Atmospheric Stability Questions and Answers

Answer: A

A lifetime of a week allows accumulation mode aerosol to cross an ocean, so Asian dust and pollution reach North America and Saharan dust reaches the Caribbean and Amazonia. The Saharan dust deposition is a significant nutrient source for the Amazon. Recognising that air pollution is a hemispheric as well as a local problem follows directly from the removal timescale.

Answer: B

Iron and phosphorus in Saharan dust fertilise both the ocean and the Amazon rainforest, with the annual deposition on the Amazon estimated at tens of millions of tonnes. The dust also reflects sunlight, cooling the surface, and the associated dry Saharan air layer suppresses but does not prevent tropical cyclone development. Each of these effects is real, which makes the question one of identifying the most significant.

Answer: A

The 2 m air temperature is usually lowest at sunrise.

After sunset the ground loses heat by longwave radiation and cools the air in contact with it. This cooling continues through the night as long as outgoing radiation exceeds what the surface receives, so the temperature keeps falling until the sun rises and solar heating begins to make up the loss. The minimum therefore comes around sunrise, or just after it.

Noon and sunset fall in the warm part of the day; the maximum normally comes in the early afternoon, after noon, because the surface keeps gaining heat for some hours. At midnight the night's cooling is only half done.

Answer: B

The Richardson number is the ratio of stability to the square of the vertical wind shear.

\[ \begin{aligned} Ri &= \frac{N^{2}}{(\partial u/\partial z)^{2}} \\ N^{2} &= \frac{g}{\theta}\frac{\partial\theta}{\partial z} \end{aligned} \]

The numerator, the square of the Brunt-Vaisala frequency, measures static stability, which suppresses turbulence; the denominator, the squared shear, measures the mechanical production of turbulence. Below a critical value of about 0.25, shear wins and the flow becomes turbulent.

The ratio uses the square of the shear, not the shear itself, and the Coriolis force does not enter it.

Answer: D

Fog favours RH 80%, clear sky, light wind and a stable atmosphere.

Radiation fog, the commonest type over land, forms on long nights with clear skies, when the ground loses heat rapidly and cools the moist air above it to its dew point. A light wind mixes the cooling through a shallow layer without dispersing it, and a stable atmosphere, often with a surface inversion, keeps the cold, moist air near the ground.

A cloudy sky reduces night-time cooling, and an unstable atmosphere mixes the moist surface air upward, so the options with cloud or instability do not favour fog.