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What causes Turbulence ? Answer Convectional Clouds and Air Instability
Temperature and Lapse Rates. Temperature in the lower levels of the atmosphere normally decreases at about 2 degrees Centigrade per 1000ft. This is known as the normal lapse rate. It does in fact vary from day to day. The standard temperature at sea level is given as 15 degrees Centigrade or 59 degrees Fahrenheit. The coldest the temperature gets is -57 Centigrade in the upper levels. It does not get colder than that. Temperature has a lot to do with stability in the air. Dew Point. Depending what the temperature is will define how much water vapour can be held in the air. When the temperature and the dew point are the same the air will become saturated. This will cause clouds to form or indeed fog if it happens at ground level. Dew point is the point where cloud or fog forms. dew point is given in weather reports. so if the temperature and dew point are the same fog could form unless there is a strong wind blowing. Dry and Wet Lapse Rates Providing the warm air remains dry when it rises it will cool at a rate of 3 degrees Centigrade per thousand feet. Once the dew point is reached and the air becomes saturated the Wet Air lapse rate is changed to 1.1 degrees centigrade per thousand feet. The air releases heat called Latent Heat. Stable Conditions
Lets look at a portion of air where the ground temperature is 16 degrees Centigrade. The lapse rate is is 2.2 degrees per 1000 Ft. Now lets say that a portion of air is heated to 20 degrees Centigrade at ground level. It therefore becomes lighter than surrounding air at 16 degrees and starts to rise. It cools at 3 Degrees Centigrade per 1000ft. When the rising air temperature reaches the same temperature at height as the surrounding temperature and both are equal the air stops rising or becomes stable. Therefore the air above this level is going to be smooth ideal for a pilot. Bearing in mind that at some point the air would have saturated and cloud formed. Providing the pilot was instrument qualified he could get above the cloud into the smooth air instead of getting bumped about flying through the rising air lower.These clouds are normally small ones. From this remember this rule. The atmosphere is stable when the lapse rate is less than the dry lapse rate for the day. The dry lapse rate remember is normally 3 degrees Centigrade per 1000 Ft In this example the air would be stable at 5000 ft and above. Unstable Conditions
Now lets look at the same example but instead of the lapse rate being 3 degrees per thousand feet it is 3.4 degrees per thousand feet. What will happen here is the temperature difference between the rising air and the surrounding air will be far greater. The air will rise rapidly in this case. The difference here is it maybe many thousands of feet before the temperatures become the same and the air is stable. This is the forming of towering Cumulus Clouds or worse the nastiest cloud of the lot Cumulonimbus. Even 747 pilots avoid these where possible they are very dangerous. They normally have ice hailstones severe turbulent air. The visibility is poor and you run the risk of getting hit by lightning. Underneath these clouds are also dangerous. Air is sucked in and up and then forced down and out. A term known as a microburst is also dangerous with many aircraft accidents happening where the air has forced the aircraft into the ground. The recommended safety distance for one of these clouds 20 miles. They are normally at their worst between lunchtime and late afternoon when the sun is the hottest.They can be easily identified. They either are anvil or mushroom shaped. these type of clouds are also associated with Cold Fronts.We will discuss fronts later on. A change of wind direction is also possible near these clouds which can be dangerous. This sudden change of wind direction is known as windshear. Temperature Inversions This means Cold Air is below warm Air This can cause problems for the pilot if not careful. This can be caused when instead of the temperature decreasing it increases with height. Sea fog is an example of this. Lets say that a parcel of warm moist air passes over the cool sea. The bottom of air may reach its dew point and fog forms. This can cause problems for a pilot trying to land at a coastal airfield. The pilot may have good visibility at 2000 ft but then descends into zero visibility at 1000 ft. Sometimes this fog is only about 100 ft high. it is enough for you not to see the runway. The next time this can happen is when there is cold air in advance of a warm front. The warm front catches the cold air up and the warm air is forced upwards. If there is a freezing level say at 5000 ft and the warm front clouds cool above it is possible the rain falls from these clouds. The rain falls into the freezing level and through it and attaches itself to an aircraft underneath. This could be very dangerous especially for a light aircraft with no anti icing systems. The pilot would then have to turn back or climb above the freezing level into the warm moist air. or even make a forced landing away from an airport. The third time this can occur is in winter. High pressure areas known as Anti Cyclones with light winds can cause a thin layer of Cumulus cloud to form. This cloud is known as stratocumulus bearing in mind heat is released when a cloud condenses this air will rise above the cloud. the sun being at its weakest would not be able to penetrate the layer of warm air so sometimes the weather stays like this for a few days. Ground mist can be caused on clear nights by warm air rising into the atmosphere as the night goes on the dew point is reached the grass gets wet. Or the air just above the ground gets wet and mist forms. In winter this water may turn into ice. We know this as frost.
I do hope now you are beginning to see how weather happens. I found this course very interesting. Some excellent photos of clouds can be found on my colleague Chuck Boudreaux's site in the AVSIM Education Resources Centre. The clouds in the weather index page photo are Cumulus Convection Clouds. As Great Britain is an island you can see that we therefore have a lot of moist air around the country. Plenty of chance for fog and poor visibility. In the next lesson we will discuss High and Low Pressure Systems.
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