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Temperature And Pressure
Radiation As we know the sun send heat waves to the earth. The heat it generates depends on the latitude of the place the wave hits. This is also affected by the time of day and the season of the year IE Spring and Summer Autumn and Winter. Hot surfaces re radiate this heat. Even objects on the ground hit by this wave act differently. For example a forest and water require more heat than rocks and sand. Some of these heat waves on clear night are reflected back into space and thus the ground is cold and frosty. When cloud is present these clouds reflect the heat waves. the clouds act like a blanket and some nights can then be humid and warm. Cloud cover by day has the opposite effect as it can prevent the earth from being heated. the result is the weather stays similar for days. Convection A result of the earth being warmed is it causes the warm air in contact with the surface to rise. This warm air expands and becomes less dense and is lighter than the cold air. As the warm air rises cold air (Wind) rushes in to replace it. The rising column of air is known as a Convection Current (thermals to glider pilots.) This is happening all the time all over the world. Air Pressure Air Pressure is defined as the weight of the air directly above the place under consideration. As a rule of thumb the average sea level atmospheric pressure is 14.7 Lbs per sq ft. This pressure is measured by the barometer. The standard sea level barometer pressure is 29.92 inches or mercury. or 1013.2 Millibars. The average standard UK setting is about 30.00 Ins. In the lower level the pressure drops about one millibar for every 30 ft. The altimeter therefore is really a barometer device. The pilot sets the altimeter to a known barometer reading and this displays the height. The altimeter detects the pressure drop and converts and displays in height to the pilot. The pilot has to be cautious as he fly's along his route towards an area of low pressure. Please Note that for every millibar the pressure drops, the altimeter will over read by 30 ft. So if the pilot was flying in cloud this could be dangerous especially if hills go higher than the height the aircraft is at. As pilots fly through the area they should use their radio to call airfields in the area to get a pressure setting from the tower or approach control of airfields they are near. A forecast Regional QNH may be given. The UK is divided up into Altimeter Setting Regions. Forecasters predict what the lowest QNH reading will be for the area for the next hour. This reading is known as QNH. QNH goes back to the days of Morse Code. Modern altimeters are accurate to about 10ft. In Flight Simulator a regular push of the B keyboard key will ensure that you have the correct QNH set at all times. Especially if you have downloaded the weather from the internet and you have been flying above transition level where all altimeters are set to 29.92 or 1013 Mbs. the QNH can vary rapidly in a short distance or change hourly. It is important to get the most accurate reading. Just like contour lines measure equal height on hills. Isobars measure equal points of pressure. From this we can then start to get a weather picture. The closer the lines are the higher the winds. Air flows from High Pressure areas to Low Pressure areas. This movement we have said already is wind. this can be demonstrated by blowing up a balloon then leaving the end open. The high pressure is then forced into the low pressure.
Humidity and Types of Air Depending where the air has travelled from will to some degree how humid the air is in a given area. For example air that has travelled over the sea from the direction of the Equator will be known as Tropical Maritime Air. It will be warm and almost saturated it will be able to hold a lot of water vapour. The further North it gets it will cool and get saturated hence the air will not be able to hold the same amount of water vapour and condenses and clouds will form. Likewise air from the Equator travelling over Africa will be dry. This will be known as Tropical Continental Air. Air coming from the Polar regions over the sea will be known as Polar Maritime Air. This air will be cold and wet. Air travelling over land from the Polar regions is known as Polar Continental Air. This is going to be cold and dry Density Last but not least is Density. We know from a bicycle pump air can be compressed. In the lower layers of the atmosphere the maximum density of air is .077 Lb.Cu Ft at the surface. The hotter the air the less dense it is. The Higher you go the less dense it is. This is important for pilots to know as this affects engine performance and take off and landing distance. If the pilot fly's a light aircraft above 3000ft then the aircraft will burn less fuel. The pilot can use the mixture leaner lever to reduce the fuel air mix. What the pilot does here is gently pull the lever out until the engine starts to splutter then just push it in slightly until the engine picks up again. This will save the fuel and the aircraft will be able to fly further than an aircraft flying at 1000 ft for example. Do not pull the lever right out the aircraft engine will stop and it has been known to ruin a good day. for those that are not familiar with light aircraft after your flight that is how you stop the engine you do not turn the ignition off like a car. A pilot has to be very careful with temperature and density as less dense air means less lift. For example Pilots flying out of Corfu Airport in the Greek Islands may for a flight to Glasgow with a 737 have to refuel on route if a full passenger load is carried on a really hot day. The time to realize this is at preparation time and not as the fence at the far end of the runway is in the middle of the windscreen on the take off run. It is accepted the runway is short as another considering factor. Airliners for the UK taking off from Skiathos have to refuel at Thessalonica. You have been warned to check your take off run length in the flight manual with the temperature. A rise 1 degree Centigrade can mean the aircraft weight must reduce by 2 tons in some cases to be within safe limits. Taking off from a Hot and High Airfield the pilot has to be extra careful as the air is less dense which means less lift and more take off distance. Mexico City would be a good example of this. Jets are very good at high level with fuel burn and guess what you are above the weather. Two good reasons why Jet airliners fly high. Having now given you all the bits that make up weather in the singular, lets now mix them up and see what happens. This will be done starting in the next lesson.
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