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Pressure Systems As mentioned earlier in the course, air flows from high pressure areas to low pressure areas. This is not 100 % true as there are other things that affect it. Because the earth is spinning (The Coriolis effect) this deflects the wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Talking about the Northern Hemisphere we have two pressure systems. High and Low Pressure systems. We said earlier that isobars, draw lines of equal pressure together, just like hills are shown on the map with equal height points. In the Northern Hemisphere air (Wind) flows around a High Pressure area in a clockwise direction. This is also known as an Anti Cyclone. High Pressure is known to be associated with good weather.
In the Northern Hemisphere air (Wind) flows around a Low Pressure area in an anti clockwise direction. The area is also known as a depression. For the pilot of an aircraft in the Northern Hemisphere if the aircraft started to get starboard drift the pilot would know he was going towards the centre area of a low pressure area.. Check this by looking at the bottom of the second image. Imagine you are flying from bottom to top If the pilot got Port Drift he knows he would have flown through the centre of low pressure and was now flying away from it towards High pressure In image one. Low pressure areas are known to produce the bad weather. Or another way starboard drift into bad weather port drift into good weather. Do not get to confident though as often there may be more than one low pressure area on route. Do not forget the altimeter over read when going towards low pressure area as the pressure drops. Imagine you are looking North on these diagrams above and the blue bit is the British Isles. You can now see that all the bad weather comes from the Atlantic which is tropical Maritime Air or Polar Maritime Air which contains the most water vapour. Another tell tell sign when High pressure is over the UK is when aircraft start taking off on Runway 09 At Heathrow or 08 at Gatwick all day. This one is not 100 % full proof though. High Pressure Area An area of High Pressure is known as Ridge. This normally gives good weather for between 12 to 24 hours. And then the conditions get worse. As the ridge moves towards you the wind speed drops the wind veers (changes compass direction in a clockwise direction) and the barometer rises. Cloud also decreases. As the ridge passes you pressure drops and the wind backs ( changes compass direction anti clockwise) and becomes stronger. The cloud starts to thicken up. Bad weather is then on its way. The Low Pressure Area is now approaching. High pressure areas move normally very slowly across the ground. Low Pressure Area The cyclone or depression as a low pressure area is known can be in three types :- 1. Deep. This has a low pressure barometer setting between 960 to 970 millibars. 2. Intense. Here the isobars are very close together this makes the wind more stronger. 3. A shallow depression. Air in this one rises more and brings more rain and cloud and thunder. If the pressure is dropping it is known as deepening. If the pressure is rising it is filling
You sometimes here the word secondary depression used. This means another system within a low pressure area it rotates anti clockwise around the main system. It moves also with the the main low system known as the primary. This is really bad for flying as it is often gales in this area. Sometimes you will hear mentioned a trough of low pressure. What this means is instead of a nice circle of equal distance isobars they distort into a V or U shape.In the middle of the V or U weather is bad normally and pilots should try and avoid this if at all possible.
The last part of this is a COL
A COL occurs between two areas of High Pressure and two areas of low pressure. The weather in the COL can vary. Winds are light. The weather depends on the history of air. In winter fog can form. In summer thunderstorms can trigger.. The Col is like the High Pressure system it will last between 12 and 24 hours. A Low Pressure system normally moves at about 20 MPH from South West to North East across the United Kingdom. So if the Low pressure off Lands End and you want to pleasure fly for while you can work out how many hours its going to take to reach you. be back in time to land safely. A pilot has to bear all these things in mind when planning a flight trying to pick the best route for his passengers. The next lesson we will look at the weather fronts that are in the Low Pressure Areas. We will discuss winds more later in its own article.
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