How is the precipitation intensity measured

Precipitation

The term Precipitation In meteorology, denotes water that falls in liquid or solid form (rain, hail, snow, sleet) from clouds to the earth. So the precipitation forms in the clouds.

These are caused by condensation of moisture in the air. In order to fall back on the earth's surface as precipitation, the size of the condensed particles must exceed a certain value. The water cycle is closed by the precipitation.

The frequency and the average amount of precipitation are characteristic of the corresponding geographic areas. Precipitation is a factor that determines the local climate. This is particularly relevant for agriculture, since successful rain-fed farming is only possible after a certain amount of rainfall and different amounts of rainfall can be assigned to specific eco-zones.

1 precipitation measurement

Most precipitation gauges collect precipitation as a point-by-point precipitation measurement in a measuring vessel. One millimeter (measuring unit) corresponds to the water height of 1 mm, which would result if no water ran off. Alternatively, the amount of water is often given in l / m² (flat surface). 1 mm corresponds to exactly 1 liter per square meter.

The fallen amounts can now also be estimated across the board using precipitation radars. This is particularly important in the field of flood management (verify or calibrate point measured values). In addition to the amount of precipitation, the intensity and duration of precipitation are particularly important.

Long-term (climatological) precipitation measurements allow statistical calculations to indicate the average frequency of different precipitation events (especially heavy rain events), which correlate intensity and duration.

2 forms of precipitation

Artdescription
rainDrops with a diameter of> 0.5 mm
DrizzleDrops with a diameter of <0.5 mm, slowly falling from about 0.01 mm
snowFrom around -12 ° C, the water vapor condenses directly into small ice crystals (so-called resublimation), which then clump together to form snowflakes
SleetIrregularly shaped, aerated and frozen granules of 2 - 5 mm in size, which could arise from strong updrafts on cold fronts, for example
hailFrozen raindrops,> 5 mm in diameter, which consist of an ice crystal core and several frozen shells and which can arise in thunderclouds with strong updrafts through repeated accumulation of a film of water and repeated freezing
dewWater vapor that condenses on plants or objects into fine water droplets at night and during the day
Riperesublimated

3 rainfall

The amount of precipitation is the height of the water layer that would have formed in the event of precipitation (rain, snow, hail, fog, etc.) on a flat surface. Factors such as evaporation, soil infiltration or runoff are not taken into account.

It is given in millimeters. 1 millimeter corresponds to 1 liter per square meter. If the amount of precipitation cannot be measured, it is referred to as “less than 0.1 mm”. In the case of snow, hail, etc., the precipitation must first be brought into a liquid form in order to be able to measure it (water equivalent). The usual information in weather reports "liters per square meter" means liters per square meter in the past hour, from which one can infer the severity and strength of the precipitation.

Two different types of measuring devices are used to measure:

  • Non-registering rain gauges (rain gauges)
  • Recording precipitation gauges (precipitation recorder, pluviograph)

The amount of precipitation can also be displayed by a precipitation radar. The scattering of microwave radiation on cloud droplets is used here.

4 Water vapor content in the air

A warmer atmosphere increases their ability to hold water vapor, so that in a warmer world there will be on average more water in the air and therefore more precipitation will fall. A rule of thumb says that the maximum possible water vapor content doubles every 10 degrees, so the relationship is roughly exponential. The widespread explanation that the air can only hold a certain amount of water and then has no more space is wrong. After all, the air is extremely thin, which can already be seen in the weight difference between liquid water and air. The limitation of the amount of water vapor in the air is not a property of the air itself, but simply the result of evaporation and condensation. At higher temperatures, molecules move faster and so more water molecules have enough energy to leave the liquid water and change into the gas phase. The rate of molecules returning to liquid water also changes. Only the overall balance ultimately leads to the maximum possible water vapor content in the air.

5 intensity of precipitation

Precipitation intensity is the quotient of the amount of precipitation and time and is given in millimeters per minute or per hour. Together with the amount of precipitation, it forms the characteristic of precipitation.

A moderate rain shower in Central Europe has an intensity of 5 mm / h (corresponding to 5 liters / m² per hour), a heavy rain of 30 mm / h. During a violent storm, the amount of rain can increase to 50 mm / h and more. Tropical storms reach values ​​of 130 mm / h and well above.

In addition to the direct calculation on site, precipitation intensities can also be determined using radar. The radar reflectivity, which depends on the intensity of the rain, is used for this purpose.

6 Duration of precipitation

The term precipitation duration stands for the duration of a precipitation event. On the basis of the duration of precipitation, a distinction is made between permanent precipitation and shower precipitation. It is also necessary for the definition of return intervals for heavy rain events and flood scenarios.

7 lessons

Experiment:

Pressurize a bottle filled with air and a little water, for example by drilling a bicycle valve through a shortened cork. Put the cork on the bottle and fill in some additional air with a pump (caution: do not overdo it, otherwise the cork will come towards you). Wait a moment, then pull the cork. With a little luck, a cloud should form.

Explanation: As the escaping air expands, it has to overcome the external air pressure. This slows it down and cools it down. The cooler air can no longer contain as much water vapor, so it condenses into small droplets.

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