Precipitation and Measurement Questions and Answers

Answer: B

Statement 1 is false and statement 2 is true. The daily rainfall depth measured at 8:00 a.m. is read from a non-recording gauge, such as the Symon's gauge, which stores the rain collected in the previous 24 hours. The float type is a recording (automatic) gauge, which gives a continuous record and is not what is read once a day. Statement 2 is true: floats thrown on a stream give the velocity of the surface water, a rough way to get the approximate stream flow velocity.

Answer: A

The correct match is P-3, Q-5, R-4, S-2, T-1.

• A current meter measures stream flow velocity by the rotation of its cups or propeller in the water.
• An anemometer measures wind speed.
• A hygrometer measures humidity.
• Symon's gauge is the standard non-recording rain gauge.
• A pyranometer measures global solar radiation.

Each of the other options assigns at least four instruments to the wrong quantity, for example pairing the current meter with rainfall or the anemometer with stream flow.

Answer: C

The correct matching is P-3, Q-4, R-1, S-5, T-2.

An isohyet joins points of equal rainfall (R-1), and an isobar joins points of equal pressure (T-2). An isobath joins points of equal depth, used in groundwater maps for equal depth to the water table (Q-4). An isochrone joins points of equal travel time to the outlet of a catchment and is used to route surface runoff (S-5). That leaves the isopleth, a general line of equal value of any variable, for evapotranspiration (P-3).

The first option swaps isopleth and isobath, the second pairs isochrone with evapotranspiration and isopleth with runoff, and the last swaps rainfall and pressure between the isohyet and the isobar.

Answer: B

The depth for the 10⁴ km² catchment is lesser than 80 mm.

Depth-area-duration curves show how the maximum average depth of a storm falls as the area over which it is averaged grows. A storm is most intense near its centre, so averaging over a bigger area brings in more of the lighter rainfall at the edges. For the same storm and the same 6-hour duration, going from 10³ to 10⁴ km² must therefore give a smaller average depth than 80 mm.

The options 'greater than 88' and 'between 81 and 88' have depth rising with area, the reverse of the DAD relation. 'Equal to 80' would mean uniform rainfall over the whole larger area, which the analysis exists to correct.

Answer: C

A plot of rainfall intensity against time is a hyetograph.

A hyetograph is usually drawn as a bar chart of rainfall intensity (or depth) in successive time intervals during a storm. It shows how the storm developed and is the input from which runoff hydrographs are computed.

A hydrograph plots river discharge against time, the output of the catchment rather than the rain. Isohyets are lines of equal rainfall drawn on a map, showing distribution in space, not time. A mass curve plots cumulative rainfall against time; 'Man curve' as printed is not a standard term.

Answer: A

A non-recording rain gauge records the cumulative depth of rainfall.

The standard non-recording gauge, such as the Symon's gauge used by the India Meteorological Department, collects rain in a bottle through a funnel. Once a day (at 08:30 IST in India) the observer measures the water collected with a graduated measuring glass, which gives the total depth of rain since the last reading.

It cannot give rainfall intensity, because it does not record when the rain fell; that needs a recording gauge. It does not measure snowmelt or cloud cover. (The Hindi text of the first two options is swapped in the paper; the English, which the Board treats as standard for technical subjects, is followed here.)

Answer: B

A recording rain gauge records rainfall in terms of depth and time.

Recording (self-recording) gauges, such as the tipping bucket, weighing bucket and natural siphon types, draw a continuous record of accumulated rainfall depth against time on a chart or a data logger. This mass curve shows when the rain fell and how fast, so rainfall intensity can be worked out from its slope.

Depth alone is what a non-recording gauge gives. Rainfall is not reported as a volume or as a specific weight.

Answer: B

Each compartment of a tipping bucket gauge holds 0.25 mm of rainfall.

In the tipping bucket gauge, rain from the funnel fills one of two small buckets mounted on a pivot. When the bucket has collected the equivalent of 0.25 mm of rain, it tips, empties, and brings the other bucket under the funnel. Each tip is recorded electrically, so the number of tips gives the rainfall depth and their timing gives the intensity.

Larger capacities such as 0.50 or 1.00 mm would make the record too coarse, and 0.15 mm is not the standard figure.

Answer: A

In windy storms the rain gauge catches less rainfall.

A rain gauge standing above the ground disturbs the wind around it. The air speeds up over the funnel and creates eddies that carry drops past the opening, so the gauge catches less rain than actually falls; the error grows with wind speed and can exceed 10% in strong winds. Wind shields and setting the gauge low reduce this.

Wind does not make the gauge catch more, and the catch has nothing to do with ice crystals or a fixed 1.25 cm.

Answer: A

The average depth is 50 cm.

In the isohyetal method each area between two isohyets is given the mean of their values, and the results are weighted by area.

\[ \begin{aligned} P_1 &= \tfrac{40 + 60}{2} = 50,\ P_2 = \tfrac{55 + 45}{2} = 50 \\ P &= \frac{100 \times 50 + 100 \times 50}{200} = 50\ \text{cm} \end{aligned} \]

500 and 550 come from adding without dividing by the area properly, and 5 cm from a slip of a decimal place.