L06.pdf

Weather Hazards to Aviation • Throughout its history, aviation has had an intimate relationship with the weather. Time has brought improvements - better aircraft, improved air navigation systems and a systemised program of pilot training. • Despite this, weather continues to exact its toll. In the a...

Weather Hazards to Aviation • Throughout its history, aviation has had an intimate relationship with the weather. Time has brought improvements - better aircraft, improved air navigation systems and a systemised program of pilot training. • Despite this, weather continues to exact its toll. In the aviation world, weather tends to be used to mean not only “what’s happening now?” but also “what’s going to happen during my flight?”. Based on the answer received, the pilot will opt to continue or cancel his flight. Weather Hazards To Aviation • Wind – Wind shear – Microburst • Turbulence • Precipitation • Thunderstorms – Lightning – Turbulence, downburst – Icing, hail • Dust, sandstorm, volcanic ash Wind • Wind is the movement of air relative to the surface of the Earth. It is caused by difference in atmospheric pressure whereby a huge amount of air flows from an area of high pressure to an area of low pressure. • Pressure difference arises from heating of the earth’s surface by the sun. The earth’s surface is made of different land and water areas, and these varying surfaces absorb and reflect the Sun’s rays unevenly. Warm air rising yields a lower pressure on the Earth, because the air is not pressing down on the Earth’s surface, while descending cooler air produces a higher pressure. Hazardous Wind Conditions • Hazardous wind conditions can cause aircraft to crash when landing or taking off. • Two such wind conditions are: – Wind shear, which is a change in wind speed/direction over a short distance. Wind shear – Microburst, which is a column of sinking air that spreads out in all directions after hitting ground. Microburst Hazardous Wind Conditions - Turbulence Turbulence is the violent, unsteady movement of air. The types of turbulence include: – Wake Vortex Turbulence is generated by aircraft and it can cause an aircraft to suddenly roll uncontrollably if it follows too closely behind another aircraft. Wake Vortex Turbulence – Clear Air Turbulence (CAT) is turbulence in the absence of any visual cues such as clouds. They occur at edges of jet streams or around thunderstorms. – Mountain Wave Turbulence occurs when a mountain obstructs the movement of wind. Turbulence forms on the other side of the mountain that is not facing the incoming wind. Aircraft may experience fluctuations in airspeed and altitude flying in this turbulence. Mountain Wave Turbulence Hazardous Wind Conditions - Warning • Aircraft can be fitted with an airborne wind shear warning system (GPWS mode 7). • Low Level Wind Shear Alert System (LLWAS) is a ground-based system used to detect wind shear close to an airport, especially along the runway corridors. This information can then be passed, in real-time, to warn pilots and aerodrome services. LLWAS GPWS Global Wind Pattern • Air above surfaces with higher temperatures, e.g., the equatorial region will begin to rise because it is less dense. As the air rises, it creates low surface atmospheric pressure. • Air above surfaces with cooler temperatures, e.g., the sub-tropical regions, sink. The sinking air creates higher surface atmospheric pressure. • Global winds do not move directly from north to south or south to north because the Earth rotates. All winds in the Northern Hemisphere appear to curve to the right as they move. In the southern hemisphere, winds appear to curve to the left. This is known as the Coriolis effect, which is the apparent shift in the path of any fluid or object moving about the surface of the Earth due to the rotation of the Earth. • Near the equator, the trade winds converge into a broad east to west area of light winds. The area is known as the doldrums because there are light winds. This belt of air around the equator receives much of the sun’s radiant energy. This area is known as the intertropical convergence zone (ITCZ), and is the area with the most active weather. Global Wind Pattern Wind – Jet streams • Jet streams are currents of air high above the Earth. They move eastward at altitudes of about 20,000 feet to 50,000 feet. Jet streams can reach speeds of 100 knots to 400 knots. • Both the Northern and Southern hemispheres have jet streams, although the jet streams in the north are more forceful. Each hemisphere has a polar and a subtropical jet stream. The polar jet streams occurs between the latitudes of 50° and 60° north and south of the equator. On the other hand, the subtropical jet stream is closer to the equator and occurs at latitudes of 20° to 30° north and south of the equator. Wind – Formation of Jet streams • Jet streams are caused by large differences in temperature and pressure at intersection of a cold air mass from the north and a warm air mass from the south, known as polar front. • When cold and warm air meets, the higher the altitude, the greater the pressure between the air masses, hence causes air to flow from the warm to the cold side. • As air begins to flow, it is deflected by the Coriolis effect, towards the right why a westerly wind is formed. Strong temperature and pressure differences intensify these wind to form a jet stream. Aircraft flying close to a Jet Stream may encounter Clear Air Turbulence (CAT) caused by Wind Shear. The CAT is strongest on the cold/low pressure side of the jet stream. Wind - Jet streams • Aircraft have difficulty flying against Jet Streams. Pilots either fly with the jet stream or above it; they do not attempt to fly against it. Aircraft flying from West to East will try to fly with the jet stream to increase ground speed. For example, flights from Tokyo to San Francisco follow the jet stream route, which is longer in distance but faster. • Aircraft flying from East to West will try to avoid the jet stream. For example, flights from San Francisco to Tokyo will take the most direct path (Great Circle route). Water Cycle Relative Humidity • Relative humidity is the amount of moisture in the air compared to what the air can "hold" at that temperature. Relative humidity is expressed as a percentage. • If air is gradually cooled while maintaining the moisture content constant, the relative humidity will rise until it reaches 100%. The temperature at which relative humidity reaches 100% is called dew point. If the air is cooled further, moisture will condense. • Warm air can hold more moisture than cold air. Hence as temperature increases, relative humidity decreases and as temperature decreases, relative humidity increases. Clouds and Fog • Clouds are small particles of water in the air. Ingredients necessary for cloud to form: – Water vapour – Cooling so that condensation can take place – Solid particles in the air such as dust, ice or sea salt for the droplets of water to stick to. These are known as condensation nuclei. • Clouds and fog form when temperature is at dew point and relative humidity (RH) reaches 100%: – Dew point is the temperature where air is saturated with moisture and can no longer hold additional moisture. – RH (%) is the moisture present in the air divided by the maximum moisture the air can hold at the current temperature x100. • Hazard of cloud and fog to aircraft: – Reduces visibility. – Cumulonimbus clouds are associated with thunderstorms. Cloud Formation Cloud Types Cumulus clouds are puffy in shape and resemble pieces of floating cotton. Cloud Types Nimbus clouds produce precipitation that reach the ground in the form of rain, snow or hail, depending on the ambient temperature. Cloud Types Cumulonimbus clouds are extremely dense, vertically developed with a low, dark base and fluffy masses that tower to great heights. They usually produce heavy rains, thunderstorms, or hailstorms. They are also known as thunderstorm clouds. Cloud Types Stratus clouds are low-level clouds characterized by horizontal layering with a uniform base. Cloud Types Cirrus clouds are high level clouds which look thin and wispy. They are made of ice crystals and not water droplets. Precipitation • Water droplets continue to accumulate in clouds until they are too heavy to be supported by air. These droplets fall out of the clouds to become precipitation. • Types of precipitation: – Rain / Drizzle – Snow – Hail • Hazards to aircraft: – Reduces visibility – Reduces braking efficiency on runway – Hail may cause structural damage to aircraft Precipitation - Thunderstorms • Thunderstorms are made up of cumulonimbus (Cb) clouds with heavy precipitation, strong wind, lightning and thunder. Most thunderstorms are formed by a cycle that has three stages: cumulus stage, mature stage, and dissipating stage. • Cumulus stage: Warm, moist air rises due to heat from the ground or due to a front pushing it up. • rising air becomes saturated and forms cloud • cloud grows in height rapidly and becomes towering cumulus cloud • super cooled water droplets form inside cloud • Mature stage: Precipitation begins to fall. - Falling rain / snow / ice creates downdraft - Updrafts and downdrafts exist side by side, creating static charge in cloud. Lightning and thunder occur. - Top of cloud begins to flatten and spread out, creating an anvil-shaped top. Precipitation - Thunderstorms • Dissipating stage: Downdrafts cool down the air beneath the cloud and weaken the updraft. Updrafts stop, lower cloud dissipates. Cumulus Thunderstorm Hazards • Hazards of thunderstorm: – Turbulence and microbursts. – Icing. – Structural damage caused by hail. – Electrical interference caused by lightning. Monsoons • Monsoons are major wind systems that seasonally reverse its direction. They blow for approximately six months from the northeast and six months from the southwest. Monsoons always blow from cold to warm regions. Northeast Monsoon – December to March Southwest Monsoon – June to September Monsoons In Singapore, the monsoon seasons are as follows: • The Northeast Monsoon from December to March. This is a wet monsoon and the prevailing surface wind is from the northerly direction. • The Southwest Monsoon from June to September. This is a dry monsoon and the prevailing surface wind is from the southerly direction. • The transition period in between the monsoons is known as the Inter-Monsoon period. For Singapore, rain fall in the InterMonsoon season is typically higher compared to that during the Southwest Monsoon season. Surface wind becomes light and variable further into the Inter-Monsoon period. Air Masses • An air mass is a large mass of air that has similar characteristics of temperature and humidity within it. An air mass acquires these characteristics above an area of land or water known as its source region. When the air mass sits over a region for several days, or longer, it picks up the distinct temperature and humidity characteristics of that region. • Air masses are classified according to latitude and their continental or maritime source regions. Colder air masses are termed polar or arctic, while warmer air masses are deemed tropical. Continental and superior air masses are dry while maritime and monsoon air masses are moist. • When two different bodies of air come together, they do not readily mix. Rather, each body of air will retain its individual properties, and a boundary forms between them. When two large air masses meet, the boundary that separates them is called a front. Air Mass Classification Fronts Boundary layers where two air masses of different characteristics (hot and cold) meet are known as fronts. Fronts Formation Process Warm A warm front occurs when warm air overtakes and replaces cooler air. The slope of a warm front is gradual and much shallower than cold fronts. Stratus clouds, rising temperature, precipitation Cold Cumulonimbus clouds, gusty wind, precipitation A cold front occurs when high pressure pushes cold, dense air along and forces the less dense, warm air upward, forming a curved leading edge that slopes upward. Fast moving cold fronts advancing into warm, moist air can trigger strong thunderstorms or lines of thunderstorms, also known as squall lines. Fronts Fronts Formation Process Stationary A stationary front occurs when the opposing forces of two air masses are relatively balanced. The boundary remains stationary for several days. Mixture of cold front and warm front weather Occluded Weather changes from warm front weather to cold front weather. An occluded front is produced when a fast moving cold front catches and overtakes a slower moving warm front.