Understanding Fog Formation and Dew Point: A Guide from Wikipedia

[rudransh verma]

I read in Wikipedia that fog is formed when the water vapor in air is condensed to form liquid water that are suspended around condensation nuclei. Fog or clouds don't move from some place but appear in places through condensation. Now there is something about dew point which has to do with the fog formation. I don't get the concept of dew point https://en.wikipedia.org/wiki/Dew_point

 

[Baluncore]

As air rises and the pressure falls, the temperature also falls due to the lower pressure. At some point the water contained in the air will begin to condense because the air has become saturated and cannot hold as much water vapour as it did when warmer. That temperature is the dew point for that air.

What do you not understand about the dew point concept ?

 

[rudransh verma]

As air rises and the pressure falls, the temperature also falls due to the lower pressure. At some point the water contained in the air will begin to condense because the air has become saturated and cannot hold as much water vapour as it did when warmer. That temperature is the dew point for that air.

What do you not understand about the dew point concept ?

So as temperature lowers air becomes saturated. Does it mean that due to low spacing between molecules there is not enough space to hold much water and so we say the air becomes saturated at lower temperature?
Water vapor coalesce to form bigger droplets and is thus seen in sky as clouds and fog. Its like Boom there is a cloud. At dew point fog appears.

 

[Baluncore]

Does it mean that due to low spacing between molecules there is not enough space to hold much water and so we say the air becomes saturated at lower temperature?

You do not need to invent a mechanism for why cold air can support less water vapour than warm air, you just have to accept that it does.

Its like Boom there is a cloud. At dew point fog appears.

Cloud that forms close to the ground is called fog. Cloud and fog form when air is cooled below the dew point.

 

[rudransh verma]

Cloud and fog form when air is cooled below the dew point.

So the fog is made of dew, dew that was water vapour before cooling of air. Condensation of water vapor happens at condensation nuclie.
Its like water is leaking from the air and remains suspended in air.

 

[Baluncore]

Fog is small droplets of liquid water that form in air that cools close to the ground.
The term “dew” is reserved for condensation that forms on vegetation or the ground.

 

[rudransh verma]

Thanks @Baluncore

 

[russ_watters]

The term “dew” is reserved for condensation that forms on vegetation or the ground.

That isn't really true. Dew point is a useful concept regardless of where it occurs. For example, the supply air temperature of air conditioners is selected based on dew point to provide a resulting room relative humidity..

You do not need to invent a mechanism for why cold air can support less water vapour than warm air, you just have to accept that it does.

There is a reason/mechanism though, and that reason is energy and vapor pressure. The same energy that makes water vapor molecules bounce around prevents them from from binding together into a liquid. More energy/temperature = higher vapor pressure/dew point.

 

[anorlunda]

It shouldn't be hard to understand. Both evaporation and condensation happen on the surface of a drop. At lower temperatures, condensation happens faster, and the drop size grows until it gets big enough to be visible. At higher temperatures, the evaporation happens faster and the drop size shrinks until it becomes invisible. The invisible border between those two cases is called the dew point temperature.

Just last week I watched clouds get thinner, then vanish right over my head as I watched. I remember thinking, "Remarkable. There's just as much water up there as there was a few seconds ago, but now I can't see it."

 

[Baluncore]

"Remarkable. There's just as much water up there as there was a few seconds ago, but now I can't see it."

I live down-wind from a mountain where there are often wave clouds. The lenticular clouds maintain their position on the standing waves, although they can be seen to form, and then to dissolve again as the air passes. It is the very slight variation in water content that makes the process visible. When back-lit at night, by the full moon, they can be fascinating.

That isn't really true. Dew point is a useful concept regardless of where it occurs.

I agree.
The "dew point" is a temperature, while "dew" is a noun, like "fog", and "cloud". Put simply, all three are liquid water that is condensed from the excess water vapour dissolved in the air. They first appear when the temperature of the air falls to the dew point.

 

[rudransh verma]

The same energy that makes water vapor molecules bounce around prevents them from from binding together into a liquid. More energy/temperature = higher vapor pressure/dew point.

So due to low spacing between molecules there is not enough space to hold much water and so we say the air becomes saturated at lower temperature is WRONG.
The air with a particular temperature has all kinds of gases + water Vapor. At higher temperatures this Vapor is bouncing around at high speeds and cannot coloease , are invisible but at lower temperatures they slow down and form bigger droplets which are visible. This is the point of condensation and saturation and thus clouds and fog are formed.

 

[rudransh verma]

@russ_watters I was taking a bath in my bathroom and turned on the hot shower. Couple of minutes later it was all foggy. Why?
Is it because all the hot water's vapor collected in the bathroom giving an impression of fog?

 

[russ_watters]

@russ_watters I was taking a bath in my bathroom and turned on the hot shower. Couple of minutes later it was all foggy. Why?
Is it because all the hot water's vapor collected in the bathroom giving an impression of fog?

Warm, humid air from inside the shower mixes with the cooler air in the bathroom. Cooling the humid air makes water condense into fog.

 

[rudransh verma]

humid air from inside the shower

How can a shower give air? It is made to give water.

 

[russ_watters]

How can a shower give air? It is made to give water.

When I said "shower" I meant "shower stall". Hot water evaporates into the air in the shower stall, making it warm and very humid.

 

[rudransh verma]

When I said "shower" I meant "shower stall". Hot water evaporates into the air in the shower stall, making it warm and very humid.

Ok! So in India there are no stalls just a shower on the wall.
When the hot water evaporates in the bathroom it adds vapours inside the bathroom. After saturation of cold air fog is formed.

 

[davenn]

That isn't really true. Dew point is a useful concept regardless of where it occurs.

I think you missed the point ( pun intended)

Baluncore was pointing out the difference in meanings of DEW and DEW POINT ... totally different meanings
even if the "dew" on the ground etc forms at the "dew point"

 

[davenn]

So as temperature lowers air becomes saturated.

No, the air is already saturated ... but as it cools, that water condenses

 

[Baluncore]

No, the air is already saturated ... but as it cools, that water condenses

If the air is above the dew point, it will not yet be saturated and there will be no cloud. Cloud only forms when the air cools to the dew point, when it reaches 100% RH, and is then saturated.
So long as cloud or fog is present the air is saturated with vapour. The excess water is liquid in suspension, or will fall as rain.

 

[Tom.G]

https://www.weather.gov/arx/why_dewpoint_vs_humidity

 

[rudransh verma]

@russ_watters so why do there are drops of water in the upper half of the wall of a half filled jar with water is because there is moist air above water that gets condensed due to cold walls of jar. Same thing happens when you put a cold glass of water in humid weather. And same thing happens when we are in a car in cold weather. There is moist air inside the car that gets condensed on the cold windshield and so the glass gets all foggy.

 

[Hall]

It does no harm to mention that, although the fog and clouds consist of water droplets but these water drops are well below the freezing point and we do have some ice-particles, too, in them.

 

[Fisic]

Dewpoint is a proxy for the concentration of water vapour molecules in the air (i.e. the mixing ratio). The higher the dewpoint, the more molecules of water there are per kg of air (mixing ratio is quoted in grams of water per kg of air, g/kg). At standard atmospheric pressure (1013.25 hPa), the mixing ratios of different dewpoints are listed below.

Consider a tiny speck of dust/smoke/pollen (condensation nucleus). There is always an equilibrium between condensation and evaporation of water molecules in the surrounding air onto and from its surface. This equilibrium depends on the temperature of the air and the mixing ratio.

At a certain temperature (above the dewpoint) some of the molecules have enough energy to break free of the surface tension of the droplet while other do not (the energy distribution has a finite width). As the temperature changes, the equilibrium shifts, in favour of evaporation (condensation) as the temperature rises (falls). As the temperature falls, condensation is favoured until the temperature matches the dewpoint and evaporation is suppressed enough to bring the mixing ratio of the surround air down to the value for that dewpoint. Further cooling causes more condensation and hence the droplet grows, following the Clausius-Clapeyron relationship.

Take a cloudless autumn night. Radiative cooling causes solid surfaces (e.g. a blade of grass) to cool more rapidly than the overlying air (as measured at 1.5 metres). But the air immediately surrounding that surface cools to the same temperature as the surface and, depending on the dewpoint/mixing ratio of that air, may reach the dewpoint and cause condensation to occur on the surface (i.e. dew-formation). Further cooling causes the dew droplets to grow. If the dewpoint and cooling temperature of the surface are both below 0 °C then that dew will actually be frost.

It's the same for radiation fog, except in this case the cooling to saturation occurs through a much deeper layer of air (1 to hundreds of metres). Advection fog occurs when warm, moist air moves over a cold surface.

 

[Vitina]

A demonstration of cloud formation I would give involved adding a small amount of water to an empty 2 liter pop bottle, placing it loosely capped in the freezer. Tightening cap on removal and letting it come to room temp, it is under pressure at 100% humidity. When cap is opened you get a cloud.
The three major variables are humidity, pressure, and temperature. In addition the presence of liquid water, nucleating agents, and light complicating calculations.
Has anyone seen a checkerboard sky, produced by pressure waves from separate storm systems (like two stones falling into a pond), the high pressure ridges give clear sky and low pressure troughs give a puff of cloud.
Industrially this "dew" point is tracked using the reflectivity off a chilled mirror. As the mirror is cooled it looses reflectivity, as it gets fogged the dew point is tracked. This is a critical measurement in paint systems, both electrostatic and regular.