a significant risk of serious icing. At the same time, the convection enhances any snowfall resulting in areas of extremely poor visibility.

Looming
Another interesting effect in cold air is the bending of low angle light rays as they pass through an inversion. This bending creates an effect known as “looming,” a form of mirage that causes objects normally beyond the horizon to appear above the horizon.


Ice Fog and Ice Crystals
Ice fog occurs when water vapour sublimates directly to ice crystals. In conditions of light winds and temperatures colder than -30°C or so, such as those that might be found in Cold Lake, water vapour from anthropogenic sources (man-made) can form widespread and persistent ice fog or ice crystals. In light winds, the visibility can be reduced to near zero, closing an airport for hours.

Whiteout
“Whiteout” is a phenomena that can occur when a layer of cloud of uniform thickness overlays a snow or ice-covered surface, such as a large frozen lake. Light rays are diffused when they pass through the cloud layer so that they strike the surface from all angles. This light is then reflected back and forth between the surface and cloud, eliminating all shadows. The result is a loss of depth perception, the horizon becoming impossible to discern, and dark objects seeming to float in a field of white. Disastrous accidents have occurred under such conditions where pilots have flown into the surface, unaware that they were descending and confident that they could see the ground.