Gas Laws and Atmospheric Variables Questions and Answers

Answer: A

The density of mercury and the expansion of the brass scale both depend on temperature, local gravity varies with latitude and altitude, and each instrument has its own small index error. Reduction to mean sea level is a further separate step. Latitude does enter, but through gravity acting on the column rather than through any change in the density of the mercury itself. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: B

Station pressure is converted to the value that would be observed at mean sea level using the hypsometric relation and an assumed temperature for the fictitious column below. Without this step altitude would dominate the pattern entirely and no synoptic analysis would be possible. The assumption about the fictitious column is the weak point, and it introduces appreciable error at high elevation stations. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: C

For a station at two kilometres the reduction involves a column of air that does not exist, and its assumed temperature can shift the reduced value by several hectopascals. This is why analyses over mountainous regions are drawn on a constant pressure surface such as 850 hectopascals instead. The instrument works perfectly well at reduced pressure and the gravity correction is small and well determined. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: D

The 850 hectopascal surface lies at about 1.5 kilometres and is above most of the terrain in many regions, so contours of its geopotential height give a fictitious-column-free analysis of the low level flow. Sea level and 1000 hectopascal analyses both require the problematic reduction. The 500 hectopascal level is used for the mid-tropospheric flow rather than for low level analysis. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: A

An evacuated corrugated metal capsule is prevented from collapsing by a spring, and its deflection under changing external pressure is amplified mechanically to move a pointer. It is compact and robust, which is why it is used in altimeters and barographs. Vibrating quartz and piezoresistive sensors are the basis of modern electronic barometers and offer better stability. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: B

A pressure altimeter is an aneroid barometer with a scale calibrated according to the standard atmosphere, so it reads true altitude only when the actual atmosphere matches that standard. Consistency between aircraft matters more than absolute accuracy, which is why all aircraft above a transition level use the same setting. A radio altimeter measures true height above the terrain and is a separate instrument. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: C

Moving into a region of lower pressure without resetting the subscale means the aircraft descends to maintain the indicated altitude, so it is lower than the instrument shows. The old aviation adage runs from high to low, look out below. The same applies to flying from warm to cold air, which is why cold weather altitude corrections are applied in mountainous terrain. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: D

The gas law and every other thermodynamic relation require absolute temperature, since ratios and products of temperature appear throughout. Using Celsius in place of kelvin is one of the commonest arithmetic errors in the subject. Potential temperature is itself expressed in kelvin and is a derived variable rather than a scale.

Answer: A

The two scales have the same size of degree and differ only in their zero point, with 0 degrees Celsius equal to 273.15 kelvin. Absolute zero is therefore minus 273.15 degrees Celsius. The rounded value 273 is adequate for most purposes but the full figure should be used where precision matters, particularly in radiative calculations involving the fourth power of temperature. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: B

The screen shields the thermometer from direct and reflected radiation while allowing free ventilation, so that it registers the temperature of the air rather than its own radiation balance. Standard height is 1.25 to 2 metres. Without such shielding a thermometer in sunlight can read many degrees too high, which is why unscreened readings quoted in the media are often misleading. The shared properties explain why these belong to one family, and the differences explain why more than one exists.