Gas Laws and Atmospheric Variables Questions and Answers

Answer: A

Reduced transpiration means less of the available energy is used for evaporation and more goes into sensible heat, warming the near surface air over vegetated land. Runoff increases rather than decreases, since more of the rainfall reaches the rivers. The effect is modest but is included in modern land surface schemes and contributes to the projected pattern of continental warming.

Answer: C

The order is [N₂] \(\gt\) [CO₂] \(\gt\) [Ar].

At equilibrium each dissolved concentration is the gas's partial pressure times its solubility (Henry's law). In sea water at 20 °C the solubilities differ widely: CO₂ is roughly 0.03 mol per kg per atm, argon about 0.001 and nitrogen about 0.0005, so

• N₂: about \(0.78 \times 0.0005 \approx 4 \times 10^{-4}\) mol/kg
• CO₂: about \(4 \times 10^{-4} \times 0.03 \approx 1.3 \times 10^{-5}\) mol/kg
• Ar: about \(9.3 \times 10^{-3} \times 0.001 \approx 1 \times 10^{-5}\) mol/kg

Nitrogen leads by far; CO₂'s much higher solubility lifts it just above argon despite its 23 times smaller mole fraction. With no chemical activity, CO₂ does not form bicarbonate, so it does not exceed nitrogen as it would in real sea water.

Answer: C

The Kelvin scale contains no negative numbers.

The Kelvin scale starts at absolute zero (0 K = -273.15 °C), the lowest temperature possible, at which the thermal motion of particles is at its minimum. Since no temperature can be lower, every kelvin temperature is zero or positive.

Negative numbers occur on the Celsius and Fahrenheit scales, which have arbitrary zeros. 'No positive numbers' is the reverse of the truth. Zero is the lowest point of the scale, but absolute zero cannot actually be reached, so the defining feature of the scale is the absence of negative values.

Answer: A

The mean atmospheric pressure at sea level is 1013 mb (hPa).

Standard sea-level pressure is 1013.25 hPa, equal to 1013.25 millibars, 101.325 kPa, 760 mm of mercury or one standard atmosphere. One millibar equals one hectopascal, which is why the two units are interchangeable in meteorology.

984 hPa is close to the global mean pressure at the actual land and sea surface, which is lower than sea-level pressure because much of the land stands well above sea level. 1000 hPa is a round reference level used on upper-air charts, and 950 hPa is the pressure some hundreds of metres up.

Answer: A

The global mean surface pressure is about 984 mb.

Mean sea-level pressure is about 1013 mb, but much of the Earth's land surface stands well above sea level (the Tibetan Plateau, the Andes, Antarctica), where the surface pressure is much lower. Averaged over the actual surface of the whole Earth, the mean pressure is about 985 mb, which is also a measure of the total mass of the atmosphere.

1013 mb is the mean pressure reduced to sea level, not at the surface. 1000 mb is a round reference level, and 950 mb is too low for a global average.

Answer: A

The equation of state is \(p = \frac{\rho R}{m} T\).

Dry air behaves as an ideal gas. Writing the ideal gas law per unit volume, with \(\rho\) the density, \(m\) the mean molecular mass of dry air (about 28.97 g/mol) and \(R\) the universal gas constant:

\[ \begin{aligned} pV &= nRT = \frac{M}{m} RT \\ p &= \frac{\rho R}{m} T = \rho R_{d} T \end{aligned} \]

Here \(R_{d} = R/m\), about 287 J/(kg K), is the gas constant for dry air. Options with \(\alpha\) use the specific volume \(\alpha = 1/\rho\); the correct form with it would be \(p\alpha = R_{d}T\), so \(p = \alpha RT\) and \(p\alpha = R/m\) (which drops the temperature) are both wrong, as is \(p = \frac{\alpha R}{m}T\).