This explanation might seem more complicated, but bear with me, because it helps understand the effect of various dewpoint spreads very nicely, and helps you anticpate changes on a given day:
Air will hold water in suspension- that is, as vapor. Vapor is not mist or fog or cloud. Normally you can't see it.
The little water molecules are all mixed up with the molecules of various gases we call "air"... and this is possible because of temperature.
The warmer air is, the more its molecules jump and zoom around. They bump into each other, too... producing more heat, and also increasing the space between molecules.
Into these spaces, attracted by molecular forces, rush free water molecules, usually released from their liquid state by the same heat that warmed the air.
This is how stuff dries out.
Okay- now that you see how water can get mixed with air, you see what "humidity" is.
How humid it gets depends on temperature, even more than saturation with water- hence really "muggy" weather. The air can be hard to breathe... it seems thick, but in fact the air-the gaseous stuff- is thinner; its pressure has gone up and it's spreading out. And if there's water vapor in there, the ratio of water to gas in a given volume can get quite high... so there's less air to breathe.
Your airplane engine notices it, too... although heat-induced air pressure is a greater factor.
But let's say that water-soaked mass of air cools. The air molecules start to settle down, getting closer to their comrades. The water molecules, having weaker attraction to the gas particles than to each other, start to clump together, forming mist or fog. Cloud and fog levels are determined by temperature at various levels, and this is why.
Water droplets form out of air when there is some particle or other solid thing for the water vapor to cling to, where it will condense more easily: a smoke or dust particle (at the heart of every raindrop and snowflake), or perhaps an airplane.
Now you see why dewpoint as well as temperature is an important indicator of icing conditions, also. Due to its higher velovity, slipstream air is usually cooler than the ambient temperature, so even innocent-looking mist can turn to water, then ice, on the surface of wings, etc. very quickly.
This is also why carb ice can occur, even when there is no visible moisture in the air or ice on the airframe. The air rushing into the carb inlet is at a much higher velocity, lower pressure, and thus lower temperature than the air just outside the engine.
Anyway, this point at which dew will form is the point where the temperature is low enough for the existing humidity level to cause formation of mist or droplets as that water is "squeezed" out. It will depend on how much water is present, which is why the dewpoint changes regularly in a given area.
So for flight planning, the closer the current temp. is to the estimated dewpoint temp., the things you watch out for are: poor visibilty and/or clouds, and carb or airframe ice, including pitot and static ports, etc.
If the dewpoint is getting closer and closer to the ambient temp, all this gets worse. If it's spreading, that means better weather.
Posted: Tue Jan 29, 2008 1:45 pm
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