Precipitation Formation Mechanisms Questions and Answers

Answer: C

Thresholds vary between national services but lie around two centimetres. Damage to property and crops rises very steeply above that size. Stone size correlates directly with updraught strength. Warnings are issued on stone size for exactly that reason.

Answer: D

A stone several centimetres across falls at speeds comparable to a car on a motorway. This is why an updraught of similar magnitude is required to keep it aloft while it grows. Only severe storms sustain updraughts of that magnitude.

Answer: A

The first two are correct and are the essential requirements. The third is wrong, since deposition is far too slow to build a stone of centimetre size in the time available. Accretion of supercooled water is the only viable route.

Answer: B

Fresh snow density ranges from under fifty to over two hundred kilograms per cubic metre. A single depth figure therefore says little about how much water actually fell. Water equivalent is the quantity that hydrology requires.

Answer: C

The ten to one ratio corresponds to a snow density near a tenth that of water. Actual ratios range from about five to one for wet snow to twenty to one for very dry snow. Temperature during the fall is the main control on density.

Answer: D

Both are true and the reason gives the variability, and cold dry conditions give much lighter snow. Forecasting depth therefore requires forecasting density as well. Depth forecasts are consequently less reliable than water equivalent.

Answer: A

Each tip corresponds to a fixed increment of rainfall and is recorded electronically. The design underestimates very intense rain, because water falling during the tip itself is not counted. Corrections for this loss are applied at high rates.

Answer: B

That dependence is a property of radar rather than of a gauge. The other three are the standard systematic errors in gauge measurement, of which wind undercatch is much the largest. Snow is affected far more severely than rain.

Answer: C

Both are true and the reason gives the mechanism, and losses can exceed half the true fall in strong wind. Shielded gauges reduce but do not eliminate the problem. Intercomparison experiments have quantified the residual loss.

Answer: D

A gauge measures accurately at one point while a radar infers rainfall over a wide area. Combining the two, using gauges to adjust the radar field, is standard operational practice. Neither instrument alone gives an adequate rainfall field.