Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Dew point temperature

  1. Aug 4, 2008 #1
    Does dew point temperature remain constant over altitude, pressure, and density? I have to admit that I am having difficulties fully understanding dew point temperature. I understand that dew point temperature is simply the temperature to which a given parcel of air must be cooled under constant pressure, for it to condense/deposit. Simple.

    However, how does this relate to the actual atmosphere? In this case the International Standard Atmosphere. As altitude increases, temperature, pressure, and density changes. If dew point remains constant, I can understand how clouds form, since temperature decreases the first 11 km (by 0.0065 K/m) which would allow water vapour to condense/deposit. However, is dew point constant over altitude? or does it decrease along with temperature? Then how can clouds form at all?
  2. jcsd
  3. Aug 4, 2008 #2


    User Avatar
    Science Advisor

    Dew point is a temperature, not dependent on temperature. It will depend on pressure. The altitude dependence is a result of pressure dependence.
  4. Aug 4, 2008 #3
    Great! Is there then an algorithm for calculating dew point from pressure (and some other variables)? The only ones I have found so far have been as a function of temperature and relative humidity, basically just conversions.
  5. Aug 4, 2008 #4
    Dew point temperature is a temperature that is measured with a different thermometer than the air temperature.

    Going back to your definition: I understand that dew point temperature is simply the temperature to which a given parcel of air must be cooled under constant pressure, for it to condense/deposit. In other words the air is saturated. An example is if air temperature Ta were 75 F (24 C) with dew point temperature Td of 50 F (10 C), it would be necessary to 50 F (10 C) in order to cause saturation. Therefore the spread between Ta and Td is an indication of the degree of saturation. The less the spread the nearer the air is to saturation. Note Td can also increase, with Ta staying the same, to reduce spread or a combination of the two.

    Td can be equal to Ta but not greater. If they are equal at ground level then fog, mist or dew often forms. The degree of saturation is also often expressed in terms of relative humidity. Water vapor in the air is what causes most weather. There are phase changes that occur with condensation, evaporation and sublimation heat and cool surrounding air.

    Now for the change with altitude: Twice a day a radiosonde balloon is released from many locations in the world. These balloons measure pressure, Ta, Td, moisture wind speed and direction. A copy of one is shown below. The red lines on the left are Td and the red on the right is Ta. As can be seen Td may either increase or decrease with altitude. This is just a snapshot at one time. As can be seen by the blue lines from 12 hrs previously that Td and Ta have changed.

    A note on decrease of temperature with altitude, called a lapse rate. It is seldom the same as the International Standard Atmosphere. Also once clouds form the lapse rate decreases.

    “Then how can clouds form at all?”
    Clouds can form from several means, but basically if the Td and Ta are the same, or close to the same, and if there are enough nuclei for condensation to occur, then they will form. It would require a study of weather (meteorology). Below is a link to a web that shows an online manual on weather. In Module 2 under Stability it gives info on lapse rate.
    Last edited by a moderator: Apr 23, 2017
  6. Aug 4, 2008 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Dew point temperature depends on the partial pressure of water vapor present in the atmosphere. The atmospheric pressure (and hence, altitude) does not affect it.
  7. Aug 4, 2008 #6
    Thank you nucleus! That was very enlightening ^_^

    And thank you Redbelly98, that sorts out a few things!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook