Gas Laws and Atmospheric Variables Questions and Answers

Answer: C

Saturation vapour pressure is a property of water at a given temperature and would be the same in a vacuum as in air, so the air is not holding anything. The correct statement is that the equilibrium vapour pressure over a water surface rises with temperature. The phrase gives the right answer for the wrong reason, which matters as soon as one tries to reason about situations it does not cover. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: D

The dew point is the temperature at which the existing vapour pressure would become the saturation value, reached by cooling at constant pressure and constant mixing ratio. The wet bulb temperature involves adding water as well as cooling and therefore lies between the dew point and the actual temperature. The frost point is the corresponding quantity with respect to ice. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: A

In unsaturated air the three are ordered with the actual temperature highest and the dew point lowest, and they converge to a single value at saturation. The wet bulb lies between because evaporating water cools the thermometer but also raises the vapour pressure. Checking this ordering is a quick way of validating a set of observations, since any other arrangement indicates an error. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: B

A psychrometer consists of two thermometers, one with its bulb covered by a wet muslin wick, ventilated so that evaporation cools it to the wet bulb temperature. The difference between the two readings gives the humidity through psychrometric tables. Hair hygrometers respond to relative humidity directly, and a shielded ventilated thermistor measures the dry bulb temperature. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: C

Because water vapour is lighter than dry air, moist air is less dense than dry air at the same temperature and pressure, and virtual temperature is the temperature adjustment that accounts for it. Using it allows the dry air gas constant to be retained in the equation of state. The adjustment reaches a few kelvin in the humid tropics and is negligible in polar air. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: D

Since water vapour is lighter than dry air, adding it lowers the density, and the equivalent dry air must therefore be warmer, so the virtual temperature always exceeds the actual temperature. The correction is about 0.6 kelvin for every gram of vapour per kilogram of dry air. It is largest in the warm humid tropics, where it can reach 3 kelvin and cannot safely be neglected. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: A

Expanding the exact expression for small mixing ratio gives \( T_v \approx T(1+0.61w) \), where \( w \) is in kilograms per kilogram. The coefficient 0.61 is \( (1-\epsilon)/\epsilon \). With \( w \) of 20 grams per kilogram and \( T \) of 300 kelvin the correction is about 3.7 kelvin, which is a substantial buoyancy effect and explains why virtual temperature matters for convection. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: B

Buoyancy depends on density, so a parcel must be compared with its environment using virtual temperature rather than temperature, and neglecting the correction can change the sign of the buoyancy. In the tropics the difference is comparable with the temperature excess of a convective plume. Humidity measurement and precipitation calculations use other quantities and do not require the correction. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: C

Using virtual temperature allows the constant 287 to be retained for all moist air, which is why the device was invented. The alternative of a humidity dependent gas constant is mathematically equivalent but far less convenient. Dalton's law is of course also true, and it is the starting point from which the virtual temperature formulation is derived rather than an alternative to it. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: D

Atmospheric pressure supports a column of mercury about 760 millimetres high, and the height is read against a scale after corrections for temperature, gravity and the instrument's own index. The final option describes an aneroid barometer, which is the more common modern instrument. Mercury barometers remain the reference standard against which others are calibrated despite being displaced from routine use. Checking the units of each quantity catches a wrong formula before the arithmetic begins.