The most common rainfall measurement is the total rainfall depth during a given period, which is expressed in ________.

Correct — A. Rainfall is reported as a depth — how deep the water would stand if none of it drained away, soaked in or evaporated — and a depth is a length, so the unit is millimetres.

1 mm of rain is 1 litre per square metre, which is why the figure does not depend on how wide the mouth of the rain gauge is.

Why the others are wrong

• (b)
  hectopascal (hPa) — Hectopascal measures pressure, and it is the unit weather charts use for atmospheric pressure, which runs near 1013 hPa at sea level. Pressure is force per unit area, not a depth.
• (c)
  millibar (mbar) — Millibar is the older name for the same pressure unit, with 1 mbar equal to 1 hPa. Renaming a pressure unit does not turn it into a measure of rainfall.
• (d)
  kilopascal (kPa) — Kilopascal is pressure again, with 1 kPa equal to 10 hPa. It turns up in engineering and in tyre pressures, never in a rain gauge reading.

Rainfall is measured with a rain gauge: a funnel of known diameter feeding a collecting bottle, whose contents are read off against a scale in millimetres.

Because the reading is a depth and not a volume, gauges with different mouth sizes at the same spot give the same number. The volume collected rises with the collecting area, and the two cancel.

The same millimetre figure underlies every rainfall statement you meet — an annual total, a daily fall, or a season's departure from normal.

Key facts

• Rainfall is expressed as a depth in millimetres.
• 1 mm of rainfall is equivalent to 1 litre of water per square metre.
• Hectopascal and millibar are equal units of pressure, 1 hPa being 1 mbar.
• Rainfall in India is recorded by rain gauges and reported by the India Meteorological Department.

Common traps

• Reading rainfall as a volume and reaching for litres or cubic metres
• Picking a pressure unit because it sounds meteorological

No directly related past PYQ was found.