Why is there always a "football" on the meteorological station?

Undergraduate roommate, who is quite funny to a northeast person, has often stressed that he is a fellow villager of Bingbing recently. Let's travel to Huangshan Mountain, overlooking the top of the mountain. Other mountains look short under the sky. , very magnificent. However, you can't climb to the top of the mountain, because there is a weather station, and only the people in the station and the invited people can enter.

There is such a weather station at the top of Huangshan Mountain, which is old-fashioned!

This weather station has been around since 1956. It was expanded more than ten years ago and has become what it is now.

There is a newborn football on it. What the hell?

That's not football, it's radar.

Come on, I've seen radar. Like a pot. Look inside the red alert:

Choose the Soviet camp in the game and build this radar. You will get a small map view and reconnaissance plane options in the upper right corner (from the game Command and Conquer: Red Alert 2).

There are many kinds of radars, and that is the most conventional radar in the Red Police. Draw a model. The radar here is called Doppler radar. In fact, it is like a pot inside, and there is a special shell outside to protect the antenna. A slightly larger weather station now has this high-tech product.

Is this something like football? Is it so powerful?

In other words, Doppler radar is powerful. ...

Oh, I see. It should be more like this thing. Always overbearing!

The weather controller, the "special weapon" of the allied camp in the game, can summon a "lightning storm" to bring a fatal blow to the enemy base (from the game Command and Conquer: Red Alert 2).

This is our civil weather forecast radar, not a weather controller, and it can't summon lightning. ...

So what's the use of this "football"?

This is very useful. Listen to me slowly.

Radar, you may have heard of it. There will be something in the popular science books of primary schools: human beings invented radar by studying the principle of bat hunting.

Radar can emit electromagnetic waves or sound waves, and estimate the position and size of an object by calculating the time difference between the transmitted signal and the reflected signal. Radar can be widely used in war, agriculture, meteorology, aerospace and other fields. This is a very common device.

The most well-known shape of radar is something like a cauldron, with an antenna in the center, which looks up at the sky obliquely and detects what flying objects are in the air while turning, just as depicted in the game.

However, adding a "Doppler" in front of the radar may be unheard of by many people. At the same time, people can notice that there is a mysterious spherical or hemispherical building on the tower next to many weather stations and airports, which looks like a football.

Sichuan Meteorological Bureau, Shenzhen Meteorological Bureau, Pudong Airport Tower

(Image from image.baidu.com)

So, what is "Doppler radar"?

This involves a physical concept in middle school-Doppler effect.

Doppler effect refers to the phenomenon that when an object moving relative to a reference emits a wave that is not perpendicular to the direction of motion, the wavelength of the wave will change accordingly.

The most common Doppler effect in life is the Doppler effect of sound. When a car whizzes past you, its sound will change from high to low, and the tone of the sound is related to the wavelength of sound waves. The higher the pitch, the smaller the wavelength and the higher the frequency of sound waves.

Therefore, for objects with the same sound wave propagation direction, the tone will become higher and the wavelength will become shorter; Moving objects in the opposite direction of sound waves will have lower pitch and longer wavelength.

Doppler effect (from Wikipedia)

Knowing the Doppler effect, it is easy to understand the detection principle of Doppler radar. It will constantly send out electromagnetic wave signals or acoustic wave signals, and obtain a frequency difference by detecting the frequency of electromagnetic wave/acoustic wave bounced by the object, and then infer the moving speed of the object. According to the reflection results of different emission frequencies, the moving direction and distance of the object can be inferred.

The biggest difference between it and ordinary radar is that it needs to be turned on all the time, and then the effective information can be distinguished by using the frequency difference of waves reflected by integrated circuits, computers and other objects. Ordinary radar calculates the motion information of the object by calculating the time difference between the transmitted signal and the received reflected signal, and the obtained information is more concise.

Doppler radar used for meteorological observation usually emits microwaves (an electromagnetic wave, which is emitted in a microwave oven). By calculating the velocity of airflow at different heights by Doppler effect, the vertical distribution of wind speed, water vapor and other information in the nearby area can be obtained.

For different observation objects, the emission frequency of meteorological Doppler radar will be different, because the reflectivity of different objects to electromagnetic waves will change with the change of frequency.

Generally speaking, the working frequency range of a single radar is relatively limited, and the specific working frequency is more accurate only when detecting particles of a specific size. For example, cloud radar is suitable for detecting cloud particles with smaller particles, and is used to detect middle and high-level clouds and measure the height of cloud bottom and cloud top. Rain radar has a long wavelength and low frequency, which is suitable for detecting raindrops and low-level rain clouds with large particles, and also for tracking clouds that may produce precipitation, so as to achieve the purpose of real-time weather observation, so it is also called weather radar.

There is also a short-wavelength VHF/UHF radar, which can be used to detect atmospheric turbulence and charged particles in the atmosphere, and is suitable for monitoring lightning and other phenomena.

When Typhoon Tian Ge landed on 20 17, the thicker the clouds, the redder the color.

So why is the weather station radar spherical?

This spherical shell is called "radome".

In fact, the real core of Doppler radar is the shape of cauldron and antenna, which is not obviously different from ordinary radar. Adding a football-like shell is mainly used to protect the internal antenna and prolong the service life. Because Doppler radar needs to be turned on all day, it is more useful to add a radome.

Radome can also play the role of evenly distributing signals. But at the same time, the radome will lead to signal attenuation and poor resolution, so some radars with high resolution requirements will not install this radome.

Radome of a base in Japan (from Wikipedia)

Doppler radar is widely used in meteorological and military fields and is an extremely important facility. According to industry statistics, there are currently 270 new generation weather radars (it is estimated that in 2020, all of them will be Doppler radars), X-band radars (used in emergency support or major events) and wind profiler radars (69 by 2016 * *) connected to the same network, forming a national meteorological observation network.

References:

1. Weather Radar Detection and Application (edited by Hu Mingbao, Meteorological Press, 2007. 1 1)

2. Radar Meteorology (edited by Zhang Peichang and others, Meteorological Press, 2005.438+0)

3./question/ 1 187230 14 . html

6. Q&A of China Network: What are the special plans for the development of weather radar (20 17-2020)?

Author: Zhang Yuheng

Editor: Mars