Great article from our friends at International Paints on the importance of checking and recording relative humidity and surface temp before painting. This really makes it easy to understand, how the paint absorbs the heat from the surface being painted and as the solvents evaporate from the paint, it rapidly cools the air next to it and if it reaches dewpoint, puts moisture on your surface:
http://www.yachtpaint.com/sgp/diy/ask-the-experts/whether-to-paint-depends-on-the-weather.aspx
So when can we paint?
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First rule as said before is that the surface temperature of what we want to paint should be 3 degrees above the Dew Point.
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Generally paint when temperatures are rising to avoid getting caught by Dew Point. In some circumstances, if trying to coax paint or resin into a timber surface for example, applying product when that surface is in the stage of cooling down may help improve the products ability to penetrate. But be aware of Dew Point conditions.
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Do not apply paint above 85% RH.
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Preferably apply paint, especially two pack polyurethanes when RH levels are at or below 70%. One reason for this is that isocyanate curing agents react with moisture to various degrees. If this happens with your paint to any large degree then an improper cure can result. In the worst-case scenario yellowing of white paints and premature loss of gloss may occur.
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If you have sprayed walls and floors with water to lay down any dust, check that your local RH has not increased over the general RH outside.
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Keep within the temperature guidelines for application as stated on product data sheets remembering that the best and easiest application temperatures are generally those in the middle of the recommended ranges i.e. temperate conditions rather than extremes.
This is also a great youtube video, it teaches you how to obtain and use your temps then convert the information using two simple graphs. Using this method you will never end up with a flat, dull paint job due to moisture (dewpoint). Anyone following a Quality Assurance methodology already uses this system.