What does it mean when the weather forecast says how many millimeters of rain will fall?

Meteorological Tips

What is rainfall?

A few days ago, readers wrote in asking: "How is rainfall measured? (How is 20 mm of the 20 mm of rainfall recorded in Huadu determined?)"

The depth of rainwater that falls from the sky to the ground and accumulates on the water surface without evaporation, infiltration, or loss is called rainfall (in millimeters), which can visually represent the amount of rainfall.

At present, commonly used instruments for measuring rainfall include rain gauges and measuring cups. The diameter of the rain gauge is generally 20 cm and contains a funnel and a bottle. The diameter of the measuring cup is 4 cm, and it is used together with the rain gauge. When measuring, pour the rainwater from the rain gauge into the measuring cup, and you can know the amount of rainfall that day based on the scale on the cup.

The China Meteorological Administration stipulates that the rainfall within 24 hours is called daily rainfall. Any daily rainfall below 10 mm is called light rain, 10.0-24.9 mm is called moderate rain, and 25.0-49.9 mm is called heavy rain. , heavy rain is 50.0-99.9 mm, heavy rain is 100.0-250.0 mm, and anything exceeding 250.0 mm is called extremely heavy rain. Due to the vast territory of our country, a few areas have different regulations based on the specific conditions of Guangdong Province. For example, in rainy Guangdong, a daily rainfall of more than 80 mm is called a heavy rain; in Shaanxi's Yan'an area with little rainfall, a daily rainfall of more than 30 mm is called a heavy rain. (

Precipitation: refers to the total amount of rain, snow, hail, etc. falling from the atmosphere to the ground, as well as dew, frost, etc. formed by condensation of water vapor, the unit is mm.

Rainfall intensity: refers to the amount of rainfall in a unit period, measured in millimeters/minute or millimeters/hour. The rainfall intensity standard generally adopted by my country’s meteorological department is: light rain: less than 5 millimeters of rain in 12 hours, or less than 10 millimeters in 24 hours. ; Moderate rain: 5-14.9 mm of rainfall in 12 hours, or 10-24.9 mm of rainfall in 24 hours; Heavy rain: 15-29.9 mm of rainfall in 12 hours, or 25-49.9 mm of rainfall in 24 hours. Quantitative standards are not consistent across regions and depend on specific circumstances. Meteorological regulations generally stipulate that heavy rains are divided into three levels according to intensity: Heavy rain: 12-hour rainfall equal to or greater than 30 mm, or 24-hour rainfall equal to or greater than 50 mm; Heavy rainstorm: 12-hour rainfall. Equal to and greater than 70 mm, or 24-hour rainfall equal to and greater than 100 mm; extremely heavy rain: 12-hour rainfall equal to and greater than 140 mm, or 24-hour rainfall equal to and greater than 250 mm.

How to measure rainfall?

The depth of the water layer that falls from the sky to the ground without evaporation, infiltration, or loss and accumulates on the water surface is called rainfall (in millimeters), which can be expressed intuitively The amount of rainfall. Currently, the commonly used instruments for measuring rainfall include rain gauges and measuring cups. The diameter of the rain gauge is generally 20 cm, and it contains a funnel and a bottle. The diameter of the measuring cup is 4 cm. It is used in conjunction with the rain gauge. When measuring, pour the rainwater from the rain gauge into the measuring cup, and you can know the rainfall of the day according to the scale on the cup.

The China Meteorological Administration stipulates that the rainfall within 24 hours is called daily rainfall. The amount of daily rainfall is less than 10 mm, it is called light rain, 10.0-24.9 mm is called moderate rain, 25.0-49.9 mm is called heavy rain, heavy rain is 50.0-99.9 mm, heavy rainstorm is 100.0-250.0 mm, and anything exceeding 250.0 mm is called extremely heavy rain. Due to the vast territory of our country, a few areas have different regulations based on the specific conditions of the province. For example, in the rainy Guangdong area, a daily rainfall of more than 80 mm is called a heavy rain; in the Yan'an area of ??Shaanxi Province, a daily rainfall of more than 30 mm is called a heavy rain.

If you don’t have a rain gauge on hand, don’t worry. You can make one yourself using some common utensils, and the effect is quite good. Take a disposable plastic or paper bowl with a diameter of 20 cm (optional). (a suitable size paper bowl for instant noodles), drill a small hole slightly larger than a corn kernel in the bottom, and then place the bowl on a jar without a lid. There is a glass bottle in the jar, and the mouth of the bottle is connected to the small hole in the bottom of the bowl. . The simple rain gauge is ready. After the simple rain gauge is completed, it can be placed 70 cm above the ground (the distance between the mouth of the tube and the ground) to receive rainwater.

After the rain stops, use a scale to weigh the water in the bottle. 30 grams of water is equivalent to 1 mm of rainfall