Collection: Rain gauge

Why Measure Rainfall

A rain gauge tells you how much rain has actually fallen. That sounds obvious until you try to judge it by eye, at which point most gardeners are out by a wide margin. A shower that soaks the paving may have delivered a couple of millimetres, which does nothing for a bed.

The decision it informs is watering. A greenhouse gets no rain at all, but the beds and containers around it do, and knowing whether last night counted saves both wasted water and plants left short.

What the Numbers Mean

Rainfall is measured in millimetres, and one millimetre equals one litre per square metre. That conversion is what makes a gauge useful rather than merely interesting: five millimetres over a bed of four square metres is twenty litres, which is roughly two full watering cans.

• Under 2 mm wets the surface and reaches almost nothing below it.
• Around 5 mm is a useful soaking for shallow rooted crops.
• Over 10 mm reaches depth and usually means watering can be skipped.

Reading it after each spell, rather than only when you remember, builds a picture of the season that no amount of judgement replaces.

Siting the Gauge

Position matters more than the instrument. A gauge under a tree, beside a wall or in the lee of a greenhouse will read low, sometimes by half. It needs open sky above it and clear ground around it.

Mount it at a height where you can read it without kneeling, and where a mower or a barrow will not catch it. Post mounted and push in models both work; the post is easier to keep clear of overhanging growth.

Reading and Maintenance

Empty it after each reading, or the next figure will be cumulative and useless. Check it is level, since a tilted gauge collects less than it should.

In winter, a gauge left out can freeze and crack. Bringing it in during hard frost extends its life considerably, and you lose little data since watering decisions are not being made anyway.

Alongside Other Measurements

Rainfall tells you about outside. For what is happening under glass, a maximum and minimum thermometer records the overnight low and midday peak, which determines whether you need to vent, shade or heat. Where beds under cover run dry, the raised bed range holds moisture better than containers do.

The wider temperature control range covers what to do once you know.