Dew point temperature

by tuoni
Tags: point, temperature
tuoni is offline
Aug4-08, 12:05 PM
P: 63
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?
Phys.Org News Partner Physics news on
The hemihelix: Scientists discover a new shape using rubber bands (w/ video)
Mapping the road to quantum gravity
Chameleon crystals could enable active camouflage (w/ video)
mathman is offline
Aug4-08, 03:25 PM
Sci Advisor
P: 5,941
Dew point is a temperature, not dependent on temperature. It will depend on pressure. The altitude dependence is a result of pressure dependence.
tuoni is offline
Aug4-08, 05:12 PM
P: 63
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.

nucleus is offline
Aug4-08, 07:24 PM
P: 171

Dew point temperature

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.
Redbelly98 is offline
Aug4-08, 07:34 PM
Redbelly98's Avatar
P: 11,989
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.
tuoni is offline
Aug4-08, 08:06 PM
P: 63
Thank you nucleus! That was very enlightening ^_^

And thank you Redbelly98, that sorts out a few things!

Register to reply

Related Discussions
Kelvin Temperature - Celsius Temperature Change General Physics 12
Matlab function to compute an approximation to the temperature at any point in the... Math & Science Software 2
Why triple point of water is higher than freezing point Classical Physics 7
Calculating point X on the Golden Spiral knowing a point and a distance Calculus 0