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The night of the rain

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Last night it rained for three hours without stopping. The rain gauge in the yard showed sixty millimetres by morning, and a normal month here gives about ninety.

Last night it rained for $3$ hours. The gauge showed $60 mm$ by morning; a normal month gives about $90 mm$ .

rain $R$ , month $M$ : $t = 3 h, R = 60 mm, M \approx 90 mm$ .

So a single night brought two thirds of a month of rain. That is twenty millimetres every hour, enough to turn the yard into a pond.

One night brought $60 \div 90 = \frac{2}{3}$ of a month: $60 \div 3 = 20 mm$ every hour.

$R \div M = \frac{2}{3}$ and $R \div t = 20 mm/h$ .

How much water is that? One millimetre of rain on one square metre is exactly one litre. Our roof is fifty square metres, so it caught sixty times fifty litres, which is three thousand litres.

Fact: $1 mm$ on $1 m^{2}$ is $1 L$ . Our roof is $50 m^{2}$ , so it caught $60 \times 50 = 3000 L$ .

$1 mm \times 1 m^{2} = 1 L$ , roof $50 m^{2} \Rightarrow 60 \times 50 = 3000 L$ .

A rain barrel holds two hundred litres, and three thousand divided by two hundred is fifteen. One roof, one night, fifteen full barrels, and we had only one barrel.

A barrel holds $200 L$ and $3000 \div 200 = 15$ . One roof, one night, $15$ barrels; we had $1$ .

barrel $200 L$ : $3000 \div 200 = 15$ ; our barrels: $1$ .

Letters of words: 519 → 171 → 32.

How to read the signs

$\times$ times
$\div$ divided by
$\frac{2}{3}$ two thirds
$\Rightarrow$ so; it follows that
$\approx$ about
$mm$ millimetres
$m^{2}$ square metres

The night of the rain

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