1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Applying the ideal gas law in the Earth's atmosphere

  1. Mar 15, 2015 #1
    Hi there,
    I'm considering how the ideal gas law applies in practice in our planet's atmosphere. In particular, I'm considering this form of the law:

    P = ρRT (1)

    where P is pressure, ρ is density, R is the gas constant and T is the temperature.

    I also know that, to a good approximation, the atmosphere is in hydrostatic equilibrium in which case a second equation is:

    dP/dz = -ρg (2)

    which is basically a statement saying that the gravitational force on a parcel of air is balanced by the vertical pressure gradient force.

    My question has to do with cause and effect changes in, say, the pressure and temperature at a point on the surface. In particular, what I'm trying to detangle in my mind is which of the three variables in (1) actually drives and manipulates the other 2. I want to say that temperature, which is set by other environmental factors such as surface temperature and insolation determines atmospheric pressure and pressure and temperature together determine the density.

    For instance, I understand that, if the atmospheric temperature profile is known, one can substitute (1) into (2) and integrate the resulting differential equation over the height of the atmosphere to determine the surface pressure. In this way, it's clear that the surface pressure must depend on temperature, not just at the surface, but through the entire atmospheric column and so changes in temperature (as determined by changes in solar insolation, cloud cover etc...) can cause changes in pressure.

    What I'm trying to detangle in my mind is whether pressure can ever directly change temperature. (By direct, I mean, considering just the change in pressure, not changes in things like cloud cover). For instance, if a low pressure centre enters the region and the air pressure drops, then applying the ideal gas law (1) suggests that either temperature or density must also change. In this scenario would the air temperature change or would it simply be the density that would change? I feel that density is the factor in (1) that is always dependent on the other two, but wanted to confirm whether my intuition is correct.

    Thanks for your time!

  2. jcsd
  3. Mar 15, 2015 #2
    Yes, the density follows from the pressure and temperature. And, as you are aware, there are local meteorological changes in both the pressure and the temperature profiles. The meteorologists include both dynamical and thermodynamic effects in modeling these changes. Under average conditions, at least in the troposphere, the temperature varies vertically (approximately) according to the adiabatic lapse rate, which provides a simplified model of these factors. Basically, to apply the hydrostatic equation, you need to know the vertical temperature profile and the pressure at the surface.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook