G190 Weather & Climate Prof. Constance Brown Indiana University 1PRESSURE Gas Pressure • Gas molecules inside a volume (e.g. a balloon) are constantly moving around freely. • They frequently collide with each other and with the surface of any enclosure. Figure 1: The internal gas pressure in a balloon, PB, is given by the impacts of moving gas molecules, as they collide with the skin of the balloon from the inside. • The force of impact of a single one such collision is too small to be sensed. • Large number of impacts of gas molecules exerts a considerable force onto the surface of the enclosure • The larger the number of collisions per area of enclosure, the larger the pressure:  G190 Weather & Climate Prof. Constance Brown Indiana University 2 • There is not only gas inside the balloon, there is also gas on the outside.Figure 2). Figure 2: The atmospheric pressure outside a balloon, PA, is given by the impacts of moving gas molecules, as they collide with the skin of the balloon from the outside. • The rate, at which the skin of the balloon is bombarded by air molecules, is dependent on how tightly the gas molecules are packed:  󰇛󰇜 󰇛󰇜    Kg/m3G190 Weather & Climate Prof. Constance Brown Indiana University 3 • Gas is compressible, so its density depends on the force that is used to compress it • In the atmosphere, the force that compresses the air at the surface is just the weight of all the air in the atmospheric column above it (Figure 3). Figure 3: Surface pressure and density in an air column • The higher we go in the atmosphere, the less air remains in the column above us. • Atmospheric pressure always decreases with height. • Air density decreases with height, because the overload to compress the air gets less and less, as we go higher. Why are there more air molecules close to the surface?G190 Weather & Climate Prof. Constance Brown Indiana University 405000100001500020000250000 200 400 600 800 1000 1200Height (m)Pressure (mb)05000100001500020000250001 10 100 1000 10000Height (m)Pressure (mb)1) Gravity - attracts air molecules - closer molecules are held more strongly 2) Compressibility of air - fluid can be compressed More molecules above more compression • Negative relationship - decreases with height • Linear graph paper - ( exponential curve) • Semi-logarithmic graph paperG190 Weather & Climate Prof. Constance Brown Indiana University 5Units of Pressure (read: Lab Manual Lab 3 notes) Atmospheric Science we use a) millibars (mb) 10-3 - milli bar - pressure Sea level pressure - 1013.25 mb b) Pascals (Pa) kPa - kilo Pascal 103 - kilo hPa - hecto Pascal 104 Sea level pressure 101.325 kPa = 101325 Pa = 1013.25 hPa = 1013.25 millibarsG190 Weather & Climate Prof. Constance Brown Indiana University 6Ideal Gas Law (or Equation of state) Use three variables to describe the atmosphere Variable Unit Temperature (T) K kelvin Pressure (P) Pa Density (ρ) kg m-3 Temperature scales Kelvin (K) Celsius (°C) Fahrenheit (°F) Boiling Point 373 100 212 Freezing Point 273 0 32 BP - FP 100 100 180 Relative size of 1 degree 1 1 ~0.55 Density = Mass Volume Ideal Gas Law P=ρRT R - Gas constant 287 J kg-1 K-1 J - jouleG190 Weather & Climate Prof. Constance Brown Indiana University 7 What does this state? a) Temperature is set constant (T=ct) P=ρRT b) Density is set constant (ρ=ct) P=ρRT c) Mass is set constant (m, T = ct) RTVmP = Example of calculation What is the pressure of air when the temperature is 15°C and the density is 1.225 kg m-3?G190 Weather & Climate Prof. Constance Brown Indiana University 8G190 Weather & Climate Prof. Constance Brown Indiana University 9What does this means in the atmosphere? If we take a parcel of air e.g. "balloon" Assume: can expand or contract freely but cannot exchange heat • Space occupied by the air molecules within the parcel defines the air density (m/v) • Average speed to the molecules directly related to the Temperature • Molecules colliding against the sides of the parcel determine the air pressure inside Surface: Temperature of the parcel is the same as the surrounding air Now lift the air parcel to region where the air pressure is lower Lower pressure outside - • No other energy source apart from that inside the baloon- uses some of its own energy to expand the parcelG190 Weather & Climate Prof. Constance Brown Indiana University 10• Referred to as an ADIABATIC Process - air molecules move more slowly after parcel expands • Decrease in average molecular speed ⇒ lowers parcel T If parcel comes back to the