Dew point - what's it for?

MissFitz

MissFitz

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Dew point - what\'s it for?

Please excuse ignorance, but looking at the met observations, it always gives the dew point temperature, & I've just realised I don't know what the relevance of this is. Does it indicate the humidity of the air? Or, er, what? And what's the practical application of it?
 
Re: Dew point - what\'s it for?

Dew Point is the temperature at which the moisture in the air will condense into visible droplets. A good experiment is to take a large glass, fill with several chunks of ice then a large shot of gin, tonic and a slice or whatever your fancy. As you place it on the table you will note there is moisture appearing on the outside of the glass. This is condensing water from the air as the glass is now below the dew point temperature of the air in the room. By the way this experiment should be repeated as many times as necessary /forums/images/graemlins/smile.gif
 
Re: Dew point - what\'s it for?

Sorry, on a more serious note you can plot a graph of daily sea temperatures and dew point temperatures and when the two curves meet you will be able to expect sea fog to form. Useful off the Grand Banks and South West Africa.

You can also use the dew point to decide when to ventilate a space.
 
Re: Dew point - what\'s it for?

Also use it to decide when to paint / varnish / antifoul. Normal rule is don't paint when air or surface temperature is within 3C of dew point. Particularly relevant when varnishing in the afternoon as air temperature dropping usually means the dew point is rising so you reach the critical point very quickly. Damp air means varnish will go milky and gloss paint becomes dull. Then it loses adhesion as well.
 
Re: Dew point - what\'s it for?

This time of year a critical point is that if the dew point is above zero when the ground temperature drops below freezing ice is very likely form, e.g a hoar frost. This is one of the factors used by highway authorities in deciding whether to grit the roads.
 
Re: Dew point - what\'s it for?

Perfectly correct old chap and an excellent experiment which I shall repeat immediately....

For a full explanation look up Boyles law and Charles law (that's what the internet is for and my G&T await's. This will tell you how the dew point varies with pressure and temperature and howthe two are related. The temperature is the most important to us sailors as it varies most and indicates the temperature at which fog forms. When warm air with high humidity meets the cold sea surface it drops in temperature and water forms fog over the oggin just like wot you get from drinkin too many o' they G&Ts (Hic)

From Wikipidithing

The dew point is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation point.

When the dew point temperature falls below freezing it is often called the frost point, as the water vapor no longer creates dew but instead creates frost or hoarfrost by deposition.

The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates that the dew point is equal to the current temperature (and the air is maximally saturated with water). When the dew point stays constant and temperature increases, relative humidity will decrease.

At a given barometric pressure, independent of temperature, the dew point indicates the mole fraction of water vapor in the air, and therefore determines the specific humidity of the air.

The dew point is an important statistic for general aviation pilots, as it is used to calculate the likelihood of carburetor icing and fog, and estimate the height of the cloud base.
 
Re: Dew point - what\'s it for?

Can also be used as a very very rough estimate for cloud height in lower levels, every 1deg difference between temp and dewpoint equates to approx 400ft, so a 3deg difference would give a very approximate 1200ft cloudbase.
 
Re: Dew point - what\'s it for?

Another related advantage of examining the full glass of gin is that once the breeze lifts the mixture over the side of the glass it is already past the time to put that reef in! Known in some quarters as gindrift. /forums/images/graemlins/cool.gif
 
Re: Dew point - what\'s it for?

Thank you all for replies, very interesting indeed.

Clearly I need to lay in an extra bottle of gin - for the purposes of experimentation, naturally /forums/images/graemlins/smile.gif
 
Re: Dew point - what\'s it for?

Practical application:-
If the dewpoint temperature of the air is higher than the sea temperature then condensation will start to form on the inside of the hull and over seacocks.
Knowledge of this fact - particularly with regard to seacocks - will help eliminate "what if my seacocks have suddenly become porous or leaky?" type anxieties.
 
Re: Dew point - what\'s it for?

[ QUOTE ]
And what's the practical application of it?

[/ QUOTE ]

temperature at which the dew drop forms on the end of your nose. Indicates the need to have a hanky or a handy coat sleeve midshipman style.
 
Re: Dew point - what\'s it for?

You can't really get full use of the dew point without a simple understanding of Adiabatic Lapse Rates, which are pretty straight forward. Basically the ALR is the rate at which air cools as it rises, nominally given as around 10C per 1000 m. It actually varies though of course, and changes dramatically once the air is saturated (i.e. the Dew Point has been reached).

The Saturated Adiabatic Lapse Rate (sometimes Wet ALR) is around 4C per 1000m which might not sound very exciting but it's one of the main reasons you get Thunderstorms.

One of the most horrible things ever invented was the Tephigram, beloved of clever people like Dr. Simon Keeling PhD, MSc (Appld. Met. & Climatology). Unfortunately they're also one of the most useful ways of understanding going on (or having it explained to you). If you take them one step at a time they are actually quite logical.

If you dig just a little bit deeper you'll find have the basics behind Convection, Stability and Instability (and therefore Sea-breeze fronts), Inversions, and also sorts of stuff. It's fundamental to an aviation guy but doesn't seem to be covered in the same way all the time in the sailing world, where fronts and weather systems are concentrated on far more.


Couple of links if you want to look further:

http://www.metoffice.gov.uk/education/higher/lapse_rates.html

http://www.iac.ethz.ch/education/bachelor/atmospheric_physics/box_feeder/The-Tephigram.pdf

http://www.absoluteastronomy.com/topics/Lapse_rate#encyclopedia
 
Re: Dew point - what\'s it for?

[ QUOTE ]
You can't really get full use of the dew point without a simple understanding of Adiabatic Lapse Rates,

[/ QUOTE ]

You can if you only want to "huff" on your specs to damp them before you polish them!
 
Re: Dew point - what\'s it for?

Absolute rubbish. Without a full understanding of dry and saturated lapse rates, the RH of the breath etc. you wouldn't have a clue how far away to hold your specs would you?? /forums/images/graemlins/tongue.gif
 
Re: Dew point - what\'s it for?

The dew point used to be a standard method for humidity measurement.
A glass bulb would be cooled down untul condensation formed and the temperature was noted. The atmosperic pressure and temperature was also noted and a set of tables gave the relative humidity of the air.

It was all built into an instrument called a wet and dry,hygrometer,psychrometer or thermometer or something. It was all a long time ago!!
 
Re: Dew point - what\'s it for?

Dr. Simon Keeling PhD, MSc (Appld. Met. & Climatology) here (by the way, you missed FRMetS off the end /forums/images/graemlins/wink.gif )
Nah, the tephigram/skew-t is one of the most beautiful diagrams ever invented. Once understood (and all it takes is 30-minutes and then looking at regularly) so many more answers to those weather questions you always had are answered.

Meteorologists use dewpoint to assess how moist the air is. For sailors if is most useful for assessing fog formation, especially when a tropical maritime air mass is approaching. For example, monitoring the dewpoints i the southwest may reveal the dewpoint depression with relation to temperature narrowing. This indicates more moist air moving into the region and may highlight and increasing risk of mist and fog forming.

Pilots use it to roughly calculate the cloud base above ground level, and sailors can do this too. It is a simple formula of: (Temperature-Dewpoint) * 400 = cloudbase in feet (Above Ground Level).
 
Re: Dew point - what\'s it for?

And compressor manufacturers use it (dewpoint - not the tephigram) to determine how hot they need to have the discharge from their compressor - for any given discharge pressure - to avoid condensation forming in the oil seperation systems.
This is applied to both simple air compressors and hydrocarbon gas compressors - gas compressors have MUCH more complex dewpoint problems!
 
Re: Dew point - what\'s it for?

I started reading this thread at 2244 last evening ..... and only just woke up !! /forums/images/graemlins/smile.gif

Only joking - it's quite interesting really. I am finding that fully understanding climatic conditions and forecasts is all about practice and doing it as often as possible.
 
Re: Dew point - what\'s it for?

If you really want to understand what is going on, I can recommend "Atmospheric Processes and Systems" by Russell D Thompson. I have it on my desk at the moment and I got it in my local library. Really good.
 
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