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:
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.
Sometimes the precipitation forecast is also expressed as a percentage. In this case you may see the phrase "Chance of rain" or "Precipitation probability" 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:
Given these two factors, meteorologists make a final probability of precipitation, which is already applicable to any point in a given area, including the user point that falls within that area.
The higher the probability value, the more confident you can say there will be precipitation at your point.
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