In theory, but as said before, it has to have dust or something floating in the air to stick to. I've seen 2 and 1 degree spreads that haven't formed any visible moisture before. It just means that it's likely to become visible if it finds something to form on (such as a cloud of dust)----------------------------------------------------------------John MorganReal World: KGEG, UND Aerospace Spokane Satillite, Private ASEL 141.2 hrs, 314 landings, 46 inst. apprs.Virtual: MSFS 2004"There is a feeling about an airport that no other piece of ground can have. No matter what the name of the country on whose land it lies, an airport is a place you can see and touch that leads to a reality that can only be thought and felt." - The Bridge Across Forever: A Love Story by Richard Bach

>>When dew point and temperature are within 2 degrees of each other you >>get fog.Not sure your statement makes much sense. As mentioned by several people, you will not get dew/fog/mist/cloud if the dew point is lower than the OAT so it doesn't make much sense to for a statement about the dew point being 2 deg higher than the OAT. In fact, the dew point in the UK is usually about 2 deg lower than OAT. Futhermore I have noticed that the dew point doesn't go above the OAT anyway. I suspect this is because of the release of latent heat into the atmosphere when moisture condenses.

Bruce,Hmmm. Interesting, but doesn't stand up to a simple empirical out of the window experience. Cloud forms when the OAT meets the dew point. So, in a low pressure system air convects, as it convects it cools (OAT reduces), as the pressure drops the dew point reduces less than the OAT, eventually the two meet and you get cloud (visible moisture).Precipitation on the other hand is a different matter. There are currently two theories on this and it seems both maybe correct under different circumstances. One is that visible moisture droplets continure to convect and collide with other increasing their size and eventually fall as rain. The more violent the convection the harder the rain, if it goes high enough it comes down as hail...think thunderstorms. The other is that crud in the atmosphere attracts droplets to merge, when they get to big to be supported by convection that fall as rain. You maybe thiking of the latter case.>>If the air is extremely clean, like at very high altitudes, the air may >>become "supersaturated" with the relative humidity exceeding 100%.Actually, the situation is totally the reverse. Air at high altitudes is extremely dry, with relative humidities in single figures. This is why air is recirculated in airliners, so that the amount of water that needs to carried to moisten the air is reduced as people's breathing will raise the humidity in an airliner to about 15%-20%

well I stand corrected on a couple of points: Bruce is correct about the need for nuclei, although there may be other processes too. I knew this and oversimplified in my haste...drat! more seriously, it seems that my skepticism concerning gardner pilots was misplaced. There do seem to a number of them (growing dill, no less!) and thus Martin, in his orginal post, might be on to something about the reason for the dew point in a METAR :-) It now occurs to me that one reason visibility is also included is that many pilots probably also paint, and this information could be useful if they planned to paint a landscape at the airport that day. :-)

My idea of gardening is to turf over the flower beds cos all I have to do then is mow - in fact I don't even do that, just pay for somebody else to do it :)

Thanks for all thin info. Lots to digest.I never saw the point of downloading real-time weather up to now. But now I realise just how much it can add to the immersion. I'm dying to run into a fog bank caused by the dewpoint dropping below the air temp.Thanks againMartin

perfect timing. HiFi Sim just released ActiveSky 6. buy it and you will be amply rewarded. Enhances the nature of the sky you encounter visually and also models all these little weather aspects in excruciatingly realitic detail (and solves some bugs with the default weather). Once you have ActiveSky, you pay a great deal of attention to what appears in the METAR.

I rarely pay attention, even when flying the big ironic. I only want to know what active runway my departure and destination is using, I enjoy surprises!

This topic seems to be moving away from FS General Discussion and into meteorolgy, specifically cloud mirophysics (probably more suited to the hanger chat forum).You mentioned many different topics in your reply, all related to my simple comment that water can be supersaturated before if condenses in the air, not on a surface (the rose petal).So without going into details, suffice it to say that water vapor will supersaturate in nature to levels near 120% when cloud condensation nuclei (CCN) are not present. The surface tension of a liquid water droplet is very resistent to the collection of water vapor, even when the vapor pressure equals the saturation vapor pressure, i.e., 100% relative humidity. I'm talking about simple condensation here, not the collision and coalescence you mentioned in your dicussion on precipitation. I thought it was interesting that in the laboratory, Wilson had the ability to purify the air to the point that he achieved 800% supersaturation. That is an amazing feat.With the availability of CCN, which can range from microscopic to sizeable, supersaturation only occurs at a few tenths of a percent before condesation occurs. Anyone looking closely at a ray of sunlight entering a room will see bits of dust and particles reflecting the sunlight as they float about the room. These are some of the particles that water vapor likes to seek out rather than try to stick to itself.So I was not referring to the Bergeron

I have just bought (would you believe it?) wxRE. I wanted it to work with FS2002 as we;; as with FS2004, and I was concerned about framerates with something as complex as AS6 - my computer is just about at its limit with my FS2004 setup. I will get AS6 (or by this time next tear version 10) one day.Martin

Thanks Bruce. I'm reading a few books on avaition weather so your explaination is helpful to me (and timely!). of course, explaining the weather is easy, the real challenging part of the Martin's orginal post is trying to explain why pilots would garden :-)

Don't know if AS6 works with FS 2002, but the improvement of ASV, ASV/E, or AS6 (the recent products) over WxRE is substantial. I owned them all. Woth upgrading if do-able