Shulou(Shulou.com)11/24 Report--

You must have a lot of travel plans this holiday.

But with typhoon "du Suri" coming

There has been a lot of heavy rain all over the country.

It is often followed by lightning and thunder

Lightning seems to have always been synonymous with summer.

When the lightning cut through the night sky

Instantly illuminate one side

Flash through a beautiful arc

Attracted many photography enthusiasts to stop.

I hope to capture that amazing flash photo.

Many people follow in the footsteps of lightning

Just to see its true appearance under a high-speed camera.

But at this time,

Are you scared, too?

What if it splits down?

Encounter thunderstorm weather

Where on earth is safe?

Always say don't stand under the tree on rainy days

So does lightning have to split only the tallest objects?

In the season of frequent thunderstorms

How to travel safely?

With these questions

We walked in together.

This makes us both love and fear lightning.

Unveil its mystery.

1. What is lightning is a natural phenomenon caused by electrostatic discharge, which occurs between two charged regions with different charges. When the electric field between the two charged regions is strong enough and the local voltage reaches the breakdown voltage of the air, the air between the two regions can be continuously ionized, and the air will be broken down to form a plasma, so that the current will pass through the originally insulated air to form lightning. According to the occurrence area, lightning can be divided into three types: intra-cloud lightning, inter-cloud lightning and cloud-to-ground lightning. The first two are the most common forms of lightning. Cloud-to-ground lightning accounts for only about 25% of all lightning, but it has the most research because its electricity has a great impact on human production and life.

2. How is lightning formed? and how is lightning formed? Before we answer this question, let's take a look at how the charge in the cloud is generated.

The detailed process of cloud electrification has not yet been determined, but it can be roughly understood as follows:

In the temperature range of-15 ℃ ~-25 ℃, there are a large number of ice crystals and soft hail, which collide with each other in the process of movement. due to friction electrification, ice crystals are positively charged and soft hail is negatively charged, the mass of ice crystals is small, the positive charge moves upward, the mass of soft hail is large, and the negative charge moves downward. As a result, the negative charge accumulates in the lower part of the cloud and the positive charge accumulates in the upper part of the cloud.

Image source: reference 3 the positive and negative charges in the cloud are separated, forming a strong electric field.

Image source: reference 1 if these two areas discharge relative to each other, lightning will be generated in the cloud.

Image source: reference 1 most of the lightning we usually see belongs to intra-cloud lightning or inter-cloud lightning, like this:

Image source: lightning like reference 1 can only see bright light, but can not see the path of lightning, because these tracks are hidden in the clouds, these lightning will not reach the ground and have less impact on us. We will not study these lightning for the time being.

Let's take a look at the formation of cloud-to-ground lightning.

If there are more negative charges gathered at the bottom of the cloud, a positive charge will be induced on the ground, and a strong electric field will be formed between the cloud and the ground, and the air between the cloud and the ground will be ionized to form a conductive channel, and the negative charge in the cloud will flow to the earth to form cloud-to-ground lightning.

Source: let's analyze this process in detail below in Ref. 1.

A cloud-to-ground lightning consists of multiple discharge pulses, and each discharge pulse includes a "pilot" and a "return stroke". Before the first discharge pulse burst, there is a preparatory stage: cascade pilot discharge.

Figure source: reference 1 under the action of a strong electric field between the cloud and the ground, the negative charge moves downwards. In the process of movement, the air is continuously ionized to form a luminous discharge channel. In the process of the first electric pulse, the discharge is not continuous, but into a ladder, which is called a step leader. The length of each stage is about 50 meters, and there is an interval of about 50us between the two steps, with an average speed of 150km/s, about 1/2000 of the speed of light, and a radius of about 1 to 10 meters. Because the leader often forks, lightning looks like the shape of a tree branch.

When the step leader is about to reach the ground, the positive charge on the ground increases, the electric field between the ground and the cloud increases, and the positive charge on the ground moves upward and meets the negative charge, thus forming a path between the cloud and the earth. The strong current flows from the ground to the cloud layer at a very fast speed. This is the first discharge pulse. This process is called "return stroke" and takes about 70us time. The speed is about 1/10 to 1/3 of the speed of light.

Image source: reference 1 in the return phase, multiple positive beams will be generated and want to meet the downward negative beam, but once a channel is closed, all the charge will be transferred to that channel, forming a powerful lightning.

Image source: reference 1 generally high energy in the cloud, a pulse can not release all the energy, it will produce a second conductance and return shock. Since the first cascade leader has opened the energy transmission channel, the subsequent leader reaches the ground directly along this channel, which is called the "straight channeling leader", so the time is faster. This process is repeated until the charge in the cloud is depleted. This is the flicker we see.

Image source: reference 13, classification of lightning as mentioned earlier, according to the area where lightning is generated, lightning can be divided into: intra-cloud lightning, cloud-to-cloud lightning and cloud-to-ground lightning.

At the same time, according to the direction of charge, lightning can be divided into positive lightning and negative lightning. From the clouds to the ground is a positive charge called positive lightning, and vice versa is negative lightning.

Image source: reference 1 the energy of general lightning is about 300 million volts, 30,000 amperes, because the distance of positive lightning is much longer than negative lightning, the energy and temperature are much larger than negative lightning, the temperature near positive lightning can reach 30000 ℃, which is more than 5 times the surface temperature of the sun (5600 ℃), so positive lightning is more dangerous than negative lightning.

In general, positive lightning has only one return stroke (that is, only one flash), while negative lightning has multiple return strikes (that is, flickering), which can be used to determine whether a lightning strike is positive lightning or negative lightning.

So the following editor will test you to see which of the following two types of lightning belong to?

Figure source: reference 1 answer: negative lightning

Picture source: reference 1 answer: positive lightning

The next time you encounter lightning, you can also judge what kind of lightning it is while taking photos.

4. Must Lightning only split the tallest objects? it's actually very simple to answer our questions, starting with the tip discharge.

We learned in high school physics that the distribution of charge on the surface of a conductor, the more sharp the charge is, the denser the charge is, the stronger the electric field is, and the easier it is to produce tip discharge. The lightning rod takes advantage of this principle.

Buildings or protruding objects on the ground, such as hilltops or tree tops, are "tips" relative to the ground, so when the clouds are close to the ground, these "tips" are more likely to gather electricity. the air between the cloud and the tip is more likely to be pierced, resulting in an electric shock.

Although lightning prefers high objects in reason, it is not absolute. After all, lightning is a random natural phenomenon, and there are many examples of people who are killed by lightning every year while driving or on ships.

5. Some little tips about lightning have talked so much that you can't wait to know how to avoid lightning outdoors. In fact, the relevant methods are already familiar, but the editor wants to tell you that there is no safe place outdoors on thunderstorm days.

What if you are struck by lightning outdoors? In the absence of indoor or other protective facilities, the ultimate way to protect you from lightning is:

Immediately stay away from any highest point, such as a mountain top, ridge, or roof.

Stay away from the open land and avoid becoming the commanding heights

Don't lie flat on the ground.

Don't hide under an isolated tree.

Stay away from water sources or conductors

Don't hide in shallow holes or ravines.

Lying down or squatting is not a good way to avoid lightning strikes. To reduce the probability of lightning strikes, you must minimize the height and the contact area between your body and the ground, but squatting is just an impossible way, and this will not significantly reduce the probability of being struck by lightning. On the contrary, it may make you miss the best opportunity to escape. So the best way to avoid lightning is not to stay outdoors.

Never wait until you are about to be struck by lightning to react. When your hair is sticking up or tingling, there may be no way to avoid an electric shock. If that happens, all you can do is run to safety as quickly as possible. Cars, buildings and other interiors are all safe places.

Therefore, the editor suggests that everyone check the local weather conditions in advance before going out, pay attention to the weather forecast in time, and avoid traveling in thunderstorm weather. When you encounter lightning, the distance of lightning can be roughly judged by the difference between the propagation velocity of lightning and thunder. We know that the speed of sound in the air is 340m/s, and the speed of light is 300000km/s. If no thunder is heard within 20 seconds after seeing lightning, it means that lightning is far away from us. On the contrary, it means that lightning is closer to us and we need to find buildings to hide as soon as possible. 30 minutes after the last thunder, the danger of lightning can be basically relieved.

Reference:

1 、 How Lightning Works-Weird World of Lightning

2. Lightning (Natural phenomena) _ Baidu Encyclopedia (baidu.com)

3. Lightning-Wikipedia

4. Complete Guide to Lightning Protection in the Field-zhihu.com

5. Lightning Faqs (weather.gov)

6 、 Lightning Safety Tips and Resources

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: Xiao Cong

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