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Guest R_Driscoll

Weather

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I found some of the technical talks at AVSIM on aircraft helpful, and thought I should try and contribute something back on weather. Possibly this post could start a thread on the topic. I'm not especially expert, but am trying to see FU3 reach the highest standards possible. So I thought I'd start the ball rolling by talking about clear air! This is meant to be practical and useful - not just academic!I have put here some notes on theory, then how this is implemented in FU3, then finally how you could change the settings to your own taste. So here goes.We fly in the troposphere, a word meaning "turning sphere", referring to large scale vertical mixing due to its unstable nature - hot air at the bottom, cold air at the top. The instability is due to convective currents - hot air is less dense so tries to rise. Thetroposphere extends to about 30,000 feet (I'll use feet as conventional for aviators) at the north pole to about 60,000 feet at the equator, the bulging caused by the Earth's spin.This contributes to the sky radiation at ground level being less intense at higher altitudes (greater depth of air), although I think higher lattitudes are a lighter shade of blue, the opposite of what sea colour does - less light means darker water.Temperature, pressure and air density decrease with height, temperature decreasing at a close to linear rate (the famous 2 deg C per 1000 ft which is a good rule of thumb for static air) from about 20 deg C to -60 deg C. Pressure and density decrease in a different manner (exponential), with about a half of the earth's atmosphere below 18,000 feet.Above the unstable troposphere is the stratosphere ("stratos" being greek for "layer"), stable due to air temperature increasing with altitude (due to radiation). Some long term weather effects can affect the stratosphere, for example causing "mother-of-pearl" high altitude clouds - generally the stratosphere is dry and cloudless.The colour of the air is affected by the light source (the sun during the day), air properties (absorption caused by Rayleigh scattering off particles such as water and dust) and the object being looked at (scattering of scattered light). Low wavelengths are absorbed by oxygen, nitrogen and ozone in the upper amtosphere (stratosphere), but white light is relatively unaffected. So basically white light enters the troposphere. The light we see will then be affected by our altitude, what we are looking at, air properties etc. This is modelled in FU3 by seven parameters:1) sky_RayleighScatterRedHW 0.012) sky_RayleighScatterGreenHW 0.0233) sky_RayleighScatterBlueHW 0.054) sky_RayleighScatterRedScatterHW 0.00045) sky_RayleighScatterGreenScatterHW 0.0026) sky_RayleighScatterBlueScatterHW 0.0037) sky_RayleighOpticalDepthHW 5000As I didn't write FU3 I have had to guess and experiment as to how these are used, and I am usually wrong. But this is how I think they work:HW simply indicates Hardware values (i.e. graphics card settings), rather than calculating colours by software (direct program code).The first three (1-3) refer to normal light scattering as it travels through the air, indicating relative amounts of scatter. We see the scattered light. I.e., if you were is space, space looks black because particle density is too low to give sufficient scattering. Unless you look directly at the source! (As we increase the amount of space junk up there, this may change );Thus blue is scattered most (sky_RayleighScatterBlueHW = 0.05), then green, then red. We see mainly blue and green, giving light blue skies. You want green sky, then drop the blue.The next three (4-6) refer to scattering of scattered light off objects (eg clouds). So to increase the greenness of clouds, you would expect to increase sky_RayleighScatterGreenScatterHW. Unfortunately it seems to work in reverse! So if my clouds look too red (with haze off), I simply increase sky_RayleighScatterRedScatterHW. The default value here is a bit low, so I suggest increasing it to 0.001 - but you should experiment for your particular monitor/card setup.The final parameter refers to absorption by air. Clouds have an optical depth of about 10 m in FU3, meaning that any light travelling 10 m through a cloud has lost about 63% of its intensity. Air has an optical density of 5000 m in FU3 (I guess they factor in height here somehow as well), but you could change that if you lived, say, in a city. So more than 5000 m of air thickness will start to look light blue (settings 1-3). This causes an effect a bit like high level haze, except the haze is sky blue in colour!To put this into practise, you could1) turn off haze (we are only looking at sky colour).2) choose noon, ground level.3) change first three parameters until sky colour matches what you like.4) change next three parameters (4-6) until high clouds have a colour that you like (light fluffy clouds should look very white; cumulous a tinge yellow, darker undernearth; stratospheric clouds pearly white; cumulo-nimbus dark greys depending on distance.5) don't bother changing optical density unless you are real real keen. Its pretty good as is.6) Put the haze back on (Alt-O, then push slider bar until it points at the "H" for haze is what I do).Sunset effects are caused by low-level haze, and I won't try and cover them here. Go see Laurie's site for some good sunset advice, or play with the sunset settings yourself! You should be able to get brilliant orange, red or yellow sunsets almost at will with a bit of practise. Or green. Of course, just as in the real world the daytime haze will have the same hue, but increased saturation so that it looks whiter. But make the haze too thick and you'll have sunset colours at noon. More on this another time.I seriously welcome any constructive feedback, amendments, improvements, insights or contributions of any sort to this thread! Please don't leave me standing here with my head sticking out all alone ...More advanced:I recommend using specular as the haze option for everything. The fluffy white clouds will then colour correctly at evening (although they are a bit too white during the day!) - you can always make them smaller (see annotated configuration file in AVSIM library) if you want. Mixing haze methods is not a good option - you will end up with different aspects of the world being coloured in a different way, for example models looking green or black rather than blending in correctly.Depending on feedback, I will try to write a similar piece about clouds at some stage. No promises!Or someone else can write it ...Rob D

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Hi, Rob. First, thank you for the annotated config file in the library. Second, thanks for this post. I am a weather nut myself :), and I take every opportunity to improve on it if I can.One question if you will: have you found out which parameters affect the shape of the anvil-looking free-standing clouds that repeat themselves over the horizon? I think these are what you called fluffy whites(?) in your post above. Any way to change the shape?Cheers.

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Naji,I agree that it would be nice to have a bit of variety in the shape of those clouds. The distribution of these clouds is well modelled in FU3, but not every cloud is the same size and shape in the real world.As for actually USING complex weather in FU3, I will have to wait for a more powerful PC. With graphics details at MAXIMUM, and 4xS anti-aliasing in operation, even a 1.2 Ghz Athlon Thunderbird struggles ! Of course, having a 2.4 Ghz Pentium 4 may help you somewhat in this department :-)Chris Low,ENGLAND.

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Naji and Chris,I assume you mean the fluffy whites at low altitudes that appear/disappear as you fly towards them! (doesn't "anvil" usually refer to the big CN clouds that reach the jetstream and get blown into that characteristic pointed shape?).The cloud models do allow variation. I'm sorry but I haven't experimented much with this. Give me some time to think about this and I'll get back to you!But a few random thoughts:- At some point I wanted to make the white fluffy clouds much bigger, and the configuration file I uploaded implements this. LGS intended them as small blue sky fluffies I think (i.e. light low cumulus as you get in the tropics). I will have to read back over what I did - I don't remember now how to control the size (sign of old age).- Several different cloud shapes are implemented in one of the RES files and can be viewed using GP's resviewer. I will look up details tonight as to which res file etc. But I think you only see these models at a distance or if you switch off the puff-model clouds - someone will correct me if I'm wrong. Again, I will try to check tonight.- There are at least two parameters that affect randomness in puff cloud models, I will check their names but they are something like puff-puffx and puff-puffz (sounds like a steam train) - those names aren't quite right but I will check and give you proper details - they are supposed to affect the shape of the puff model clouds.- I have never seen variation in shape in the low alt white puffy clouds, and don't think its possible. So I hope someone disagrees with me on this and shows me how to do it because it would be nice!- The amount of cloud coverage in the typical default weather files is very low. So you are better to master the weather program from Markus at site: http://www.geocities.com/in_04/Here you can define multi-layers, higher oktas, wider geographic ranges etc.Now it would be nice if some people systematically developed a range of good weather conditions (by good, I also mean rough!) properly named. Rob D

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>Of course, having a 2.4 Ghz >Pentium 4 may help you somewhat in this department :-) >Chris, with the weather I fly in, even the 2.4 is hardly catching up :-lol.Have a look in this thread over at BFU for screenshots of the weather I flew in a few days back:http://ftp.avsim.com/cgi-bin/dcforum/dcboa...rum=DCForumID43I'm starting to build up my case to convince my wife to let me get the new 3.06 :-eek. And then the new nVidia NV30(?) card is coming out in a month. So I'll have to make another case then :-lol. It is never gonna end, hehe.

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Yes, the new 3.06 Ghz Pentium 4 (with hyper threading technology) has caught my eye recently. The only problem is the excessive cost of the CPU itself. As for the NV30 graphics chip, this will be completely wasted on FU3. Remember, it's the processor that's important for our little flight sim world :-)Chris Low,ENGLAND.

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