Adiabatic Processes and Lapse Rates Questions and Answers

Answer: C

The rule follows from the difference between the dry adiabatic rate of 9.8 and the rate at which the dew point falls on ascent, about 1.8, giving a convergence of 8 kelvin per kilometre. That inverts to 125 metres per kelvin. Deriving it once makes it far easier to remember than treating it as an arbitrary number.

Answer: D

The mixing ratio is conserved during dry ascent but the pressure falls, so the vapour pressure falls and with it the dew point, at about 1.8 kelvin per kilometre. The temperature falls much faster, which is why the two curves converge and saturation is eventually reached. Their rate of convergence is what produces the 125 metre rule.

Answer: A

A ceilometer determines the cloud base directly by timing a laser pulse reflected from the cloud, and its readings agree well with the lifting condensation level for convective cloud. A radiosonde gives the saturated layer but only twice a day. Satellite imagery sees the cloud top and radar sees the precipitation, so neither gives the base. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: B

By the afternoon the boundary layer is well mixed, so a parcel with the mean properties of that layer is a better representation than the surface value alone. Mixed layer and most unstable parcel methods are both used, the latter being appropriate for elevated convection. The morning surface parcel is unrepresentative of afternoon conditions. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: C

Where a stable layer separates the surface from the convecting layer, the relevant parcel is the one with the greatest equivalent potential temperature aloft rather than any surface value. Nocturnal elevated thunderstorms are the classic case. Choosing the wrong parcel can make a genuinely unstable situation appear stable, so the choice deserves explicit thought. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: D

The lifted index is the environmental temperature at 500 hectopascals minus the temperature of a parcel lifted to that level, so negative values indicate instability. It is quick to compute and was widely used before convective energy became routinely available. The K and total totals indices use combinations of temperature and humidity at fixed levels instead. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: A

The index is defined as environment minus parcel, so a buoyant parcel warmer than its surroundings gives a negative value. Values below minus 6 indicate strong instability. The sign convention catches people out because instability is usually thought of as a positive quantity, which is why the definition should be recalled rather than the sign memorised. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: B

An index evaluated at a single level says nothing about how deep the unstable layer is or whether a cap prevents the parcel from reaching it. Two soundings with the same lifted index can behave quite differently. This is why convective energy and inhibition, which integrate over depth, have largely replaced them in operational practice.

Answer: C

The K index adds the 850 to 500 temperature difference to the 850 dew point and subtracts the 700 dew point depression, giving a measure of both instability and mid-level moisture. Values above 30 indicate a substantial thunderstorm risk. Its inclusion of mid-level humidity is what distinguishes it from the purely thermal indices.

Answer: D

A hodograph plots the tips of the wind vectors from successive levels and joins them, so its shape shows how the shear varies through the depth of the sounding. Curvature indicates the shear vector turning with height, which produces streamwise vorticity. It is the standard tool for assessing storm type alongside the thermodynamic diagram.