Thermal came from a greek word which means "heat". For all gliders, thermal will be the fuel and power source for it. Without it, gliders will not be able to soar in the open skies. And even birds rely on it too. A very good example of birds which soars are eagles, hawks, cranes and even swift.
The main source for thermal is the sun. The higher it stand in the sky, the more light and warmth it emit to the irridated surface below. The sun irridates everything around it be it water, soil, trees, roads and even houses. Anything that reflect light will also reflect heat and eventually thermals are form . As thermal gathered together it will develop into a bigger thermal called thermal columms. Apart from that, there are also thermal called "Boomer Thermal" which you will only encounter it at high altitude and it will take you several meters in a minute.
Back in the 1954, Carmichal started out with 3 standard models:
Strong thermals
Weak thermals
Wide thermals
Strong thermals refer to thermal updraughts that produce maximum rate of climb of 6 meter per second. The radius of such thermals are small, Charmichal's measurement gave it a maximum of 200m.
Weaker thermals are less than 3m per seconds for the rate of climb and in the center of the thermal, it can reach up to 5m per second. Weaker thermals have slightly a larger radius and it can be up to 300m wide.
Besides the irridation of the sun, the temperature is also one of the key factor. As we all know at our latitude temperature is much warmer compared to the to higher altitude. When warm air break loose, it will raise up till it cools down when it reaches certain altitude. So if the ground temperature is low and the differences are minimal, then you won't get any good thermals. In the tropical place such as Singapore, Malaysia or event Thailand, you will get thermal almost everywhere. It's only a matter of how wide and how frequent the thermal are produce.
There two main different areas which causes thermal develop which is dark and light areas. Light area consist of area such as corn field, open field, dry grass field, stone quarries, buildings, housing estate...etc whereas dark areas are humid meadow, forrest, empty car parks, mash,..etc
As we all know that here in singapore we do not corn fields, but we can take advantages of our surroundings such as open land ( tuas area ) or open field ( old holland road or even bukit batok )
The best time to hunt for thermal will be between 10.30am to 4pm. During this time, there will be lots of indicators to show you whether there is a thermal near you or far from you. A very good example of thermal indicator are the birds call "swift". They are masters in finding the core of the thermal as they feed while in the thermals. As we all know that whatever in the thermal's way will rise, so does insect which will be the "swift" food. You can tell whether if there is a thermal by seeing the "swift". If the "swift " flies chaoticly means there is thermal bubbles or a none rotating thermals and if it a rotating thermals, the "swift" will normally fly in an accordingly to the direction of the thermal rotating.
Another way to locate thermal is to see a sky predator ( eagles & hawks ) circling the skies. If you happen to come close to the eagles or hawk and start following them, you will notice that your glider will start to ascend.
What if there is no "swift" or eagles in the sky? Is there still thermal around? Yes, there is. How? The clouds!..clouds ( Cummullus ) will give you a very good indication of thermal activities around you. Try observing the clouds, you will notice there is a new cloud and old clouds. Thermals are normally present in the birth of a clouds. But the cloud are decepating so does the thermals.
What about in the evening, does it still have thermals around? Yes, there are and it comes from the dark areas like the forrest, sand quarries, roads, emptied car park. These dark area absorbed the heat during the day and released it during the evening time during sunset. But thermal that you get from them are aomewhat weak. If you are flying a 1.5m glider than you may have the chance to catch the thermals.
I won't go in details on how to read the clouds as it's indeed a very large episode. Anyway, I've got this book called " The Secret of Thermal Soaring" by Markus Lisken and Ulf Gerber. The book explain the fundamental of thermalling as well as giving you the better understanding of thermals. I got mine from RCS ( Radio Control Sports ) and it was the last copy of it. I believe you could get it from the net. Besides books, there is also some DVD's from Radio Carbon Art which you could get from www.soaringcomposites.com
Apart from that I have also some website which I have browse thru and it will actually help you to have a better understanding about thermals.
http://www.quicktechhobby.com/articles/thermal_surfing%20part%201.htm
http://www.quicktechhobby.com/articles/thermal_surfing%20part%202.htm
http://www.rc-airplane-world.com/thermal-soaring.html
There are many more websites in the net which i didn't mention about thermals and soaring. Try searching the net and you will be surprise that there are lots of information about thermals.
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