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6 Answers

Dew Point

Asked by: 32795 views Student Pilot

Hello !

I am a little unfamiliar with the dew point...

when i read in a METAR : .............. BKN080   13/10  

 

13/10 is the temperature and the dew point ...

What is the Dew point ..and why do i get these information together ?

Thanks  a lot 

P.G.

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6 Answers



  1. Brent on May 24, 2012

    Dew point is the temperature at which the moisture in the air will condense into liquid water. The information is presented with the temperature, because dew point is usually only interesting in relation to the current temperature. A low dew point spread, such as the one in METAR you’ve posted, could be indicative of the formation of fog. If the dew point is higher than the temperature, and the temperature is below freezing, then there is an icing risk.

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  2. Micah on May 24, 2012

    The temperature/dew point spread can be useful to indicate all types of cloud formation, including fog, and is useful to predict cloud formation and height, and is an indicator of the type of air mass you’ll be entering. 

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  3. Kris Kortokrax on May 24, 2012

    Brent,
     
    When the temperature and dew point are equal, the relative humidity is 100%.  Dew point will not be higher than the ambient temperature.

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  4. Wes Beard on May 24, 2012

    When the dew point is higher than the temperature, the air is said to be super saturated and rarely happens.
     
    Icing can form in temperatures above freezing up to +10°c.  The biggest factor in whether or not icing will form is the size of the water droplets.  Super Large Droplets (SLD) will hit the aircraft and travel backwards.  They will cool quickly and freeze in weather above freezing.  If the droplets are quite small, they will not freeze no matter what the temperature is.
     
    You can remember what the dew point is by remembering that on early morning when you walk outside there is dew (or visible moisture) on the vegetation outside your house.  As Brent said, when the temperature falls to the same temperature as the dew point… dew will form.  In the air we call those clouds. 

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  5. Brian on May 25, 2012

    I like to think of this with a science brain rather than a typical verbatim definition. Early on we learn that heat causes expantion and cooling has an opposing effect. Remembering this is easy if we consider how we react to temperatures: In cold weather we curl up to warm up (contract) and in warm weather we spread out to cool off (expand).
     
    Air particles are no exception, following this same pattern of thermal expansion/contraction. Except a particle of air is not simply made up of one element, but many; including solids, liquids, and gases. When temperature is decreased, all of these elements contract. However, gases tend to expand and contract at a higher rate than solids and liquids. In other words, when that particle of air is cooled it will shrink in size faster than the liquid and solid elements contained within it. Eventually it will no longer be larger than these liquid and solid elements, the dew point will be reached, and visible moisture will be present.
     
    Some common examples are seeing your breath on a cold day, watching water form on a cold glass of your favorite beverage on a warm summer day, or the already mentioned exmple of dew. All of these show particles of air which have been cooled to a point where they could no longer contain the liquid and solid elements they once housed.
     
    To side track for a moment before I depart, the supercooled discussion Brent speaks of can be ignored for your purposes. It’s an upper atmosphere phenomina that rarely occurs. If a ground instrument is reporting a dew point higher than temperature it’s time for maintenance. 🙂

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  6. Kelly Smith on May 29, 2012

    P.G.–
     
    As you can tell, much of the dew point provides you clues you can use for the weather–e.g. fog. However, you can also use it for other issues as well. Higher humidity will increase the likelihood of any kind of venturi icing, e.g. carburetors, as the temperature decreases. As most can recall, carb icing can occur well above the freezing point. Therefore, be cautious of high humidity as you climb toward the freezing level.
     
    A little more of the “so what” to your question is that you lose some performance of the a/c. Humid air is worse for performance. AOPA has some articles you can review (http://www.aopa.org/search/searchresults.cfm?q=humidity&go.x=0&go.y=0&go=Go), but I can’t remember if you have to be a member to see these.
     
    Fly Safe, Cheers,
    Kelly

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