Meteorology and Dispersion Questions and Answers

Answer: B

Lightning produces NO, nitric oxide.

The lightning channel heats the air to around 30,000 K, hot enough for atmospheric nitrogen and oxygen to combine:

\[ \mathrm{N_{2} + O_{2} \rightarrow 2NO} \]

The NO is then oxidised to NO₂ and finally to nitrate, which rain carries to the soil. This is the natural, non-biological route of nitrogen fixation.

CO and CO₂ need a carbon source and come from combustion and respiration, not from heating air. Ammonia comes from biological processes such as decay and animal waste.

Answer: A

The atmosphere is absolutely stable.

Stability is judged by comparing the environmental lapse rate (ELR) with the dry adiabatic lapse rate (about 9.8°C/km) and the moist (saturated) adiabatic lapse rate (about 6°C/km). If the ELR is less than the moist rate, it is also less than the dry rate, so a rising parcel, saturated or not, always cools faster than its surroundings, becomes denser and sinks back. The air is stable for every parcel.

Absolute instability needs an ELR greater than the dry rate. Conditional instability occurs when the ELR lies between the two adiabatic rates, stable for dry air but unstable for saturated air.

Answer: B

Wind speed is measured with an anemometer.

The common cup anemometer has three or four cups on arms that spin about a vertical axis; the rate of rotation is proportional to wind speed. Vane, hot-wire and sonic anemometers measure the same quantity by other means.

The other instruments measure different things. A hydrometer measures the density or specific gravity of a liquid. A pH meter measures the acidity or alkalinity of a solution. A piezometer measures pore-water pressure or the water level in an aquifer, which gives the hydraulic head.

Answer: B

Atmospheric pressure is measured with a barometer.

A mercury barometer balances the weight of a column of mercury (about 760 mm at sea level) against the pressure of the air. An aneroid barometer uses a sealed, partly evacuated metal capsule that flexes as the pressure changes.

A tachometer (spelt 'Tacometer' in the paper) measures the speed of rotation of a shaft. A thermometer measures temperature. An anemometer measures wind speed. Only the barometer measures pressure.

Answer: C

Temperature is measured in a Stevenson's screen.

The Stevenson screen is a white wooden box with louvred sides, mounted about 1.2 m above the ground. It shields the thermometers (dry bulb, wet bulb, maximum and minimum) from direct sunlight and rain while letting air circulate freely, so they read the true air temperature in the shade.

A sunshine recorder (the Campbell-Stokes type burns a trace on a card) measures the duration of bright sunshine. A pan evaporimeter measures evaporation from an open water surface. A piezometer measures groundwater level or pore-water pressure. None of these measures air temperature.

Answer: B

Mixing height is determined to assess the vertical dispersion of pollutants.

The mixing height is the depth of the lowest layer of the atmosphere in which vigorous vertical mixing takes place, usually capped by a stable layer or inversion. Pollutants released near the ground are diluted through this layer, so the mixing height, multiplied by the mean wind speed in the layer, gives the ventilation coefficient used to judge a site's capacity to disperse pollution.

Emission rates come from source data, such as fuel use and emission factors, not from mixing height. Air temperature and wind speed are measured directly and are inputs used to find the mixing height, not its purpose.

Answer: B

A hygrometer estimates the water vapour content of air.

A hygrometer measures humidity. The common wet-and-dry-bulb hygrometer (psychrometer) uses the cooling of a wet thermometer by evaporation: the drier the air, the bigger the difference between the two readings, from which relative humidity is read off tables. Hair hygrometers and electronic capacitive sensors do the same job.

Soil water content is measured by gravimetric sampling, neutron probes or TDR. Capillary potential (suction) of soil water is measured with a tensiometer. The specific gravity of a liquid is measured with a hydrometer, a name easily confused with hygrometer.

Answer: B

The correct match is a-ii, b-i, c-iv, d-iii.

1. Valley wind: by day the slopes heat up and air moves from the plains into the valley and up its sides (ii).
2. Land breeze: at night the land cools faster than the sea, so air moves from land to sea (i).
3. Sea breeze: by day the land heats faster, so air moves from sea to land (iv).
4. Heat island: the warm city air rises, and cooler air moves in from the surrounding countryside to the city (iii).

The option that matches a-i, b-ii and so on takes the lists in order and pairs land breeze with the valley wind. The other two options give the land and sea breezes the wrong directions; the last one reverses them outright.

Answer: A

Both (A) and (R) are true and (R) is the correct explanation of (A).

A wind rose is a circular diagram for a site. Spokes or bars point in the directions from which wind blows, their length shows the percentage of time the wind came from each direction, and bands within each bar show the frequency of each wind-speed class. So the Assertion is true: a wind rose is a graphical representation of wind data.

The Reason is also true and says exactly what that representation consists of, frequencies of wind speed and direction for a location, so it explains the Assertion. Wind roses are used in siting industries and stacks so that emissions are carried away from towns.

Answer: A

In an inversion, the temperature of the Earth's surface is less than at higher altitudes in the atmosphere.

Normally air temperature falls with height, about 6.5 degrees C per km. In a temperature inversion this is reversed: a layer of warm air lies above cooler air near the ground, for example after rapid radiative cooling of the ground on clear, calm nights, or when air subsides in a high-pressure system. The warm lid stops vertical mixing and traps pollutants, as in smog episodes.

A warmer surface than the air above is the normal lapse condition, not an inversion. Equal temperatures describe an isothermal layer. Temperatures alternating with altitude do not define an inversion.