How do we measure precipitation and what does a chance of rain, say 30%, actually mean

Precipitation is an atmospheric moisture that falls on the ground in the form of rain, snow, or hail. Usually it is calculated in in exact values, namely millimeters (mm) or inches (in). But our knowledge on this most issue of meteorology does not end there, it just begins. In this short post, we'll figure out exactly how we measure precipitation.

First of all, millimeters or inches of precipitation is a layer of water that would have formed from precipitation if it had not penetrated into the soil, did not flow, or evaporate.

As a general rule in meteorology and weather forecasting, 1 mm (0.03 in) of precipitation equals one liter (0.21 gallons) of water per 1 square meter (10.7 sq foot) of area.

The amount of precipitation is measured over a certain period of time, for example, per hour, day, a few days, a week or a month, or a year.

To measure precipitation meteorologists use tanks — rain gauges. These are, most often, simple metal barrels. A funnel with an area of one square meter is installed above the tank. When it's time to measure precipitation — calculate by how much the water has risen. By the way, when solid precipitation — snow or hail — melts, the experts measure the height of the resulting water.

So, if it is raining, according to the forecast, should you take an umbrella, or should you wait it out altogether? Take into account this approximate amount of precipitation per day:

Light rain gives up to 2–4 mm (0.07–0.15 in) of precipitation;
Moderate rain gives 5–6 mm (0.19–0.23 in);
Rain or strong rain gives up about 15–20 mm (0.59–0.78 in);
Rainfall gives more than 30 mm (1.18 in).

To realize how large this amount of water is, you should know that 1 mm (0.03 in) of precipitation per 1 ha of area gives 900 buckets of water.

What does a chance of rain (precipitation probability) mean

Sometimes the precipitation forecast is also expressed as a percentage. In this case you may see the phrase "Chance of rain" or "Precipitation probability (PP)" / "Probability of precipitation (PoP)" in the forecast. This seems simple to understand, but in general, calculating the probability of precipitation is a complicated thing, which has recently been actively debated on social networks.

If we summarize this discussion, meteorologists take into account the following two things when calculating the probability of precipitation:

The immediate likelihood of rain forming. Let's say a powerful cyclone comes to our area tomorrow, so it's very safe for the meteorologist to say that precipitation will occur. They determine this probability based on their knowledge, experience, and the observed situation in the atmosphere.
Expected areal coverage of precipitation. Weather models calculate weather forecasts for particular points, which together make up a grid or spatial resolution of a model, not for each locality individually. And it turns out that the forecasts take into account what percentage of your area is captured by the predicted precipitation.

Given these two factors, meteorologists make a final probability of precipitation, which is already applicable to any point in a given area, including the one you live in.

So, the bottom line is that this indicator means exactly what it means, without complications — the probability of rain, expressed as a percentage, for the predicted area and time. The higher the probability value, the more confident you can say there will be precipitation at your any point in your area.

Conversely, the probability of rain does not mean that it will rain a certain percentage of the time (say, 25% — 6 out of the next 24 hours) or that it will cover a certain percentage of the area. Both are fundamentally wrong.

This post was originally published in the Windy.app Meteorological Textbook on June 25, 2019.

Text: Windy.app team, Mariya Kolennikova and Ivan Kuznetsov

Cover photo: Paolo Candelo / Unsplash